A supernova remnant (SNR) is the remains of a
explosion. SNRs are extremely important for understanding our
Galaxy. They heat
up the interstellar
medium, distribute heavy
throughout the Galaxy, and accelerate
|Two Example Supernova Remnants
|Cygnus Loop in X-rays
||Crab Nebula in X-rays|
How do we Classify Supernova Remnants?
- Shell-type remnants:
- The Cygnus Loop (above left) is an example of a shell-type
remnant. As the shock wave from the supernova explosion plows through
space, it heats and stirs up any interstellar material it encounters,
thus producing a big shell of hot material in space. We see a
ring-like structure in this type of SNR because when we look at the
edge of the shell, there is more hot gas in our line of sight than
when we look through the middle. Astronomers call this phenomenon
- Crab-like remnants:
- These remnants (also called plerions) resemble the Crab Nebula
(above right). These SNRs are similar to shell-type remnants, except
that they contain a
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:
- These SNRs appear shell-type in the
radiation). In X-rays, however, they appear
crab-like, but unlike the true crab-like remnants their X-ray
spectral lines, indicative of hot gas.
- Plerionic composites:
- These SNRs appear crab-like in both radio and X-ray wavebands;
however, they also have shells. Their X-ray spectra in the center
do not show spectral lines, but the X-ray spectra near the shell do
have spectral lines.
How do we Know a Supernova Remnant's Age?
Naturally, if the supernova explosion was recorded in history, as is the
case of many SNRs less than a few thousand years old, we know the age of the
corresponding SNR. However, sometimes historians are not certain if a recorded
star" was a supernova or was the same supernova as a
corresponding remnant. It is therefore important to be able to estimate the
age of SNRs.
An easy way to guess the age of a SNR is to measure the temperature of
the hot gas using X-ray
From this observation we can estimate the velocity of the shock wave, and then
infer the age from the shock velocity. This works because the
velocity of the shock slows down with time as it engulfs more material
and cools. This is easy to do, but not very
accurate, because there are a number of complicated processes that can
heat up or cool down the gas which are independent of shock velocity.
A better way, which works well for the youngest SNRs, is to measure a SNR's
expansion over time and apply the equation
rate x time = distance.
For example if we observed a supernova remnant both 20 years ago and today,
we would have two images 20 years apart. Comparing the sizes of the two
images and dividing the difference by 20 years, yields the rate at which the
SNR is expanding. For example, if we found that the supernova remnant
expanded by 5% over the 20 year period, then the the rate of expansion would
rate = 5/ 20 years = 0.25 /year
Because the SNR expanded 100% since it exploded, its age can be calculated
in the following manner:
time = 100/ (0.25 /year) = 400 years
With the above example, it is safer to say that the supernova explosion
happened less than 400 years ago, because it is quite likely that the SNR's
expansion has slowed down since the explosion (whereas it is unlikely to have
sped up). An age calculated according to this method is more likely to be
accurate when calculated for the fasting moving features in the supernova
remnant or the result agrees with historical records.
Why are Supernova Remnants Important to Us?
Supernova remnants greatly impact the ecology of the Milky Way.
If it were not for SNRs there would be no Sun or Earth.
All the elements heavier than boron were made in
either a star or a
supernova explosion. So, how did these elements come to be on Earth? Through
the action of supernova remnants.
The gas that fills the disk of the Milky Way is called the interstellar
medium (ISM). In the parts of the galaxy where the ISM is most
dense (for example
in the Galaxy's spiral arms), the ISM gas can
collapse into clumps. Clumps that are above a critical mass (somewhere
between the mass of Jupiter and the Sun) will ignite
fusion when the clumps gravitationally collapse, thus forming stars.
Therefore, the chemical composition of the ISM becomes the chemical
composition of the next generation of stars.
Because, supernova remnants mix supernova
the newly formed elements) into the ISM, if it were not for supernova remnants
our solar system, with its rocky planets, could never have formed.
What Else Do SNRs Do to the Milky Way?
In addition to enriching our Galaxy with heavy elements, supernova
remnants add a great deal of energy to the ISM (1028
supernova). As the shock wave moves outward it sweeps across a large
volume of the ISM, impacting the ISM in two primary ways:
||The shock wave heats the gas it encounters, not only raising the
overall temperature of the ISM, but making some parts of the Galaxy
hotter than others. These temperature differences help to keep the
Milky Way a dynamic and interesting place.|
||The shock wave accelerates
ions (via the
Fermi acceleration process) to velocities very close to the speed
of light. This phenomenon is very important, because the origin
cosmic rays is one of great outstanding problems in astrophysics.
Most astronomers believe that most cosmic rays in our Galaxy used to
be part of the gas in the ISM, until they got caught in a supernova
wave. By rattling back and forth across the shock wave,
these particles gain energy and become cosmic rays. However,
astronomers still debate to what maximum energy SNRs can accelerate
cosmic rays -- the current best guess is about 1014
What are the Stages of a SNRs Life?
The stages of a SNR's life represent an area of current study; however,
basic theories yield a three-phase analysis of SNR evolution.
||In the first phase, free expansion, the front of the
expansion is formed from the shock wave interacting with the ambient
ISM. This phase is characterized by constant temperature within the
SNR and constant expansion velocity of the shell. It lasts a
couple hundred years.|
||During the second phase, known as the Sedov or
Adiabatic Phase, the SNR material slowly begins to decelerate
by 1/r(3/2) and cool by 1/r3 (r being the radius
of the SNR). In this phase, the main shell of the SNR is
Rayleigh-Taylor unstable, and the SNR'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 SNR shell. This phase lasts 10,000 - 20,000 years.|
||The third phase, the Snow-plow or Radiative
phase, begins after the shell has cooled down to about
106 K. At this stage, electrons begin recombining with the heavier atoms (like Oxygen) so the shell can more efficiently radiate energy.
This, in turn, cools the shell faster, making it shrink
and become more dense. The more the shell cools, the more atoms can recombine, creating a snowball effect. Because of
this snowball effect, the SNR quickly develops a thin shell and
radiates most of its energy away as optical light. The velocity now
decreases as 1/r3. Outward expansion stops and the SNR
starts to collapse under its own gravity. This lasts
a few hundreds of thousands of years. After millions of years, the
SNR will be absorbed into the interstellar medium due to
Rayleigh-Taylor instabilities breaking material away from the SNR's
outer shell. |
For more general information on SNRs, see:
Astronomy, The Cosmic Perspective by Zeilik and Gaustad
A good book on SNRs and supernovae, written for the non-scientist is:
The Supernova Story, by Laurence A. Marschall, ©1988, Plenum Press,
For more information on types of SNRs, see:
Weiler K., and Sramek, R. 1988. Ann Rev. Astron. Astrophys. 26: 295-341
For more information on SNR evolution, see:
Chevalier, R.A. 1977. Ann. Rev. Astron. Astrophys. 15: 175-96
For more information on SNR and soft X-rays, see:
Gorenstein, P., Tucker, W.H. 1976. Ann. Rev. Astron. Astrophys. 14: 373-414