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Ancient Cosmic Explosions
Ancient Cosmic Explosions
Background Science
Author: Haley Gomez
Background Science - Faulkes Telescope Project
Ancient Cosmic Explosions
Background Science
Supernovae
Supernovae are the violent explosions of stars occurring at the end of their lives. On
average, one supernova goes off every 50 years or so in our Galaxy. There are two main
types of supernovae - Type Ia and II. Type II are the explosions of very massive stars
with mass greater than 8 times the mass of the Sun. Type Ia are the explosions of stars
similar in mass to the Sun, which have a binary companion and become unstable.
Type II -These stars 'live fast - die young', using up all of their Hydrogen and Helium fuel
in only a few million years, thousands of times faster than the Sun burns its fuel. When
the fuel supply is exhausted the star must burn heavier and heavier elements until, finally,
when it can do no more to keep itself alive, the inner parts of the star collapse to form a
neutron star or black hole, and the outer parts are flung off in an explosion we call a
supernova.
Ancient Cosmic Explosions
Type Ia - A star similar in mass to the Sun reaches the end of its life as a white dwarf star,
which is a hot, dense core (see the Lifecycle of Stars programme). If this hot core has a
companion star (so the two stars are orbiting around a common centre of mass - known
as a binary system), the white dwarf will pull matter from the companion star onto its
surface, until it becomes unstable.
The enormous explosion from these stars ejects material into the surroundings at very
high velocities, sweeping up the surrounding gas into a shell or a giant bubble. This is
known as a supernova remnant. The ejected material and the swept-up compressed
gas are very hot. The shell (or bubble) shines at different wavelengths, mainly in the Xray, optical and radio.
Supernovae release more energy in a single instant than the Sun will produce in its whole
lifetime! If the nearest massive star, Betelgeuse, in the constellation Orion, were to go
supernova it would (for a short time) be brighter than the full moon.
Below are some images of supernovae captured with the Hubble Space Telescope and
the Chandra X-ray Observatory.
Credit: Dr CBurrows, ESA/STScI and NASA; NASA/CXC/ASU/J Hester et al.; NASA/CXC/SAO.
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Multi-wavelength Supernovae
Supernova remnants are studied at many different wavelengths from optical light to Xrays. Different things are happening in different wavelengths; when we observe in, say
the X-ray, we are looking at the parts of the shell that are much hotter than the areas
shining in the optical.
The X-rays come from the extremely hot material at around 10 million degrees Kelvin.
These high energy rays are emitted from the chemical elements in the gas, for example,
from silicon, iron, magnesium, calcium etc. At such high temperatures, the electrons in
these elements are ripped out of the atoms and emit X-rays. Using X-ray images of
these objects, we can find out about the distribution of the different elements and how
much of these elements are present. This tells us important things about the material in
the explosion.
The optical emission comes from the interaction between the outwardly moving shell and
the material that surrounds the supernova. The material is compressed and heated to 10
thousand Kelvin. At optical wavelengths we are looking at the shocks caused by the
expanding material as the ejected material sweeps outwards at high velocities.
Ancient Cosmic Explosions
Radio emission is a different story. Radio waves in a supernova remnant come from high
energy electrons which are heated up by the energy released in the explosion and whiz
about in the strong magnetic field of the remnant.
In this project you will either use observations of one supernova remnant already studied
by Faulkes or you can also make your own observations from a list of three remnants
(see the list below). However, be warned, these objects are very faint and require long
exposure times!
Information about the three supernova remnants chosen for this project are listed below.
The X-ray pictures are from the Chandra Observatory, optical from the Hubble Space
Telescope and radio from the Very Large Telescope (VLT) and Australian Telescope
Compact Array (ATCA) telescopes.
Name
Type
Constellation
Cas A
Distance
(light years)
10,200
II
Cassiopeia
Kepler
15,000
II or Ia?
Ophiuchus
N49
160,000
II
Dorado, in the Large Magellanic
Cloud
Note: 1 light year is 9.5 x 1015 m.
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Cassiopeia A (Cas A)
Cassiopiea A is the brightest radio object in the sky so it is very well studied (and very
beautiful)! The massive star at the centre exploded about 300 years ago into a Type II
supernova. It is around 11,000 light years away and measures about 8 arcmin across.
Credit: NASA/CXC/MIT/UMass Amherst/M D Stage et al; NRAO/AUI/NSF; NASA/ESA/Hubble Heritage
The X-ray image shows red for low intensity X-rays, Green for medium intensity and blue
for high intensity X-rays. These show the X-rays from hot iron and silicon, which gives us
information about the chemical elements in the explosion.
Ancient Cosmic Explosions
Kepler
Kepler’s remnant was discovered by the astronomer Johannes Kepler 400 years ago. It
is 15,000 light years away and is 5 arcmin across in the sky. The Sun is very small in
comparison to this remnant. You would need 1, 000, 000, 000, 000 Suns placed side by
side to stretch out the whole length of our Galaxy but you would only need 6000 Kepler
supernova remnants placed side by side! We do not know whether Kepler is a Type Ia or
Type II supernova, as there is evidence that supports both!
Credit: NASA/CXC/NCSU/S Reynolds et al; VLA; NASA/ESA/ R Sankrit and W Blair
Notice how Kepler’s optical image is very faint (this is because it is not as energetic as
Cas A’s explosion was). You can already see some differences between Kepler and Cas
A. In the X-ray, radio and optical the remnants look very different, even though the same
process is thought to have formed them!
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N49
N49 is seen in our nearest neighbour galaxy, the Large Magellanic Cloud, about 160, 000
light years away. It is the brightest supernova in this galaxy and is believed to be more
than 5000 years old! The shell is 1 arcmin across, and looks incomplete.
Credit: ATCA/I. Ill/J. Dicket et al; NASA/STScI/UIUC/Y.H.Chu & R.Williams et al;
NASA/CXC/Caltech/S.Kulkarni et al.
Ancient Cosmic Explosions
N49 has a strange, lop-sided filamentary structure. It was a Type II Supernova which
formed this remnant.
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