SUPERNOVAE REMNANTS (SNRs)
Stars are self-luminous gaseous spheres. They evolve and finally
die. Their source of energy is the nuclear burning of the material
they are made of. At the first stages of their lives they contain only
Hydrogen (H) and Helium (He). With a series of nuclear reactions they
convert H and He to heavier elements [e.g. Oxygen (O), Iron (Fe),
This graph shows the structure of an evolved massive star of 20 solar
masses. The heaviest elements are closer to the core of the star
(e.g. Fe), while the outer shell still comprises of H and He. The core
of the star is very compact and heavy. Once a star has reached this
structure, the nuclear burning stops; the iron core will not ignite to
induce further nuclear burning as has happened earlier to the He, C,
O, and other cores. The reason is that Fe can't burn. The fact that
iron does not burn leads to the collapse of the star's iron core.
Extremely hot, massive stars (with masses more
than about 10 times that of the sun) burn their supply of hydrogen and
helium very quickly. Once this fuel runs out, they end their lives in
a spectacular explosion, known as a supernova, blasting their outer
layers out in to interstellar space. Lower mass stars do not burn their fuel
as quickly as the heavier ones, and end their lives in a more quiet
The above two photographs are of the same part of
the sky. The photo on the left was taken during the supernova
explosion (SN 1987A), while the right hand photo was taken
beforehand. Supernovae are one of the most energetic explosions in
nature, making them like a 1028 megaton bomb (i.e., a few octillion
Supernova remnants (SNRs) are the remainders of the
massive stars that exploded. The explosions eject hot shells of gas at
speeds reaching 20,000 km/s. This explosion throws out into space
the outer layers of the star. These layers are composed largely of
elements produced by nuclear fusion, such as carbon, oxygen, neon, and
silicon. It also produces a shock wave speeding out ahead of the
ejected material, that sweeps up the material around the star and heats
it to about 10 million degrees. The `shocked' interstellar medium is
so hot that it is invisible to the naked eye, but glows in X-ray
light. The interstellar gas is heated up by the shock to temperatures
high enough to produce thermal X-ray emission.
The resulting supernova shells are perhaps the most beautiful
objects seen in X-rays.
EXPLANATION OF ABOVE PICTURE...
X-rays are emitted by the gaseous clouds while they
remain hot, but they rapidly fade as the gas cools. Since the cold gas
gives out very little visible light, the shells are difficult to
detect with optical telescopes. The only evidence for the existence of
an old supernova remnant may be a rapidly rotating neutron star (a
pulsar), which may also emit X-rays.
What happens to the star after the
Depending on the initial mass of the star, the
supernova might leave behind the core of the star, altered
dramatically by the explosion. One possible form the core can take is
a neutron star, a star as massive as the sun, but compressed to a
diameter of approximately 20 kilometres (13 miles), and a density
comparable to that of an atomic nucleus. Many neutron stars produced
in this way are pulsars, sending out narrow beams of radio waves and
X-rays. As the star rotates, we observe periodic radio and X-ray
pulses as the beam crosses our line of sight. A second possible form
for stars larger than about 20 times the mass of the sun is a black
How Often Do Supernovae Occur?
Although many supernovae have been seen in nearby galaxies,
supernova explosions are relatively rare events in our own Galaxy,
happening once a century or so on average. The last nearby supernova
explosion occurred in 1680. It was thought to be just a normal star at
the time, but it caused a discrepancy in the observer's star catalogue
which historians finally resolved 300 years later, after the supernova
remnant (Cassiopeia A) was discovered and its age estimated. Before
1680, the two most recent supernova explosions were observed by the
great astronomers Tycho and Kepler in 1572 and 1604 respectively.
In 1987 there was a supernova explosion in a
companion galaxy to the Milky Way. Supernova 1987A is close enough to
continuously observe as it changes over time thus greatly expanding
astronomers' understanding of this fascinating phenomenon.
a list of historical supernovae
What Causes a Star to Blow Up?
Gravity gives the supernova its energy. A
supernova explosion will occur when there is no longer enough fuel for
the fusion process in the core of the star to create an outward
pressure which combats the inward gravitational pull of the star's
great mass. After the H and He burning finishes in the star's
core, the star will swell into a red supergiant (on the outside). On
the inside, the core yields to gravity and begins shrinking. As it
shrinks, it grows hotter and denser. A new series of nuclear reactions
begin to occur (creating the heavier elements), temporarily halting
the collapse of the core.
When the core contains essentially just iron, it
has nothing left to fuse (because of iron's nuclear structure, fusion
of iron does not release energy, but requires energy as input) .
Fusion in the core ceases. In less than a second, the star begins the
final phase of gravitational collapse. The core temperature rises to
over 100 billion degrees as the iron atoms are crushed together. The
repulsive force between the nuclei overcomes the force of gravity, and
the core recoils out from the heart of the star in an explosive shock
wave. The shock then propels the matter out into space,creating
the supernova remnant.
Why do we study SNRs?
They tell us
how elements escape from the stars
Many of the more common elements are made through
nuclear fusion in the cores of stars. Supernovae scatter these
elements out in to the interstellar medium. These are the elements
that make up stars, planets and everything on Earth -- including
The above picture show how various elements (e.g.
Ne, Fe,) are distributed within the SNR Cassiopeia A (Cas-A). Cas-A is
a young supernova remnant, with a 15 light years diameter. It is situated
10 thousand light years away. It is the remains of a massive star which,
having exhausted all its hydrogen fuel, exploded 320 years ago.