1. Introduction
ver the course of the last couple
of centuries, scientists have amassed large amounts of
evidence which have led them to conclude that the Universe
is about 12-14 billion years old and was formed in the
primordial event that scientists now call the Big Bang.
However, in the last fifty years, an offshoot of
Fundamentalist Christianity has grown up (mainly in, but
not limited to, the US) called Young Earth Creationism.
Adherents, called Young Earth Creationists (YECs),
vehemently reject most of modern science, on the basis that
it contradicts their own version of Christianity, which is
based on a strict literal interpretation of the Bible (and
in particular, the early chapters of Genesis). Perhaps
their most strident (and famous) opposition is to Darwin's
Theory of Evolution.
YECs believe that the Universe, and therefore the Earth
and all on it, including Humanity, were created by the
Biblical God, Yahweh, in only six days approximately 6,000
years ago.
While the majority of YECs are involved in trying to
refute the findings of modern science in biology and
geology, a few look to astronomy and cosmology to support
their beliefs. One of their approaches deals with
supernova remnants, the remains of the exploding
stars known as supernovae. YECs make two claims
regarding supernova remnants:
- There are not enough supernova remnants observed in our
Galaxy to support an ancient Universe - the numbers
observed actually are indicative of a young Universe.
- There are no ancient supernova remnants thus the
Universe is actually young.
This FAQ is written with the dual intention of both
providing a general introduction to supernovae and
supernova remnants, and addressing the
YEC claims. To
this end, it is split into six main parts:
- Sections 2 to 4
provide a general introduction to supernovae, along with a
detailed description of what actually happens in a
supernova, and also some examples of past supernovae.
- Sections 5 to 7
provide information on supernova remnants, the by-products
of supernovae. Once again, examples of supernova remnants
are provided.
- Section 8 looks into the
relationship between supernovae and us, and the dangers to
the Earth posed by supernovae.
- Section 9 dives briefly into the
strange world of the phenomena known as
hypernovae.
- Section 10 deals with the
assertions of the YECs.
- Sections 11 to 13 detail notes, references and other
materials used in preparing the FAQ.
2. What are
Supernovae?
supernova: a star
that explodes and becomes extremely luminous in the
process
That's it. Literally, a supernova is an exploding star.
The star explodes in a massive explosion, resulting in an
extremely bright and short-lived object that emits vast
amounts of energy, typically as much as an entire galaxy.
As well as visible light (i.e. optical radiation),
supernovae emit huge amounts of various types of radiation:
X-rays, ultraviolet, infrared, gamma rays, neutrinos,
cosmic rays and radio waves. The remains of the matter that
is exploded away from the star during the supernova is
known as a supernova remnant. Supernovae were first
proposed as a distinct class of objects in 1934 by the
astronomers Fritz
Zwicky and
Walter Baade.
3. What are the different types of
Supernova?
The taxonomy of supernovae is quite complicated.
Astronomers use observational criteria, not theoretical
criteria, to type supernovae. Type 1 supernovae do not have
hydrogen lines in their spectra1, but Type II do. Each Type is
broken down into further subclasses, depending on their
light curves (Figure 1), progenitors and location -
Type I into Types 1a, 1b and 1c, and Type II into Types IIL
and IIP (Cappellaro
& Turatto 2000).
As with most other classifications, there are
exceptions. The spectra and/or light curves of a few
supernovae differ sufficiently from the standard types to
lead astronomers to suggest several new subclasses (Panagia et al. 1986; van Dyk et al. 1993; Baron et al. 1995, Benetti et al. 1998; Lentz et al. 2000; Filippenko 2000; Li et al. 2000; Howell 2000).
3.1 Type I Supernovae
Type Ia supernovae occur in a binary system, where one
component is a white dwarf 2. The gravitational attraction of
the white dwarf is so intense that it is capable of
siphoning off material from its companion star (Hachisu & Kato 2001).
This causes the star to exceed its limit of stability - the
Chandrasekhar limit3 -
causing it to go into thermonuclear instability. At this
point, thermonuclear incineration of the white dwarf
ensues, although how exactly this occurs is still under
debate, as the physics of thermonuclear burning in the
degenerate matter that makes up a white dwarf is complex
and still not completely understood, although there is much
research going on in this area (e.g. Woosley & Weaver 1994;
Branch et al. 1995;
Hillebrandt &
Niemeyer 2000; Hillebrandt et al.
2000; Branch 2000; Ghezzi et al. 2001).
Whatever the exact mechanics, however, the result is a
massive explosion that produces an extremely massive
outburst of energy, some 1051 ergs, with an
absolute magnitude of about -19.5 (Sandage et al. 1996; Saha et al. 1996)4. The star literally blows itself
to bits, leaving nothing behind except a rapidly expanding
remnant.
Type Ib and Ic supernovae are actually similar to Type
II supernovae (they were named before astronomers really
understood what they were). They occur as a giant star of
about 20 solar masses evolves and loses its hydrogen
envelope (the outer layers of the star) via either stellar
winds (the extremely weak flow of charged particles,
consisting of mainly protons and electrons, which stream
from a star's outmost layer into interplanetary space) or
to a binary companion (van Dyk
et al. 1996); then the exposed helium core
explodes. As with Type II supernovae, the explosion is
triggered by the collapse of its iron core. Type Ib and Ic
supernovae are (slightly) less spectacular than Type Ia
supernovae. Type Ib supernovae have strong Helium lines in
their spectra, whereas Type Ic supernovae have weak or no
Helium lines in their spectra (Baron et al. 1996). The
relationship between Type Ib and Ic supernovae and Type II
supernovae is such that several Type II supernovae have
been observed to have transformed into Type Ib/Ic
supernovae (e.g. Finn et
al. 1995; Matheson
et al. 2001).
The standard Type Ia supernovae light curve shows an
early peak followed by a sharp drop, then a linear decline
after 50 days at a rate of 0.015 magnitudes per day. The
light curves of Type Ib supernovae, though being dimmer
than a Type Ia at maximum, show a similar sharp drop.
However, the subsequent exponential decline differs
markedly from that of Type Ia supernova, with the rate of
decline less for Type Ib supernova than Type Ia, being
about 0.010 magnitudes per day. The light curves of Type Ic
supernovae are identical to that of Type Ib supernovae.
3.2 Type II Supernovae
These occur when a high mass star (greater than
approximately 7.6 solar masses) no longer has enough fuel
for the fusion process5 in
the core of the star to create the outward pressure which
combats the inward gravitational pull of the star's great
mass. When this occurs, the star will swell into a red
supergiant... at least on the outside. On the inside, the
core yields to gravity and begins shrinking. As it shrinks,
it grows progressively hotter and denser. This allows a new
series of nuclear reactions to occur, forming new elements
which in turn fuse to form further new elements, and so on.
This allows the star to temporarily keep on shining
(Table 1). All these differing reactions take
increasingly shorter periods of time, and release
progressively less amounts of energy6. As these new reactions take
place, the structure of the star becomes similar to an
onion - there are shells of progressively less heavy
chemical elements surrounding the core.
Table 1 - Nuclear Burning
Processes
(data from Prialnik (2000))
Nuclear
Fuel
|
Process by
which reaction occurs
|
Threshold
(106 K)
|
Products
|
Energy Released
per Nucleon (MeV)
|
Hydrogen
|
p-p
|
4
|
Helium
|
6.55
|
Hydrogen
|
CNO
|
15
|
Helium
|
6.25
|
Helium
|
3-alpha
|
100
|
Carbon,
Oxygen
|
0.61
|
Carbon
|
C + C
|
600
|
Oxygen, Neon,
Sodium, Magnesium
|
0.54
|
Oxygen
|
O + O
|
1000
|
Magnesium,
Silicon, Sulphur, Phosphorus
|
0.30
|
Silicon
|
Nuc.
eq.
|
3000
|
Cobalt, Nickel,
Iron
|
<
0.18
|
Once the star fuses silicon into iron it hits a major
snag. As can be seen, the above reactions create energy (an
exothermic reaction). But to convert iron into
heavier elements requires energy (an endothermic
reaction, which requires approximately 2 MeV per Nucleon).
Thus, fusion halts. At the very high temperatures now
present in the core of the star (much greater than
109 K), a process known as
photodisintegration7
occurs. Due to the loss of energy that occurs due to
photodisintegration, the core starts to rapidly collapse.
Different parts of the core collapse at different rates,
with the result that the inner core decouples from the
outer core, leaving it behind. During the collapse, speeds
can reach 7,000 km s-1 in the outer core, and
within about one second, a volume the size of Earth has
been compressed down to a radius of 50 kilometres. As a
result the rest of the star is left in the precarious
position of being almost suspended above the
catastrophically collapsing core. This collapse of the
inner iron core continues until the density there exceeds
approximately 8 x 1017 kg m-3. At
this point, the material that now makes up the inner core
stiffens (as a result of the nuclei of the atoms present
becoming repelled by each other), with the result that the
inner core now rebounds somewhat, sending pressure waves
outwards into the infalling material of the outer core.
These pressure waves, when they reach the local speed of
sound, form a shock wave that starts moving outwards.
As the shock wave propagates outwards, it encounters the
falling inner iron core. The extremely high temperatures
that occur as a result of this cause further
photodisintegration, robbing the shock of most of its
energy8. If what's left of
the iron core is not too massive (less than 1.2 solar
masses), the shock will fight its way through the rest of
the outer core - which takes about twenty milliseconds, and
collide with the remainder of the outer layers of the star.
On the other hand, if the iron core is massive enough, the
shock stalls, becoming nearly stationary, with infalling
material now accreting onto it. At this point, the
neutrinos now streaming from the core (due to the
conversion of the iron core into essentially a neutron
core) superheat the material beneath the shock wave; the
resulting plumes of hot material push the shock wave
outwards and allow it to continue its march towards the
surface, driving all before it (Janka 2001). As the shock
encounters material in the star's outer layers, the
material is heated, fusing to form new elements and
radioactive isotopes (Meyer et
al. 1995; Thielemann
et al. 1996). The shock then propels the various
outer layers of the star out into space, leaving the inner
core behind. The total energy in the expanding material is
in the order of 1051 ergs (or less). Vast
amounts of photons are released, resulting in a spectacular
optical display, the equivalent of 109 suns,
giving an absolute magnitude of about -18. Due to the
radioactive decay of heavy elements produced in the
explosion (Mochizuki
& Kumagai 1998; Hernanz
2000; Wanajo et al.
2001), it then starts to slowly fade, at rate of
approximately six to eight magnitudes each year. Type II
supernovae are not as luminous as Type Ia supernovae, by a
factor of at least three. The mechanics of this type of
supernovae are dealt with in detail by Bethe (1993), Wallerstein et al. (1997), Mezzacappa (2000) and Liebendoerfer et
al. (2001).
If the mass of the core remnant is beneath about three
solar masses, it will become a neutron star9 (rapidly rotating neutron stars
are known as pulsars10).
If it exceeds about three solar masses, it continues to
contract. The gravitational field of the collapsing star is
so powerful that neither matter nor light can escape it.
The "star" then collapses to a black hole (Balberg & Shapiro
2001), a singularity or point of zero volume and
infinite density, hidden by an event horizon at a distance
called the Schwarzschild radius11. Bodies crossing the event
horizon, or a beam of light directed at such an object,
would seemingly just disappear - pulled into a "bottomless
pit". In either case, the creation of these rather exotic
objects is accompanied by a tremndous production of
neutrinos, the majority of which escape into space with a
total energy approaching 3 x 1053 ergs12.
The majority of Type II supernovae are split into II-L
(linear) or II-P (plateau) subclasses, depending on their
light curves - Type II-P display a plateau soon after
maximum luminosity.
4. Examples of
Supernovae
4.1 Past Supernovae
Since the earliest days of mankind looking up into the
skies, we have saw many bright points of light in the sky
which appeared suddenly and then slowly faded away over the
course of many months. Most of these "guest-stars", as the
ancient Chinese called them, were novae of various sorts,
but some were genuine supernovae. The most reliable records
come from Asia, where Korean, Japanese and Chinese
astronomers kept suprisingly accurate records dating back
as far as 1400 BC - Wang (1986)
reported that there were 90 probable novae and supernovae
listed in Chinese records between 1400 BC and 1700 AD. In
Europe, on the other hand, the earliest known observation
of what we now know to be a supernovae was not until the
11th century AD. As a result of intensive study of these
records, and later reports by European astronomers like
Tycho and Kepler, astronomers are now aware of quite a few
Galactic supernovae having occurred in the last couple of
thousand years (Table 2)13.
Table 2 - Historical
Galactic Supernovae
(data from various sources
including Michael Richmond's "
Information on the Historical Supernovae" and Strom (1994))
Year
|
Peak
Magnitude
|
Constellation
|
Distance (light
years)
|
|
-6
|
Centaurus
|
4,500
|
386
|
-3
|
Scorpius
|
|
1006
|
-10
|
Lupus
|
4,600
|
1054
|
-6
|
Taurus
|
6,500
|
1181
|
-1
|
Cassiopeia
|
8,500
|
1572
|
-4
|
Cassiopeia
|
10,000
|
1604
|
-3
|
Ophiuchus
|
14,300
|
|
6?
|
Cassiopeia
|
9,100
|
The first extragalactic supernova ever discovered was SN
1885A near the nucleus of M31 (the famous "Andromeda
Galaxy") on 20 August 1885. SN 1885A had an apparent visual
magnitude of 5.85 - it would have been just barely visible
to the naked eye had not the glow from M31 overwhelmed it
(de Vaucouleurs &
Corwin 1985).
Probably the most famous extragalactic supernova was
observed on the 24th February 1987 in the Large Magellanic
Cloud A blue supergiant star (of about 20 solar masses)
called Sanduleak -69 202 (its previous apparent visual
magnitude was a lowly 10.2) exploded in a burst of light
visible to the naked eye (when discovered on a photographic
plate by the astronomer Ian Shelton of the University of
Toronto at the Las Campanas Observatory in Chile, it was
magnitude 4.5 - it later peaked at magnitude 2.8 before
fading slowly over time (Shelton
1993)) - thus it was a Type II supernova. It was
designated SN 1987A17. In
the subsequent years, a bright supernova remnant was seen
to form around the star in the form of an expanding shock
wave. Only now, years later, is the shock wave reaching
rings of previously existing gas surrounding the now dead
star (Chu 2000)18. This is causing the knots of
gas to glow brightly. There are many images
of SN 1987A available on the WWW. Perhaps the definitive
review of SN 1987A is Arnett et al. (1989), although
this does not cover more recent developments.
SN 1987A was extremely important to astronomers as it
was the first supernova which astronomers could study in
great detail with modern astronomical instruments. It
confirmed a whole host of predictions that astronomers had
made regarding supernovae, including:
As observing techniques and equipment improve, more and
more supernovae are discovered each year (
Figure 2).
172 supernovae were discovered in 2000. At the time of
writing, 102 supernovae have been discovered in 2001. A
complete list of all supernovae discovered since SN
1885A is maintained by the Central Bureau of Astronomical
Telegrams (CBAT).
4.2 Potential Supernovae
Candidates
The three nearest candidates for supernovae sometime in
the near future (astronomically speaking) are all nearby
(again, in astronomical terms) red giants: Betelgeuse (in
Orion) at 430 light years, Antares (in Scorpius) at 600
light years, and Rasalgethi (in Hercules) which lies 380
light years from Earth. These will all be Type II
supernova. There is a closer red giant - the star Scheat in
Pegasus, this is 200 light years away and although this is
currently a red giant, the progenitor star is almost
certainly not large enough to go supernovae, instead, the
outer layers will slowly drift off into space forming a
planetary nebula, and leaving a white dwarf behind.
However, it is more likely that the next Type II
supernova in our Galaxy might either be the highly evolved
orange supergiant HD 179821 (Jura
et al. 2001) or the blue supergiant Sher
25. Even though both stars are extremely luminous, they
lie at considerable distances from Earth, and thus are not
visible to the unaided eye.
Sher 25 has an age of about three million years, but has
a mass of about 120 solar masses, which makes it one of the
most massive stars ever observed. As it dies, it is blowing
parts of its own outer envelope away at speeds of 20-83 km
s-1. As with Sanduleak -69 202, a bubble of gas
has formed surrounding the star, which are seen as
filaments and a ring shaped structure (Petersen 1999). Indeed, both the
star and the surrounding material resemble Sanduleak -69
202 closely, although there are some minor differences,
most probably due to differences in the environment
surrounding the star (Brandner et al.
1997b). The gases and dust around the star are nitrogen
enhanced - the sign of an evolved, very hot star rapidly
burning through its hydrogen and helium and forming other
elements in the process (Brandner et al.
1997a). Perhaps in a few tens of thousands of years, or
maybe even tomorrow, Sher 25 will explode like Sanduleak
-69 202, and provide another spectacular display of cosmic
fireworks.
Recently, astronomers have suggested that the binary
star KPD 1930 + 2752 is a future candidate for a future
Type Ia supernova event . The primary star in this system
is a subdwarf-B star, and it has an unseen companion star
that is almost certainly a white dwarf. The orbital period
is only 2 hours 17 minutes. The total mass of the system is
1.47 Solar Masses, above the Chandrasekhar limit. Maxted
et al. (2000)
proposed that the binary will merge within approximately
200 million years due to orbital shrinkage and the
evolutionary expansion of the primary), and when this
happens, because of accretion of helium and other elements
heavier than hydrogen onto the white dwarf, a Type Ia
supernova would occur. Some other astronomers have disputed
this scenario, claiming that because the B-star would form
a white dwarf before merging with its companion, the total
mass of the system would be beneath the Chandrasekhar limit
, thus no Type 1a supernovae would occur (Ergma et al. 2001).
It has also been determined that supernovae can be
responsible for the production of runaway stars (or
at least a proportion of them). These are stars that were
originally part of a multiple star system. Sometime in the
past, one of their companions went supernovae, and the
force of the blast pushed the star off into space at a very
high velocity (Blaauw 1961; Hills 1983; Stone 1991; Kaper et al. 1997; Hoogerwerf et al.
2000, 2001).
5. What are Supernova
Remnants?
A supernova remnant (usually abbreviated to SNR) is the remains of
the matter that is exploded away from a star when it goes
supernovae. This ejection of matter is much more violent
than occurs in the planetary nebula that mark the end of a
low mass star, giving expansion speeds of 1000-10,000 km
s-1. The ejected matter sweeps up surrounding
gas and dust as it expands producing a shock wave that
excites and ionises the gas, which results in the
production of X-rays, and radio waves in the form of
synchrotron radiation. This plasma may reach temperatures
of 1,000-1,000,000 K, but with densities of only about a
million particles per cubic metre. Gradually, the expansion
rate slows down, seeding the local neighbourhood with heavy
elements, but not before the remnant occupies an area of
space dozens or hundreds of light years in diameter.
5.1 The Life Cycle of a Supernova
Remnant
In the classical model of SNR evolution (Woltjer 1972; Gull 1973; Chevalier 1977), there are four
stages or phases:
- In the first phase, known as free expansion, the
front of the expansion is formed from the shock wave
interacting with the ambient interstellar medium (ISM).
This phase is characterised by constant temperature within
the remnant and constant expansion velocity of the shell.
This phase can last anywhere from 90 years to over 300
years.
- During the second phase, known as the Sedov or
Adiabatic Phase, the remnant material slowly begins
to decelerate and cool. In this phase, the main shell of
the remnant is unstable, and the remnant's ejecta becomes
mixed up with the gas that was just shocked by the initial
shock wave. This mixing also enhances the magnetic field
inside the remnant shell. This phase can last anywhere from
100-100,000 years.
- The third phase, the Snowplough or
Radiative phase, begins after the shell has cooled
down to about 1,000,000K, so the shell can more efficiently
radiate energy. This, in turn, cools the shell faster and
thus making it shrink and become denser. This makes it cool
faster still. Because of this snowball effect, the remnant
quickly develops a thin shell and radiates most of its
energy away as light. The velocity now decreases fairly
rapidly. Outward expansion stops and the remnant starts to
collapse under its own gravity. This phase can last
hundreds of thousands of years.
- The fourth phase, known as Dispersal. Here the
shell breaks up when the velocity of the "snowplough"
becomes subsonic, and what's left of the remnant dissipates
into the ISM.
This is the theoretical model, in reality astronomers have
found that most
SNRs do not follow
this standard model (
Harrus
et al. 2001). Some of the reasons for this are:
6. What are the different types of
Supernova Remnants?
There are three generally accepted types of SNRs. Note
that the categories are not set in stone - SNRs have been
observed in the process of gradually transforming from one
type to another (Sakhibov
& Smirnov 1982; Lazendic et al. 2000).
The three types are:
- Shell-type remnants: As the shock wave from the
supernova explosion ploughs through space, it heats and
stirs up any interstellar material it encounters, thus
producing a big shell of hot material in space. A ring like
structure in this type of remnant is seen because at the
edge of the shell there is more hot gas in our line of
sight than in the middle. Astronomers call this phenomenon
limb brightening. The vast majority of SNRs are of this
type.
- Crab-like remnants: These remnants, also known
as "plerions" (a term first suggested by Weiler &
Panagia (1978), and
from the Greek word meaning "full") are similar to the Crab
Nebula. They are similar to shell-type remnants, except
that they contain a pulsar in the middle that blows out
jets of very fast moving material. These remnants look more
like a "blob" than a "ring."
- Composite Remnants: These remnants are a cross
between the shell-type remnants and crab-like remnants.
They appear shell-like, crab-like or both depending on what
part of the electromagnetic spectrum one is observing them
in. There are two kinds of composite remnants -- thermal
and plerionic. Thermal composites appear shell-type in the
radio waveband (synchrotron radiation). In X-ray
wavelengths, however, they appear crab-like, but unlike the
true crab-like remnants their X-ray spectra have spectral
lines, indicative of hot gas. Plerionic composites appear
crab-like in both radio and X-ray wavebands, however they
also have shells. Their X-ray spectra in the centre do not
show spectral lines, but the X-ray spectra near the shell
do have spectral lines.
Rho & Petre (1998)
proposed a fourth class of SNRs - the so-called
"mixed-morphology SNRs". These remnants are classified as
shell-type remnants at radio wavelengths, but the X-ray
morphology is centrally peaked. In addition, the X-ray
emission is thermal which comes from the ISM, not the
ejecta making up the SNR. And finally, there is no
prominent, central, compact source in radio or X-ray bands
(i.e. there is no pulsar).
7. Examples of Supernova
Remnants
Some of the more famous SNRs (easily visible in small
telescopes) include:
SNRs are officially designated according to their Galactic
coordinates. Thus the SNR at 184.55
o Galactic
Longnitude and -5.78
o Galactic Latitude,
commonly known as the Crab Nebula, is designated as G184.6
- 5.8.
There are many galleries of SNR images on the WWW -
perhaps the two most extensive are the
ROSAT X-ray Satellite Gallery and the Chandra
X-ray Satellite Gallery. According to Green (2000), there are 225 confirmed SNRs
in our Galaxy, with another 61 possible or probable
remnants, with more are being discovered all the time (e.g.
Bhatnagar 2000; Crawford et al. 2000;
Combi et al. 2001; McClure-Griffiths et
al. 2001). There are hundreds known in other
galaxies (e.g. Danziger et
al. 1979; van den Bergh
1983; Peimbert et
al. 1988; Long et
al. 1990; Braun
& Walterbos 1993; Gordon
et al. 1993; Muxlow
et al. 1994; Yang et
al. 1994; Huang et
al. 1994; Cowan et
al. 1994; Magnier et
al. 1995; Matonick
& Fesen 1997; Dunne et
al. 2000; Schlegel
et al. 2000; Rosado
et al. 2001). There are nine known plerions in
our Galaxy, and twenty-three known Composite remnants, the
rest are Shell-Type remnants (Green
2000), although the proportion of plerions is expected
to rise in the future as SNRs which are currently
classified as Shell-Type or Composite are examined more
closely (Gaensler 2000). Since
the "mixed-morphology" category was proposed, some
astronomers have been re-examining existing SNRs with a
view to recategorising them as mixed-morphology SNRs, so
far about nine have been identified (Yoshita et al.
2001).
8. Supernovae and Us
8.1 Could our Sun turn into a
supernova?
Relax, the answer is an emphatic no! Our sun is nowhere
near massive enough to become a Type II supernova and
there's no white dwarf companion to become a Type Ia
supernova. Besides, it will take another five billion years
before our sun's supply of hydrogen is depleted. At that
time it will begin its dying process and eventually become
a white dwarf with a surrounding shell of material much
like the Ring Nebula (M57) in the constellation of Lyra,
i.e. a planetary nebulae. This is still small consolation
for us on Earth however, as in another billion years or so
the Sun will have increased in luminosity so much that the
Earth will likely become totally uninhabitable.
8.2 What would happen if a Supernova
occurred near to Earth?
In short, life on Earth would be in big trouble.
Depending on the distance and the type, the massive amounts
of radiation emitted by supernovae could mean possibly all
or most of the life on Earth would be fried. From an
article by Michael Richmond on the risks to Earth from
nearby SNRs:
I suspect that a Type II explosion must be within a few
parsecs of the Earth, certainly less than 10 parsecs (32.6
light years), to pose a danger to life on Earth. I suspect
that a Type Ia explosion, due to the larger amount of high
energy radiation, could be several times farther away. My
guess is that the X-ray and gamma ray radiation are the
most important at large distances.
Interestingly, there is a possibility that a supernova
exploded close to earth (within 100 light years) about five
million years ago (Ellis et al.
1996; Fields & Ellis
1999). Could this have caused an extinction event?
Quite possibly. Was there one that correlates to this
supernova? Probably not19.
There is also some evidence of another supernova
occurring within 600 light years of the Sun within the last
couple of million years and which was responsible for the
nearby shell of gas known as the North Polar Spur (Cruddace et al. 1976;
Hayakawa et al.
1977; Davelaar et
al. 1980; Heiles et
al. 1980; Egger
& Aschenbach 1995), although there are alternative
non-supernova explanations (Sofue 1977). There are other
similar large shells of gas in the general Galactic
vicinity (Nousek et al.
1981).
The famous Geminga
pulsar (aka 2CG195+4), which lies close to the Crab Nebula
in the sky, has also been proposed as the remains of a
supernova that occurred 300,000 years ago. Gehrels &
Chen (1993) proposed that
this supernova is the cause of the Local
Bubble20, whereas
Cunha & Smith (1996)
proposed that the supernova was the cause of the loop of
gas surrounding the star Lambda Orionis in the
constellation of Orion, again, roughly 300,000 years ago.
Innes & Hartquist (1984) also proposed that
the Local Bubble was the result of a past supernova,
whereas Smith & Cox (1998, 2001) have suggested that
instead of being the result of one supernova, the Local
Bubble is the result of repeated supernovae.
Incidentally, Geminga lies approximately 510 light years
from Earth (Caraveo et
al. 1996a; Caraveo
et al. 1996b). Although it was discovered in
1975 as a source of high-energy gamma-rays (Thompson et al. 1977;
Bennett et al.
1977), it was not until 1992 that astronomers worked
out what it actually was (Bertsch et al. 1992; Halpern & Holt 1992).
This is reflected in its name - "Geminga" is Milanese for
"That which does not exist".
As a brief aside, the Old Earth Creationist Hugh
Ross21, has stated:
According to Genesis 5 and 6, one of the many changes
God decreed at the time of the Genesis Flood was the
shortening of human lifespans from an average of 900+ years
down to a maximum of about 120 years. Besides protecting us
from intensification of evil, this change, which apparently
involved a reprograming of our cells, also protects us from
certain types of cancer. The change also involved either
the removal of some sort of pre-Flood radiation shield or,
more likely, an increase in the amount of cosmic radiation
showering the Earth.
He identifies the supernova responsible for the Vela SNR
as being one possible cause of this change of life span.
However, on the assumption that such a change in human
lifespans did take place as reported in Genesis, there are
still two major problems with this claim:
- The Vela supernova was between 800 and 1,600 light
years away (Gvaramadze
2001a)- at this distance the radiation hitting the
Earth from the supernova itself is negligible (about the
same as several hours normal sunshine at the surface of the
Earth), mainly due to distance and the protective effects
of the Earth's atmosphere22.
- In any case, such a dose of radiation as Ross proposes
would have left traces of isotopes in various sediment
layers - no such traces in these layers have been
found.
In short, there is no evidence to support this claim,
and plenty of evidence against it.
8.3 Is it true that the Earth wouldn't
exist if it weren't for Supernovae?
The Big Bang produced very little but hydrogen and
helium, with some lithium (Thielemann et al.
2001). Various other elements (heavier than carbon but
lighter than iron) are produced by fusion in the red giant
stage of stars (Table 3). Elements heavier than iron
get produced mainly in supernovae, specifically in the
explosive nuclear burning that takes place either during
the phase where the shock wave that results from the
collapse of the star's core encounters the outer layers of
the star (for Types Ib, Ic and II supernovae), or in the
general nuclear fireball that Type Ia supernova become. In
the aftermath of a supernova event, the local ISM is
saturated with these heavy elements. The supernovae and the
resulting blast wave heat and stir up the ISM. For stars
that don't go supernova, most of their heavy elements get
locked up in the white dwarf that they end up as These
elements are slowly distributed through the stellar wind
and other forms of mass loss (Vink
et al. 2001).
Table 3 - The 10 most common
elements in the Galaxy, their proportions and origins
(data from Croswell (1996))
Atomic Number |
Element
|
Abundance by
Number (Hydrogen = 1.000)
|
Produced Mostly
by
|
Ejected into
our Galaxy by
|
1
|
Hydrogen
|
1.00000
|
Big
Bang
|
Big
Bang
|
2
|
Helium
|
0.09700
|
Big
Bang
|
Big
Bang
|
8
|
Oxygen
|
0.00085
|
Helium burning in
high mass stars
|
Type Ib, Ic, and
II Supernovae
|
6
|
Carbon
|
0.00036
|
Helium burning in
red giants and high mass star
|
Planetary nebulae
and Type Ib, Ic, and II Supernovae
|
10
|
Neon
|
0.00012
|
Carbon burning in
high mass stars
|
Type Ib, Ic, and
II Supernovae
|
7
|
Nitrogen
|
0.00011
|
Hydrogen burning
in main sequence stars and red giants
|
Planetary
nebulae
|
12
|
Magnesium
|
0.00004
|
Neon burning and
carbon burning supernovae in high mass stars
|
Type Ib, Ic, and
II Supernovae
|
14
|
Silicon
|
0.00004
|
Oxygen burning in
high mass stars
|
Type Ib, Ic, and
II Supernovae
|
26
|
Iron
|
0.00003
|
Type Ia, Ib, Ic,
and II Supernovae
|
Type Ia, Ib, Ic,
and II Supernovae
|
16
|
Sulphur
|
0.00002
|
Oxygen burning in
high mass stars
|
Type Ib, Ic, and
II Supernovae
|
Astronomers investigating a class of meteorites known as
carbonaceous chondrites (so-called becuse they
contain carbon and are characterised by small
inclusions or chrondules of molten material
within them) have found by-products of short-lived
radioactive isotopes which are produced either exclusively
or mainly in supernovae (Lee et
al. 1978; McCulloch &
Wasserburg 1978; Clark 1979;
Arnould et al. 1980;
Dearborn et al.
1988; Nittler et al.
1996; Ott 1996; Timmes et al. 1996; Amari et al. 1996; Hernanz 2000). As such meteorites
are thought to be primeval remnants from the time of the
early solar system, approximately 4.6 billion years ago, it
indicates that at some point before the formation of the
solar system, a supernova occurred.
To sum up, most of the elements that make up the
computer you're using to view this article, the world
around you, the solar system and your body, were originally
produced in a supernova (Cameron & Truran 1977;
Harper 1996). As the singer Joni
Mitchell put it, "We are stardust...". So the answer is
yes - without these supernovae, it is very likely that us
(humanity) and everything else on the Earth (and also the
Earth itself) would not exist.
9. What are
Hypernovae?
It has been proposed that not all massive stars
successfully launch supernova events. If the core of a
massive rapidly rotating progenitor collapses into a black
hole and absorbs the surrounding stellar mantle without
producing a neutrino driven explosion, the result is a
collapsar, and the massive release of energy
produced is called a hypernova. Hypernovae are
typically 150-200 more massive than the Sun and explode
with a total energy output of more than 1052
ergs (Nakamura et al.
2001) - many times more than the energy output of a
typical supernova.
Hypernovae have been proposed as a way to explain the
existence of Gamma-ray Bursts23 (Woosley
1993; Paczynski 1997; MacFadyen & Woosley
1999; MacFadyen 1999).
Gamma-ray Bursts (GRBs). Astronomers have identified
several hypernova-type events that appear to be linked to
observed GRBs (Hansen 1999; Bloom et al. 1999; Chu et al. 1999; Filippenko 2000; Iwamoto et al.
2000).
However, other explanations for GRBs have been put
forward - these include
Currently, none of the hypotheses have been confirmed to
the exclusion of the others - indeed it is now apparent
that there are different types of GRBs which could be
caused by differing processes (Piro et al. 2000). More
detailed discussion of GRBs is beyond the scope of the FAQ,
but Meszaros (1999, 2001), Antonelli et al. (2000) and Piran (2001) all give good overviews of our
current understanding of GRBs.
10. Are Supernova Remnants evidence
of a young Universe?
All of the YEC literature on the WWW concerning
supernovae and supernova remnants originates from one
article written in 1994 by a Canadian, Keith Davies,
entitled "Distribution
of Supernova Remnants in the Galaxy". This article
is part of the Creation Discovery
Project. Various versions and summaries of this article
appear on various other YEC web sites including Answers
in Genesis (by Jonathan Sarfati),
Creation in the Crossfire (by Jon Colley), Creation
Online and He
Comes....24. According
to the Creation Science
Association For Mid-America, Davies' article is based
on a presentation25 he
gave at the Third
International Conference on Creationism in 1994.
The first line of the Creation Online article sums up
the YEC argument nicely:
The results of observations done by astronomers indicate
that there are not enough supernovas to justify an old
galaxy. The numbers observed are consistent with a young
galaxy of thousands of years old."
Sarfati elaborates further:
...a young universe model fits the data of the low
number of observed SNRs. If the universe was really
billions of years old, there are 7000 missing SNRs in our
Galaxy.
How do the YECs arrive at this conclusion? In Davies'
original article, he estimated the numbers of SNRs visible
in an ancient Universe (billions of years old) and in a
young Universe (thousands of years old), and compared both
values with the actual number of observed SNRs (Table
4). He used the following methodology:
- Assume a rate of supernovae occurrence in our Galaxy of
one every 25 years.
- Assume that the first stage of SNR expansion ends after
317 years, the second after 120,000 years and the third
after a million years.
- Due to radio telescope observational limitations,
assume that only 19% of first-stage SNRs, 47% of
second-stage SNRs and 14% of third-stage SNRs are
observable.
- Dividing the ages of the various states by the rate of
supernovae occurrence, work out the numbers of SNRs of each
type visible if the universe was ancient.
- Calculate the numbers of SNRs of each type visible if
the universe was only 7,000 years old.
- Compare the two results to the actual number of
observed SNRs.
Table 4 - Numbers of
Predicted and Observed SNRs according to the YEC
Model
(data from Davies (1994))
Supernova Remnant
Stage |
Number of
predicted observable SNRs if our Galaxy was billions of
years old
|
Number of
predicted observable SNRs if our Galaxy was 7,000 years
old
|
Number of SNRs
actually observed
|
First
|
2
|
2
|
5
|
Second
|
2256
|
268
|
200
|
Third
|
5033
|
0
|
0
|
Do the claims of Davies and others stand up to scrutiny?
As it happens, not very well.
10.1 The YEC Methodology
Davies claims that:
The number of Supernova Remnants (SNRs) observable in
the Galaxy is consistent with the number expected to be
formed in a Universe that is 7,000 years old.
However, using Davies' own methodology, the actual
number of observable SNRs in our Galaxy (225 from Green (2000)) gives a value of 11,970 years,
not 7,000 years. The 11,970 is the lowest possible
value for the age of the Universe as derived from his
methodology. Yet a strict reading of the lineages in
Genesis gives the date of creation as being 4004 BC, about
6,000 years ago (as famously calculated
by Archbishop Ussher of Armagh in the 17th century).
His calculations are repeated here, using his values
(for convenience, any fractions are rounded to the nearest
whole number - it makes very little difference to the
results):
- Number of first-stage SNRs in total = 317 (length of
first stage of an SNR's lifetime) / 25 = 13. Number of
observable first-stage SNRs is thus 19% of 13 = 2. This
figure is the same for both an old universe and a young
universe.
- Number of second-stage SNRs in total with an old
universe = 119,683 (length of 2nd stage of an SNR's
lifetime) / 25 = 4,787. With a young universe = 6,683 / 25
= 267. Number of observable first-stage SNRs with an old
universe thus 47% of 4,800, or 2,250. Number of observable
second-stage SNRs with a young universe = 47% of 267 =
126.
- Number of third-stage SNRs in total with an old
universe = 880317 (length of 3rd stage of an SNR's
lifetime) / 25 = 35213. With a young universe = 0 / 25 = 0.
Number of observable third-stage SNRs with an old universe
thus 14% of 35213, or 4,930. Number of observable
third-stage SNRs with a young universe = 14% of 0 = 0.
But if the second calculation is repeated with the actual
number of observed SNRs in our Galaxy, 225, then according
to his methodology, the result is 479 SNRs in total, and if
the rate of supernovae occurrence is one every 25 years,
then 25 x 479 is 11,970 years.
There is a mathematical error in Davies' calculation.
Surely if only 47% of second stage SNRs are visible, then
the number of visible SNRs in a young universe is not
267/268 but 47% of this = 126? But to quote Davies:
Total [number] of Second Stage SNRs expected to be
observed under a 7,000 year old Universe with t* =25
approx. 268.
In fairness to Davies, this is most likely a simple
mathematical mistake, but both Sarfati and the author(s) of
the Creation Online article correct this error, and give
the value of 126 for visible second-stage SNRs, without
telling the reader that they have done so, and without
pointing out the error in Davies' original article. The
other articles propagate the erroneous value of 268.
Davies devotes a large part of his article to
calculating the percentages of SNRs at different stages in
their lifetimes that should be visible. He gets the results
of 19%, 47% and 14% for first, second and third stage SNRs
respectively). However, these figures are wrong. One of the
components he uses in the calculation is the relationship
known as Sigma-D (obtained from Ilovaisky & Lequeux
1972a), i.e. the relationship between the surface
brightness at a specific radio frequency and the linear
diameter of an SNR. Although it can be used for determining
distances to SNRs (Goebel et
al. 1981; Huang
& Thaddeus 1985; Case & Bhattacharya
1998), it only works for shell-type SNRs that have the
same supernova explosion energy and mechanism, and are
evolving in identical environments, whereas Davies assumes
that it holds for all SNRs. More information on measuring
distances to SNRs is given in Section 10.7. As Green (1991) states,
It is not possible to quote a single surface-brightness
completeness limit for current catalogs of SNRs, not only
because the background emission varies in different regions
of the Galactic plane but also because different regions
have been surveyed with different instruments.
Davies also uses the radio observational limitations
from Ilovaisky & Lequeux (1972a) to give
proportions of SNRs in various stages of evolution (19%,
47% and 14%). There are many problems with Davies'
approach:
- Davies' proportions are gross simplifications - it is
simply not possible to work out accurate figures for the
proportions of SNRs visible at various stages in their
lifetimes, as there are too many external factors involved
(see Section 10.3).
- Others (e.g. Kodaira 1974; Vettolani & Zamorani
1977; Leahy & Wu
1989) have updated and corrected Ilovaisky &
Lequex's original findings - corrections Davies has
not taken into consideration, despite their papers
being published well before 1994.
- Diameters of SNRs are crucial to Davies' calculations.
But to calculate the diameter of a SNR, the distance must
be known with accuracy. Davies gives the impression that
the diameters and distances of Galactic SNRs that he uses
are accurately known, when this is not the case (see
Section 10.7 and Green (1984, 1991)
for more details).
- In radio astronomy, flux density is defined as power
received per unit area per unit frequency. The unit of flux
density is the Jansky (Jy) and is equivalent to
10-26 W Hz-1 m-2. The
amount of radiation in the radio spectrum emitted by a SNR
is measured in Jy, and a radio telescope with a low flux
density limit is much more sensitive to SNRs with low flux
densities. Davies has limited himself to SNRs with a flux
density of greater than 10 Jy. Out of the 225 confirmed
SNRs in our galaxy, only 90 (40%) have flux densities of 10
Jy or more. When Davies wrote his article, the numbers of
SNRs with flux densities of 10 Jy or more was 102 out of
176 in total. The other remnants, 74 in 1991 and 135 today,
Davies completely ignores. These SNRs are effectively
dimmer and harder to detect, and thus are probably either
further away or larger (and thus older). By ignoring these,
Davies is claiming that he finds no old remnants, but he is
doing so after eliminating large numbers of possible old
remnants from his calculations!
10.2 Rate of Supernovae
Occurrence
The main source for Davies' value of 25 years for the
rate of supernova occurrence in our Galaxy is an estimate
made in 1970 by the Swiss astronomer Gustav Tammann. The
value Tammann gave was 26 ± 10 years, calculated by
comparing our Galaxy to other similar galaxies (in terms of
size and luminosity) and working out the rate of supernovae
from observing them (Tammann
1970). Poveda & Woltjer (1968) gave a rough
estimate of 60 years, whereas Chai & van den Bergh (1970) estimated 100
years, and Ilovaisky & Lequeux (1972b) gave 50
± 25 years.
In 1994, Tammann revised his 1970 estimate for the rate
of occurrence of supernovae in our own Galaxy to 40
±10 years (Tammann et
al. 1994). Cappellaro et al. (1996) suggested that for
our Galaxy there should be 4 ± 1 Type Ia, 2 ±
1 Type Ib/c and 12 ± 6 Type II observable supernovae
per millennium, which works out at rate roughly half that
of Tammann's 1970 value. Weiler & Sramek (1988) conclude that the
average interval between supernovae in our Galaxy is
between 20 and 50 years and van den Bergh & Tammann (1991) and Turatto (1999) both give estimates in
reasonable agreement with this. The values given in
Carroll & Ostlie's
Introduction to Modern Astrophysics are 36 years for
Type I supernovae and 44 years for Type II supernovae.
Folgheraiter et al. (1997) gives an average
interval of 30 years as being "the currently accepted
value".
In the 1940s, 1950s and early 1960s, astronomers found
that supernovae occurred at different rates in different
types of galaxies, and that the rate of supernovae in
spiral galaxies is dependent on the luminosity of the
galaxy involved (Tammann et
al. 1994). In addition, the rate at which
supernovae are observed to occur in other galaxies is
dependent on the inclination of the galaxy - a lot more
supernovae are detected in galaxies that lie pole-on to us
(van den Bergh &
McClure 1990; van
den Bergh & Tammann 1991). Another determining
factor in the rate of supernova occurrence is the amount of
progenitor stars - either suitable binary systems (for Type
I) or massive giant stars (for Type II) are required.
Dragicevich et al. (1999) have proposed that
the Earth is situated favourably within our Galaxy for
viewing supernovae, thus the calculated rate for supernovae
occurrence is actually high compared with the rate
throughout the Galaxy as a whole.
Astronomers are generally quite cautious in inferring
rates of supernova occurrence from the amount of supernova
remnants. Indeed, to quote from Jones et al. (1998):
People should exercise extreme caution in inferring
supernovae rates from counts of mature and old SNRs.
And from van den Bergh & Tammann (1991):
Since the lifetimes of radio supernova remnants (SNRs)
depend so critically on environment, it will be very
difficult to derive meaningful information on supernova
rates from the statistics of SNRs.
However, on balance, Davies uses an acceptable value for
the rate of Galactic supernovae occurrence. Incidentally,
it is thought that one supernova occurs every second in the
whole Universe (Burrows
2000).
10.3 Numbers of Supernova
Remnants
YECs claim that not as many SNRs are observed as would
be expected in an old universe. Davies uses a value of one
million years for the lower end of the typical visible
lifetime of a SNR and assumes that all SNRs last this long.
He gets this figure from Ilovaisky & Lequeux (1972b). However, on
reading the original paper it is noticeable that this value
is actually for the theoretical lifetime of the
remnant, not the observable lifetime of the remnant.
Why is there a difference? Quite simply, SNRs are actually
hard to detect. Factors that seriously hinder our ability
to detect SNRs (and which Davies almost completely ignores)
are:
- SNRs can only be observed in a small proportion of our
Galaxy - our view of most of the Galaxy is blocked by large
amounts of dust and interstellar matter. Only some younger,
radio emitting SNRs would be visible through this dust (Sramek et al. 1992; Gray 1994). This largely explains why
there has been no observed Galactic supernovae in the last
300 or so years (Clark et
al. 1981; Dawson
& Johnson 1994; Hatano
et al. 1997), even though we would have expected
perhaps 5-10 to have occurred (McKee
2000).
- It is also difficult to identify much older remnants as
they either have faded beyond our ability to detect them
(they may have merged with the ISM), they have merged with
other remnants, or they have faded into the general
background "noise" (Nousek et
al. 1981; Matthews
et al. 1998; Braun
et al. 1989; Landecker et al.
1990; Normandeau et
al. 2000). Younger SNRs, or SNRs which are still
interacting with gas expelled by their progenitors are much
more likely to be detected (Jones
et al. 1998; Slavin
& Cox 1992). Shull et al. (1989) carried out a statistical
analysis of SNRs, and found that with isolated SNRs, less
than 1% last for longer than 100,000 years, and only 20%
are still intact after 50,000 years.
- The make-up of the local ISM that the supernova occurs
in is critical to the observability of the resulting SNR
(Dohm-Palmer & Jones
1996). SNRs in regions where the density of the ISM is
low (Henning & Wendker
1975; Gaensler &
Johnson 1995b) or there is little ionised gas present
(Heiles et al. 1980)
may not be readily visible. Indeed, it may be the case that
as few as 15-20% of supernova events cause observable SNRs
(Clark & Stephenson
1977; Clark 1979; Kafatos et al.
1980).
- Some young SNRs can be intrinsically faint at radio
wavelengths and thus unusually difficult to detect (Gray 1994; Duncan & Green
2000).
- SNRs are obscured by and can be indistinguishable from
other interstellar emission nebulae, and their spectra can
be similar to powerful distant radio galaxies and quasars
(White & Becker 1990;
Inglis & Kitchin
1990; Caswell &
Stewart 1991, 1992; Williams et al. 2000).
In other words, there is a lot of clutter out there, and
finding SNRs is often a tricky and difficult task. Indeed,
only a minority of SNRs are visible at optical wavelengths
(Long et al.
1990).
- The limits of the equipment used to detect SNRs
(usually radio telescopes) impinge upon our ability to
observe supernova remnants (Green 1991; Kassim 1992; Frail et al. 1994). As
this gets better in the future, the numbers of SNRs
detected will rise. This can be illustrated by the way
astronomers have detected more and more SNRs in our own
galaxy over the last few decades - in 1984, there were only
174 Galactic SNRs known, and back in 1971, only 113 (Downes 1971).
- Not all the sky has been surveyed to the same degree -
there are still large areas of the sky (mainly in the
southern celestial hemisphere) waiting to be surveyed with
more powerful instruments (Case & Bhattacharya
1998).
As a result, Davies vastly overestimates the actual number
of observable SNRs. Berkhuijsen (
1984) suggested that there
might be 1,000 to 10,000 SNRs in our Galaxy (depending on
the lifetime of SNRs), but this is the only estimate I'm
aware of that provides a figure anywhere near Davies', but
even then, Berkhuijsen's estimate is for the
total
number of SNRs, and not for the
observable SNRs.
However, Berkhuijsen's value is very much the exception.
Most other estimates for the total number of SNRs in the
Galaxy are around 1,000 (e.g. Minkowski 1964; Caswell 1970; Li et al. 1991). Leahy &
Wu (1989) give a figure for
the total possible number of radio observable SNRs in our
Galaxy within 50,000 light years of Earth to be 485
± 60/f1, where
f1 is the completeness factor for SNR
observations within 6,000 light years of the Sun (i.e. if
we have only detected 75% of nearby SNRs, then the estimate
is 486 / 0.75 or 648). Case & Bhattacharya (1996) gave 486 ±
42 as an upper limit, whereas Trushkin (1999) gives 300-1000 potentially
detectable SNRs in our Galaxy.
YECs have also invoked the number of SNRs in the Large
Magellanic Cloud to support their assertions. From
Sarfati's article:
Not only that, but the predictions for the Milky Way's
satellite galaxy, the Large Magellanic Cloud are also
consistent with a young universe. Theory predicts 340
observable SNRs if the LMC were billions of years old, and
24 if it were 7000 years old. The number of actually
observed SNRs in the LMC is 29.
The number of SNRs observed in the Large Magellanic
Cloud in 1999 is actually 37 (Williams et al. 1999),
although more are being discovered all the time - indeed it
is recognised that, just like our own galaxy, there are
many more SNRs yet to be discovered in the LMC (Milne et al. 1980; Dickel & Milne 1988; Chu & Kennicutt 1988).
The discrepancy in Sarfati's figures can probably be
explained by outdated references, and thus should not be
counted against him.
However, both Davies and Sarfati make a more serious
error. The estimate of 340 for the total number of SNRs in
the LMC is from Mathewson & Clarke (1973). However, Clark
& Caswell (1976),
Clarke (1976) and Milne et
al. (1980) all point out
major problems with Mathewson & Clarke's estimate -
basically, due to improved observations of SNRs in the LMC,
Mathewson & Clarke's estimate is no longer valid. The
true number of SNRs in the Large Magellanic Cloud is much,
much lower.
Now, Davies has read at least one of these papers (the
Clark & Caswell paper), thus he must be aware of the
status of the Mathewson & Clarke estimate. Yet he uses
this as one of the main supports of his theory, knowing
that it is at the very least in serious dispute. When
combined with the deliberate misquotation of the Clark
& Caswell paper (detailed in Section 10.9), the only logical conclusion is
that either Davies is seriously incompetent or he has
deliberately set out to deceive (and Sarfati appears to
have blindy copied from Davies' original paper, without
verifying the original calculation).
10.4 The Age of Supernova
Remnants
The other main plank of the YEC argument is the
assertion that all SNRs are less than 10,000 years old.
This can be best summed up by a section from Sarfati's
article:
According to their [Astronomers'] model, the SNR should
reach a diameter of about 300 light years after 120,000
years. So if our galaxy was billions of years old, we
should be able to observe many SNRs this size. But if our
galaxy is 6,000-10,000 years old, no SNRs would have had
time to reach this size. So the number of observed SNRs of
a particular size is an excellent test of whether the
galaxy is old or young. In fact, the results are consistent
with a universe thousands of years old, but are a puzzle if
the universe has existed for billions of years.
Additionally, from the He Comes... article:
...And if you calculate, using the observed rates of
expansion and the present radii, how long ago it was that
the shell type supernovae explosions occurred, all the
dates are under 10,000 years. Whereas, if the universe were
really old, one would expect a distribution of ages,
ranging from just a few years to over the million years
that we calculate that the expected supernova remnants
would still be strong enough to be detected via today's
radio-telescopes.
This claim is widely propagated in YEC literature26. However, it is completely
false. While one way of measuring the age of SNRs is indeed
to look at the radii and the rate of expansion, and thus
calculate the age, this can only be done for younger SNRs -
it is not applicable to older, more evolved SNRs, the ages
of which are measured in different ways. In fact, the
population of observed SNRs does show a wide distribution
of ages, from young ones to really ancient ones.
For example, one of the most famous SNRs, the celebrated
Veil Nebula in the constellation of Cygnus is approximately
14,000 years old (Levenson
et al. 1998). G89.0 + 4.7 is 19,000 years old
(Leahy & Aschenbach
1996); G6.4 - 0.1 is 58,000-110,000 years old (Kaspi et al. 1993). The
remnant G69.0 + 2.7 is at least 77,000 years old (Koo et al. 1990) and G166.2
+ 2.5 is 150,000 years old (Kim
et al. 1988). There are many other ancient
remnants (Woltjer 1972; Fich 1986; Storey et al. 1992).
Duncan et al. (1995)
report on G279.0 + 1.1, which they estimate could be half a
million years old (it is an extremely large and faint
remnant). And older SNRs are not confined to our own
Galaxy. The remnant SNR 0450-709 in the Large Magellanic
Cloud, which is 340 x 245 light years in size, is several
hundred thousand years old (Jones
et al. 1998). And with newer and improved
equipment and detection techniques, astronomers are finding
more and more ancient SNRs. It has even been suggested that
the large-scale structure known as the Origem Loop is an
ancient SNR in a very advanced stage of evolution, and
which is approximately a million years old (Hanbury Brown et al.
1960; Berkhuijsen 1974;
Kahn 1976).
As mentioned before, as time goes by, a SNR becomes more
difficult to detect, as it increases in size and the
material in the remnant gets thinner and spread out more,
and distorted by the ISM - Davies completely ignores this.
The values typically accepted by astronomers for the
average maximum visible lifetime of a SNR range from 60,000
years to upwards of 500,000 years (Cox
1972; Jones 1975; McCray & Kafatos 1987;
Leahy & Wu 1989; Dorfi 1994; de Grijs et al. 2000).
From Clark (1979):
...within a few tens of thousands of years most of the
extended remnants which have survived to 'middle-edge' are
expected to merge with the interstellar medium and be
unrecognizable.
Perhaps two of the most famous pulsars are those within
the Crab Nebula and the Vela SNR (Lorimer &
Ramachandran 1999). Astronomers are also attempting to
relate other pulsars with various SNRs . Because the age of
a pulsar can generally be computed accurately27, if it can be associated with
an SNR then the age of the SNR can also be calculated (Furst et al. 1993; Caraveo 1993; Gaensler & Johnson
1995b).
10.5 Third Stage Supernova
Remnants
One of the most important assertions that the YECs make
is that there are no third-stage, i.e. SNRs in the
radiative stage Indeed, the very presence of just one
third-stage SNR would completely destroy the YEC argument
for a young Universe, as the amount of time a SNR takes to
reach this stage is way beyond anything that the YEC time
scale allows.
So, are there any actual third-stage SNRs? There have
been dozens of papers published over the last several
decades examining and discussing actual radiative SNRs -
quite an achievement considering how, according to YECs,
they don't actually exist! Despite what the YECs say,
radiative SNRs do actually exist. A brief reading of
the relevant literature reveals the following Galactic SNRs
that are in the radiative phase (and there are others):
In addition, Matthews
et al. (
1998) reported on an
extremely faint SNR called G55.0 + 0.3:
G55.0+0.3 is amongst the faintest SNRs known. This SNR
could be just one member of a larger population of faint,
old remnants that are not currently detectable at radio
frequencies. If a significant fraction of SNR survive
longer than 50,000 years, further imaging of the Galactic
plane with high surface sensitivity and high angular
resolution should reveal more old SNRs.
The most massive stars (the ones most likely to end up
as Type II supernovae) are found in clusters. Thus most
Type II supernovae will not have been the first one to
occur in the vicinity, but more likely occur in a medium
that has been disturbed by the action of previous
supernovae (Chu 1997). The typical
lifetime of a massive star that is likely to end in a
supernova (a few 106 years) is not long enough
for the ISM to backfill the cavity left over by previous
supernovae (Jones et al.
1998). Single or multiple supernovae (in the latter
case, in the same general vicinity) can result in the
formation of a superbubble, up to hundreds of light
years across over a time scale of one to twenty million
years (Heiles 1984; McCray & Kafatos 1987;
Oey & Clarke 1997; Ehlerova et al. 2001).
There are many examples of these superbubbles both in our
own Galaxy and in other galaxies (e.g. Blades et al. 1980; Fich 1986; Meaburn & Laspias
1991; Hunter 1994; Bomans & Chu 1997; McClure-Griffiths et
al. 2000; Bond et
al. 2001). Indeed, it is likely that the Sun is
located in one of them (Hughes & Routledge
1972). Maciejewski et al. (1996) describe a structure
they have named the "Aquila" supershell, which lies about
8,500 light years from Earth, with a radius of over 520
light years, which they calculate is about ten million
years old, and the result of 10-100 supernovae. It contains
several star-forming regions. Incidentally, there is one
SNR associated with this structure, G34.7 - 0.4, with a
calculated age of approximately 20,000 years (Wolszczan et al.
1991; Shelton et al.
1999).
In addition, Davies's assumption that the adiabatic
phase of SNR evolution (i.e. the "second stage") always
lasts 120,000 years and that the radiative phase always
lasts 880,000 years is also completely wrong. As was
mentioned in Section 5.1, the evolution
of SNRs varies enormously.
10.6 The Ages of Stars
YECs such as Davies claim that the universe is about
6-7,000 years old. However, the life cycle of stars which
turn into supernovae is of the order of a few tens of
millions of years for high mass stars (Type II supernovae)
and at least a billion years (and usually much, much more)
for lower mass stars (Type I supernovae). The oldest known
stars are approximately 12.5 billion years old (Cayrel et al. 2001; Qian & Wasserburg
2001), which is consistent with the latest estimate of
the age of the Universe as a whole at about 13.5 to 14
billion years old (Lahav 2001; Ferreras et al.
2001).
Indeed, supernovae do play an important part in the
birth of new stars - when a supernova explodes near a
molecular gas cloud, the expansion of the shock front into
the cloud can:
- accelerate relativistic particles
- heat and compress the molecular gas
- change its chemistry
- produce turbulent mixing.
The condensed clumps of interstellar gas created by this
mechanism eventually end up as new stars (Assousa et al. 1977; Huang & Thaddeus
1986). The classical example of this happening is a
group of stars embedded in a reflection nebula
approximately 3,000 light years away in the constellation
of Canis Major called CMa R1, in which there are two very
young stars (Z CMa and HD 53367) which are the same age
(about 300,000 years old) as an expanding ring of gas which
appears to be an SNR (Herbst & Assousa 1977;
Shevchenko et al.
1999). This scenario been disputed by some astronomers
however as the identification of the ring of gas as an SNR
isn't confirmed completely. A much stronger case for
supernova induced star formation is the remnant G349.2 +
0.7 which is interacting with a larger shell of molecular
gas, which is likely to be an extremely ancient (four
million years old) SNR. This hypothesis is supported by the
presence of IRAS 17147-3725, a clump of dust with similar
characteristics to the proposed SNR, being ionised by an
object with the spectral characteristics of a newly-formed
star (Reynoso & Mangum
2001)
10.7 Distance to Supernovae and
Supernova Remnants
How are the distances to supernovae and SNRs measured?
Well, there are several methods available to astronomers.
As the amount of energy released by a Type 1a supernova is
quite accurately known, many astronomers have suggested
that they can be useful in measuring distances in space,
rather like a cosmic yardstick (Riess et al. 1996; Saha 1997; Riess et al. 1998; Perlmutter et al.
1998a; Regnault 2000). It
has been discovered however, that not all Type Ia
supernovae are identical and therefore not all have the
same intrinsic brightness (Cappellaro et al.
1997; Filippenko
& Riess 1999; Canal et
al. 2000; Hatano et
al. 2000; Howell et
al. 2001; Howell 2001; Garnavich et al.
2001), and some astronomers have disputed the whole
usage of Type Ia supernovae as standard candles (Drell et al. 2000),
although others contend that the differences involved are
not sufficient enough to rule out their use in measuring
the universe (Gibson &
Brook 2000; Gibson
& Stetson 2001; Richtler et al.
2001).
The Sigma-D relationship (already referred to in
Section 10.3), can be used to work out
the true luminosity (and thus distance) of some shell-type
SNRs. The emissions from the optical filaments at the shock
front of a SNR can be examined to produce the true velocity
at which they are moving at, and thus the distance can be
worked out. The X-ray emission from a SNR in the Adiabatic
phase can be measured, and from this, the actual diameter
can be worked out, and thus the distance. There are other
methods as well, including measuring the redshift of
distant supernovae, locating known adjacent objects to
which the distances are already known, and many more. For a
detailed description of these methods, see Green (1984)29. Even though these methods
cannot give us the exact distances - indeed, we don't know
the exact distances to most SNRs (indeed, Green (2000) only gives distances to a
quarter of galactic SNRs), all of them tell us that apart
from a few close to Earth, every supernova and SNR known is
more than 7,000 light years away. In fact, astronomers have
been able to directly measure the distance to SN
1987A via trigonometry. It works out at about 167,000
± 4,000 light years (Panagia et al. 1991; Panagia 1999)30. Astronomers have also have
recently detected traces of supernovae billions of light
years away (Perlmutter
et al. 1998b, Riess
et al. 1998, Riess
et al. 2000). That it has taken about 167,000
years for the light from SN 1987A to reach Earth scuppers
any idea of a 7,000 year old Universe.
So how do YECs respond to this? Well, it has been
proposed that all the light from supposedly distant objects
didn't come from those objects, but was created by Yahweh
at the time of Creation in transit - in other words, those
distant objects don't actually exist and are only
illusions. This is an extension of the Omphalos
argument (the Omphalos argument, first expounded in a book
of that name by Philip Henry Gosse (1857), argues that the
universe was created young but with the appearance of age).
Omphalos is unfalsifiable, untestable and totally
unscientific. Plus, it would relegate Yahweh to the role of
a cosmic deceiver - creating objects and events that we
observe (e.g. SN 1987A) which don't actually exist. Due to
the obvious theological difficulties of this argument, many
YECs have abandoned it (though many haven't) and proposed
hypotheses based on alternate cosmologies which allow light
to travel billions of light-years in a short period of
time31, or a variable
speed of light. Perhaps the chief exponent of this latter
idea is the YEC Barry Setterfield, who has postulated
that the rate of the speed of light is variable, and was
much higher in the past (just after Creation), thus
allowing for objects to appear farther away than they
really are.
However, all the evidence is that the speed of light has
not changed in this manner (Goldstein et al.
1973; Baum
& Florentin-Nielsen 1976; Tubbs & Wolfe 1980; Gruber et al. 1981; Ellis et al. 2000)32. The most distant supernovae
(and therefore the most ancient) show the same time scale
of radioactive decay of the elements produced (taking into
account the relativistic time dilation observed caused by
the speed of the expansion of the Universe). This confirms
that there has been no notable changes in decay rates
between then and now, which is consistent with the idea of
an ancient, expanding Universe of vast size (Leibundgut et al.
1996; Riess et al.
1997; Pranztos 1998; Filippenko & Riess
1999; Riess et al.
2000; Ellis &
Sullivan 2000; Filippenko & Riess
2000; Turner & Riess
2001)33.
10.8 Outdated References
Even though Davies' article was written in 1994, the
vast majority of his references are from the 1970s with a
few going back to the 1960s, with just a few from the 1980s
and 1990s. Here are a few examples:
- He uses Mathewson & Clarke's 1973 estimate for the
number of SNRs in the Large Magellanic Cloud - which was
subsequently questioned in 1976 and then shown to be
incorrect by 1980.
- He uses Tammann's 1970 estimate for the rate of
supernovae occurrence (this is slightly moot, as the more
recent estimates aren't all that different)
- He refers constantly to Illovaisky & Lequeux's 1972
papers, to the exclusion of many other more recent papers
which have updated and corrected this paper.
This reliance on old references in itself is somewhat
forgivable on its own, but a quick search on the WWW
reveals hundreds of scientific papers written before 1994
(the year of publication of Davies' article), which
directly contradict Davies' findings (e.g.
Leahy & Wu's 1989 paper on
the total number of SNRs in our Galaxy).
To make matters worse, the abridgment articles were
written even more recently (Sarfati's was originally
written in 1997), so there is really no excuse for others
to repeat Davies' mistakes.
10.9 Misquoting and
Paraphrasing
Davies misquotes several astronomers. For example, he
quotes Cox (1986) as saying (in
reference to a supposed lack of SNRs in the Large
Magellanic Cloud):
The final example is the SNR population of the Large
Magellanic Cloud. The observations have caused considerable
surprise and loss of confidence.
However Cox was discussing possible models of supernova
remnant evolution in the adiabatic phase, and the relevant
paragraph from the original paper is actually:
The final example is the SNR population of the Large
Magellanic Cloud. The observations (many collected in
Mathewson et al. 1983) have caused considerable
surprise and loss of confidence in simple models such as
those in this paper.
Which is actually saying something completely different
from what Davies claims that it says. He also misquotes
Clark & Caswell (1976) twice. The first:
Why have the large number of expected remnants not been
detected?
is quoted by Davies in such a way to make the reader
think his estimate of the number of Galactic SNRs is
correct. But in the original paper, this was a rhetorical
question, in the context of discussing the 1973 estimate by
Mathewson & Clarke that there should be 340 visible
remnants in the Larger Magellanic Cloud. Clark &
Caswell immediately follow this by giving several reasons
why the 1973 estimate is unreliable (the Mathewson &
Clarke estimate has been discussed earlier, in Section 10.3). The relevant
paragraph from the original paper is:
Thus two anomalies require explanation. Why have the
large number of expected remnants not been detected? Is it
reasonable that E0/n should differ so greatly
from our estimate for the Galaxy? Both anomalies are
removed if we assumed that the N(D)-D relation has been
incorrectly estimated owing to the small number of remnants
(4) used.
As already mentioned in Section 10.3, Clark & Caswell's suspicion was
subsequently proved to be correct. But Davies totally
ignores this. The second quote from this paper that Davies
uses:
The mystery of the missing supernova remnants
is actually lifted from this
sentence in the original paper:
It appears that with the above explanation there is no
need to postulate values of Eo/n differing
greatly from those in the Galaxy, and the mystery of the
missing supernova remnants is also solved.
Both quotes have been lifted out of context and mean
something completely different than what Davies says it
does. Sarfati uses these two quotations in, what appears to
be at first glance, an even more dishonest manner. He
states:
As the evolutionist astronomers Clark and Caswell say:
'Why have the large number of expected remnants not been
detected?' and these authors refer to 'The mystery of the
missing remnants'.
Whilst one could say that this appears to be a
deliberate attempt to deceive, it could be the case that
Sarfati is just a bad paraphraser. However, at the very
least Sarfati is guilty of incompetence, and also guilty of
not checking Davies' sources. Davies however, cannot escape
so lightly. The only logical conclusion from the above
trail of misquotation is that Davies appears to be
deliberately deceitful.
10.10 Conclusion
Let's briefly summarise how YECs such as Davies and
Sarfati are correct and incorrect. First, in their favour,
they are correct on:
- The frequency of supernovae occurrence in our
Galaxy
However, that's actually it. This is the only point in
their favour. In contrast, they are completely incorrect
about the following:
- The number of actual, observable SNRs in our
Galaxy.
- The typical observable lifetime of SNRs.
- The evolutionary timescales of SNRs.
- The uniformity (or lack thereof) of SNR
characteristics.
- The presence of Radiative SNRs.
- The difficulty of finding SNRs.
- The distance to SNRs.
In others words, just about everything. In addition, the
methodology that they use to calculate the numbers of SNRs
they claim should be present is hopelessly wrong. They have
also engaged in repeated misquotation, selective
interpretation of the data and indeed, selective ignorance
of data which disagrees with their conclusions. They are
quick to assert that astronomers are confused and
bewildered about the so-called "mystery of the missing
supernova remnants". This however, is an assertion
completely without foundation. There is no mystery. The
YECs are making the mystery up themselves. They are
completely wrong:
Supernovae and Supernova
Remnants are good, hard, evidence for an ancient
universe.
11.
Notes
1. The spectra of Type I
supernovae do not contain prominent hydrogen lines, whereas
the spectra of Type II supernovae do.
2. A white dwarf is the
tiny, extremely heavy and dense form of a star near to the
end of its life. Typical density for a white dwarf is in
the range of 1010 kg m-3. Matter in a
white dwarf is in the form of an electron-degenerate gas,
wherein the electrons are all stripped from their parent
atoms. Gas in this peculiar state is an almost perfect
conductor of heat and does not obey the ordinary gas laws.
Such a white dwarf no longer has any source of energy and
simply cools down forever, eventually becoming a black
dwarf - a cold, dead lump of matter hanging in space.
3. Named after the
Indian-born astrophysicist Subrahmanyan Chandrasekhar, who
first calculated it in 1930 as a means of passing the time
whilst on a boring 18-day boat trip from India to England!
It is the maximum possible stable mass for a white dwarf
star. It is equal to 1.44 solar masses - the mass of the
Sun is approximately 1.9891 x 1030 kg.
4. To illustrate the vast
amounts of energy output by supernovae, our Sun has an
absolute (how it would appear if it was 10 parsecs, or
32.616 light years away) magnitude of only +4.7 - it would
thus appear to the naked eye as a faint star, barely
visible to the naked eye. The apparent magnitude of the Sun
is -26.8. Even so, excluding neutrinos, optical radiation
only accounts for 1% of the total energy output (van den Bergh 1988).
5. The normal fuel of
stars is hydrogen. Over the lifetime of the star, this
hydrogen is gradually converted (via thermonuclear fusion
reactions) into helium by a process called
nucleosynthesis. In nucleosynthesis, light atomic
nuclei (such as hydrogen) collide with such violence and
frequency in the high-temperature and high-density interior
of the star that they fuse into heavier nuclei (such as
helium) and release vast amounts of energy (like in an
H-Bomb). In effect the lighter elements "burn" to form
heavier elements.
6. According to Arnett
et al. (1989), for a
star of 20 Solar Masses, it takes approximately ten million
years to complete the hydrogen burning state. Helium
burning requires approximately a tenth of this, 950,000
years. Carbon burning takes 300 years, and neon and oxygen
burning take 180 and 140 days respectively. Silicon burning
is completed in two days. At this time, the temperature in
the core is some 3.7 x 109 K.
7. Photodisintegration is
the stripping down of nuclei by photons into individual
protons and neutrons. This process is highly endothermic
(i.e. it requires more energy than it generates). It was
first identified by Willy
Fowler and Sir
Fred Hoyle in the 1960s. Photodisintegration can also
occur in the silicon-burning phase.
8. For every 0.1 solar
masses of iron that is broken down via photodisintegration
into protons and neutrons, the shock loses 1.7 x
1051 ergs.
9. A neutron star is a
star made almost entirely of neutrons, with a density of an
atomic nucleus (Horowitz &
Piekarewicz 2001) . Such a star contains typically the
same amount of matter as there is in our Sun, but packed
into a sphere about 10 km across. The maximum mass for a
neutron star is approximately three solar masses, the
so-called Oppenheimer-Volkoff limit (first postulated in
1939 by Robert
Oppenheimer, of A-Bomb fame, and his student, George
Volkoff), and the minimum mass about 0.1 solar masses (any
lighter neutron star that tried to form would turn into a
small white dwarf, as some of the neutrons converted
themselves into protons by a process called beta
decay). The density of matter in a neutron star is much
greater than in a white dwarf - about 1017 kg
m-3.
10. A pulsar is a
rotating neutron star, with a mass similar to the Sun's but
a diameter of only about 10 kilometres. The pulses occur
because the neutron star is rotating very rapidly: a beam
of radio emission produced by synchrotron emission from
electrons moving in the very strong magnetic field (about
108 tesla, or a billion times the strength of
the magnetic field at the surface of the Earth) of the
rotating neutron star sweeps past an observer once per
rotation. The pulses are very regular, apart from the
occasional glitch, and all single pulsars are gradually
slowing down as they lose rotational energy (van der Swaluw & Wu
2001). The time between successive pulses ranges from
1.558 milliseconds for the fastest known pulsar, PSR 1937 +
21 (Xu et al. 2001), to
8.5 seconds for the slowest observed pulsar (Young et al. 1999). The
first pulsar was discovered in 1967 by
Jocelyn Bell Burnell at Cambridge, England. There are
over 1300 pulsars known (Gotthelf et al. 2000;
Lorimer 2001), although many
more are being discovered all the time (D'Amico et al. 2000; Edwards et al. 2001; McLaughlin et al.
2001) - the most famous being the one at the centre of
the Crab Nebular, which has a spin period of 50
milliseconds (Wang et al.
2001). Although it has long been thought that pulsars
are the most common form of young neutron star, there have
been recent discoveries of other classes of objects (Gaensler et al. 2001).
These include: Magnetars - young isolated neutron stars
with extremely high magnetic fields (Duncan & Thompson
1992; Paczynski 1992),
Soft Gamma-ray Repeaters (SGRs), pulsating X-ray sources
with occasional intense Gamma-ray activity but no
detectable radio pulsations (Hurley 1999) and Anomalous
X-ray pulsars, pulsating X-ray sources which are spinning
down slowly (Mereghetti
1999). It may be the case that both SGRs and AXPs are
types of magnetars (Thompson & Duncan
1996; Frail et al.
1997; Vasisht &
Gotthelf 1997; Kouveliotou et al.
1998) or other unusual pulsars (Marsden et al. 2001b)
maybe even some other exotic type of star (Xu et al. 2000). Most
SGRs/AXPs appear to be physically associated with supernova
remnants (Gaensler et
al. 2001).
11. Named after the
German astronomer and physicist Karl
Schwarzschild, who investigated the concept in the
early 20th century. It is the radius below which the
gravitational attraction between the particles of a body
must cause it to undergo irreversible gravitational
collapse. It is equal to 2.95 x (MassBody/MassSun) kilometres.
12. Neutrinos are
elementary particles with no electric charge and almost no
mass, and they interact only very weakly with other matter.
Because they hardly interact with matter at all, neutrinos
are very difficult to detect. In one type of neutrino
detector shown to work successfully, detectors in a large
tank of water (located as far as possible underground to
block off cosmic rays which interfere with the detection
process) pick up Cerenkov radiation generated by the
interaction of electrons with solar neutrinos. Detectors of
this type made the first observation of neutrinos from a
supernova - those from SN 1987A - at 7.36 GMT on the 23rd
February 1987 (before the optical light reached Earth). The
Kamiokande II detector in Japan recorded 9 neutrinos within
2 seconds, followed by 3 more within 13 seconds, the IMB
detector in Ohio in the US detected 8 neutrinos within 6
seconds and the Baksan detector in the then Soviet Union
recorded the arrival of 5 neutrinos within 5 seconds.
13. New Scientist
Magazine of the 18th September 1999 reported on a supernova
that apparently occurred in 1320, but went
strangely unobserved at the time. The ROSAT X-ray
satellite imaged a supernova remnant in the constellation
of Vela, only 640 light years distant, and scientists have
observed a spike in the concentration of nitrates in
Antarctic Ice Cores corresponding to the year 1320 -
similar spikes were observed in 1572 and 1604, when known
supernovae occurred (Aschenbach
1998; Aschenbach et
al. 1999; Robinson
1999; Burgess &
Zuber 2000). Recently however, doubts have been
expressed about the recent dating of this SNR, and it has
been proposed that the SNR is actually thousands of years
older (Mereghetti 2001; Slane et al. 2001).
14. Chin & Huang
(1994) & Schaefer (1995, 1996) have disputed that this was
a genuine supernova, instead suggesting that it was a comet
or nova or even a combination of both; it has even been
suggested that a luminous halo of Gas thrown off from the
planetary nebula He 2-111 was the culprit (Webster 1978), but Trimble &
Clark (1985), Strom (1988) and Thorsett (1992) treat it like a genuine
supernova. There is a SNR nearby, but this has not been
authoritatively confirmed as being associated with the
original supernova; Rosado et al. (1996) claimed that the SNR was
too far away and thus too old, whereas Smith (1997) found a much lower distance,
and thus a much younger age for the SNR.
15. Astronomers are not
100% sure which supernova remnant is the result of this
supernova - there are three candidates, which are all in
the Tail Area of Scorpius, although both Green et
al. (1988) and Reynolds
et al. (1994) have
suggested the SNR G11.2 - 0.3, and the distance given is
from Strom (1994).
16. This supernova was
inexplicably missed by astronomers at the time. This was
most probably because of its faint visual magnitude, which
was probably due to clouds of obscuring interstellar dust
and gas lying between us and it (Searle 1971) - but the resulting
SNR, known as Cassiopeia A (which is the youngest known
SNR), is the strongest radio source in the sky apart from
the Sun. It's brightness is declining by about 1-2% per
year (Dickel & Greisen
1979; Raymond 1984).
In the absence of any confirmed sighting, the exact date
that the supernova occurred cannot be known with complete
certainty, although the date given in the table (1671) is
from Thorstensen et al. (2001). Other suggestions
include 1658 (Kamper
& van den Bergh 1976b; van den Bergh & Kamper
1983) and 1667 (Kamper & van den
Bergh 1976a). These dates have generally been
calculated by measuring the proper motion of various knots
of ejected gas over several decades, thus enabling the
expansion rate of the remnant to be worked out and from
this extrapolating the convergence date and position.
In 1680, the famous English astronomer Sir
John Flamsteed observed a 6th magnitude object in
Cassiopeia, whose position (R.A. 23h 21m 55s; Dec
+58o 32'.3) nearly coincides with that of
Cassiopeia A (R.A. 23h 21m 11s; Dec +58o 32'.3).
Flamsteed's 1725 star catalogue labelled this object as the
star 3 Cassiopeiae. However , this star has not been seen
by anybody since Flamsteed, and does not appear on modern
maps - when Francis Bally was preparing his corrected 1835
edition of the Flamsteed catalogue, he noted that 3
Cassiopeiae was missing from the sky, and he was unable to
find any observation of it in the Greenwich astronomer's
records. It has been speculated that this object was in
fact the supernovae responsible for Cassiopeia A (Ashworth et al. 1980),
which would put the date of the supernova back to the years
1677-1680. Although the association between the two is
somewhat speculative, and indeed many have argued against
it (e.g. Kamper 1980, Hughes 1980), it is still considered
a possibility, as an explosion date of around 1680 is by no
means impossible (Thorstensen et al.
2001). as it would only require a slight deceleration
in the expansion rate (approximately 1%), as is the case
with the remnant resulting from the 1592 supernova (Raymond 1984).
17. Supernova are named
after the year in which they are discovered and in the
order they are discovered, therefore SN 1987A was the first
observed supernova of that year. If more than 26 supernovae
are discovered each year (as has been the case for since
the mid-eighties), the 27th is given the suffix "aa". The
28th is thus "ab". Once all the "a"s have been exhausted,
"b"s are used, i.e. "ba", "bb", and so on. The last
supernova discovered in 1999 was known as SN 1999gv.
18. That the progenitor
star of SN 1987A was a blue supergiant (Arnett 1987; Podsiadlowski 1992) was at
the time somewhat of a puzzle to astronomers - the standard
models of stellar evolution indicated that it was
red supergiants that turn into supernovae, although
it had been theorized previously that blue supergiants were
capable of going supernovae (Lamb
et al. 1976; Brunish & Truran
1982a, 1982b). It
is now known that in the past, some 40,000 years ago, the
progenitor was actually a red supergiant, which then
evolved into its pre-supernova form of a blue supergiant,
losing mass through the process of stellar wind (Weiler & Sramek 1988;
Woosley et al. 1988;
Woosley 1988; Saio et al. 1988a, 1988b; Dar
1997). It is thought that this mass loss is responsible
for the inner ring (Masai et
al. 1988; Luo &
McCray 1991; Chevalier & Dwarkadas
1995; Panagia et al.
1996). What caused the outer rings is still disputed
and various explanations have been put forward to explain
their origin (Blondin
& Lundqvist 1993; Martin & Arnett 1995;
Burderi & King 1995;
Burrows et al. 1995;
Meyer 1997; Soker 1999). For years, astronomers
searched (without success) for the neutron star or black
hole that theory predicted would be formed in the remains
of SN 1987A (Chevalier 1992;
Apparao 1993; Percival et al. 1995).
In 1997, a candidate for an optical pulsar was finally
found (Middleditch et
al. 1997; Nagataki
& Sato 2001). The debris from the supernova is
currently moving outwards at a rate of several thousand km
s-1 (Jansen
& Jakobsen 2001).
19. Although according
to
Dr Fields:
There is fossil evidence for a couple of
mini-extinctions during the Cenozoic Era," Fields said.
"One occurred about 13 million years ago; the other
occurred about 3 million years ago. Marine animal families
near the bottom of the food chain -- years such as
zooplankton and echinoids -- were impacted most. The
pattern of extinction is consistent with a major reduction
in marine photosynthesis.
Hughes and Routledge (1972), and Russell (1979) came out in favour of a
nearby supernova being the cause of the extinction of the
dinosaurs 65 million years ago. This was all of course,
before the discovery in 1980 by the geologist Walter
Alvarez that an asteroid
was the probable culprit (Alvarez et al. 1980; Russell 1982). Overall. there is
little or no evidence directly linking nearby supernovae
and extinctions throughout the Earth's history (van den Bergh 1994).
20. The Local
Bubble is a hot, low-density (compared to its
surroundings) region of interstellar space, in which the
Sun and a few other nearby stars sits. The precise location
of the sun is in a small irregular condensation, or cloud,
within the bubble. The bubble is several hundred light
years across (Frisch 1998, 2000).
21. Unlike YECs, Ross
accepts the Big Bang and the antiquity of the Universe and
the Earth (he was an astrophysicist before turning to
theology) but he largely accepts the YEC viewpoints on the
validity of Theory of Evolution and the origins of
Humanity.
22. For the exact
calculations see
this post to the newsgroup talk.origins by Sverker
Johansson on the 18th December 2000.
23. Gamma-ray Bursts
are the most energetic events known in the Universe (apart
from the Big Bang of course). A GRB takes the form of an
occasional burst of Gamma Rays coming from a seemingly
random direction in space (Fishman
1995), lasting varying lengths of time, from a fraction
of a second, to many minutes. They were initially
discovered by accident. In the late 1960s, Ray Klebesadel
of the Los Alamos Laboratory in the US was working on a
project to monitor compliance with a nuclear test ban
treaty using a series of U.S. military satellites
designated "
Vela". During 1969, he was reviewing gamma-ray data
recorded on 2nd July 1967 when he noticed an unusual spike.
In the following three years to July 1972, some 16 bursts
of gamma-rays were recorded (Klebesadel, Strong & Olson
1973). At first, GRBs, of which several thousand have
been observed (Lu 2000), were thought
to originate in the Solar System or in the local area of
our own Galaxy, but it was not until 1991 that instruments
aboard the orbiting Compton Gamma-Ray Observatory confirmed
that they were extragalactic in origin (Burrows 2000). On 28th February
1997, a satellite called BeppoSAX successfully an optical
afterglow from a GRB. In recent years, astronomers have
successfully detected many of these optical afterglows -
typically they have a visual magnitude in the range 18 to
20 (Jha et al. 2001). It
is estimated that there could be as many as 1,000 GRBs
occurring each year (Cheng &
Lu 2001). Incidentally, the amount of energy released
in a GRB is such, that if one occurred at a distance of
1,500 light years, the Earth would be bathed with an amount
of energy roughly equivalent to 10,000 megatons of TNT
(roughly equivalent to the world's stockpile of nuclear
weapons)! This would essentially destroy the ozone layer,
which, while not threatening the existence of human
civilisation, would have massive side-effects on the
environment (Thorsett
1995).
24. Davies has written
two other articles on how various aspects of astronomy
support a young Universe. Both these articles have been
debunked and show a pattern of misquotation and deliberate
ignorance of conflicting data. One article is on the
population of red dwarfs in Globular Clusters (original
article and rebuttal) and the
other is on the Age of the Sun (original
article and rebuttal).
25. Davies' claims are
also found in a wide range of YEC literature - for example,
Darren Gordon's "Creation
Science FAQ" and Danny Faulkner's "The
Current State of Creationist Astronomy".
26. For instance, in an
article entitled "
Isn't the Universe Billions of Years Old?" by the
well-known YEC Walt Brown. His article contains the
completely untrue statement:
...Besides, all supernova remnants we see in our galaxy
appear to be less than 10,000 years old. This is based on
the well-established decay pattern of a supernova's light
intensity in the radio-wave frequency range. [Keith Davies,
personal communication.]
27. However, recently
some doubts have been expressed about the techniques used
to measure the ages of pulsars. In a recent paper in Nature,
Astronomers Bryan Gaensler and Dale Frail re-examined the
pulsar PSR B1757 - 24, which is associated with the
supernova remnant G5.4 - 1.2, and which was previously
though to have an age of 16,000 years (Manchester et al.
1991; Lyne et al.
1996). However, they re-calculated the proper motion of
the pulsar and found that if it is associated with the
remnant, then it must be at least 39,000 years old, and
more probably 170,000 years old. If Gaensler and Frail are
correct, then it is more bad news for YECs, in that it
means that the ages of pulsars, and thus, their associated
supernova remnants, are being seriously underestimated (Gaensler & Frail 2000;
Marsden et al.
2001a; Gvaramadze
2001b). Even with conventional dating, the majority of
pulsars are very old (Kijak 2001),
with a typical lifetime of around 10 million years (McLaughlin et al.
2001).
28. Gaensler and Frail
have contended that G5.4 - 1.2, with the revised dating of
PSR B1757 - 24, is thus in the radiative phase (Gaensler & Frail
2000).
29. Green (1984) is very dismissive of using the
Sigma-D relationship to measure distances to
SNRs.
30. The exact mechanics
of how the distance to SN 1987A was measured are explained
in a comprehensive
article by Todd S Green.
31. The alternative
cosmology which has had the most uptake in YEC circles is
by an American Nuclear Physicist Dr. Russell Humphreys and
published in 1994 a book called "Starlight and Time". It
has been comphrensively
debunked on multiple
occasions by many astronomers and cosmologists. Ironically,
at the forefront of this effort has been Hugh Ross, who has
pronounced it "irremediably flawed" and that the book
itself "exhibit[s] profound misunderstandings of relativity
theory and cosmology".
32. Setterfield's claim
has been shown to have
been based upon a flawed statistical analysis of
measurements of the speed of light over the last three
hundred years, and indeed has been rejected
by some YECs.
33. More information on
this can be found at the
website of Dave Matson. Incidentally, the American
astronomer Halton
Arp has hypothesised in
many papers over the last several decades that redshift
is not actually a function of speed of recession due to the
expansion of the Universe - that low-redshift galaxies and
high-redshift quasars which appear close together as seen
from Earth are physically connected by gaseous bridges,
which is obviously an impossibility if the quasars are
billions of light-years further way However, it has been
shown that Arp's bridges are almost certainly nothing more
than either photographic artifacts or statistical anomalies
(e.g. Sharp 1985, 1986; Newman & Terzian 1995;
Wehrle et al. 1997;
Hardcastle et al.
1998; Crawford et
al. 1999; Hardcastle
2000).
12. References
12.1 Books
The following books were used in preparing this FAQ.
Zelik & Gregory's Introductory Astronomy &
Astrophysics (4th Edition) is a good textbook for
people wanting to get into the more detailed aspects of
astrophysics. It does contain a few equations, so a
reasonable grounding in mathematics and physics would be
helpful.
- Bradley W. Carroll &
Dale A. Ostlie, "An Introduction to Modern Astrophysics",
Addison-Wesley Publishing Company 1996, ISBN
0-201-54730-9.
- David H. Clark, "Superstars: Stellar
Explosions shape the Destiny of the Universe", J.M. Dent
& Sons Ltd 1979, ISBN-0-460-04384-6.
- Ken Croswell, "Alchemy of the
Heavens", Doubleday 1996, ISBN 0-385-47214-5.
- John Gribbin, "Companion to the
Cosmos", Little Brown Company 1996, ISBN
0-316-32835-9.
- Michael Hoskin (Editor), "The
Cambridge Concise History Of Astronomy", Cambridge
University Press 1999, ISBN 0-521-57291-6.
- David H. Levy, "Observing Variable
Stars: A Guide for the Beginner", Cambridge University
Press 1998, ISBN 0-521-62755-9.
- Dina Prialnik, "An Introduction to
the Theory of Stellar Structure and Evolution", Cambridge
University Press 2000, ISBN 0-521-65937-X.
- Christopher Walker (Editor),
"Astronomy Before The Telescope", British Museum Press
1996, ISBN 0-312-15407-0.
- Michael Zelik & Stephen
A. Gregory, "Introductory Astronomy & Astrophysics (4th
Edition)", Saunders College Publishing 1998, ISBN
0-030-06228-4.
12.2 Technical Papers
The vast majority of these are available online, from
either the Los Alamos E-Print Archive
(LANL), or the NASA Astrophysics Data
System (ADS). LANL papers are either in Adobe Acrobat
or Postscript format, and ADS papers are in GIF format.
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13. Credits
Thanks to Grant Bazan, John Boggan, Bobby Byrant, Robert
Carroll, Dave Chapman, Sarah Clarke, Mike Dworetsky, Manny
Edwards, Bryan Gaensler, Dave Green, George Greene, Todd
Greene, Martin Hardcastle, Richard Harter, Mike Hopkins,
Mark Issak, Sverker Johansson, Joseph Lazio, Andrew MacRae,
Bill McHale, Kevin O'Brien, Marshall Perrin, Phil Plait,
Ezra Poetker, Michael Richmond, Matt Silberstein, Michael
Thorsley and Stuart Weinstein for information, and also for
reviewing and commenting on draft versions of this FAQ.
This article would not have been possible without the use
of NASA's Astrophysics Data System Bibliographic
Services.