Supernovae
Supernovae are simply
stars which explode. It turns out there is more than one way for a star
to explode, and hence we have more than one type of supernovae. To understand
supernovae, we need to understand stars.
Our sun was formed out
of clouds of dust and gas. In these clouds, little clumps collapsed, and
their gravity continued to attract more and more material until the material
heated up and became dense enough to start the fusion of Hydrogen to Helium.
A star spends most of its life converting its Hydrogen to Helium - we think
our sun will do this for a total of about 10 billion years. In case you
are worried, we have about another 5.5 billion years left before the sun
goes into its next stage, burning Helium to Carbon. In this stage our Sun
will swell up hundreds of times its current size, swallowing the Earth,
and glowing hundreds of times brighter than it is now. We call this phase
of a star its red giant phase. The sun eventually runs out of Helium to
burn, and this is the end of the road for the Sun. Its core collapses into
a very dense star known as a white dwarf, and its outer layers are released
to form a planetary nebula (one of the most spectacular sights in the sky).
The planetary nebula soon disperses, and the white dwarf is left to cool
for eternity, fading into oblivion.
Stars more than 10 times
the mass of our sun have much more exciting lives. They form in much the
same way as our sun, but are a million times brighter, and subsequently
burn their Hydrogen much quicker than our sun. The good life does not last
long (it doesn't take an astrophysicist to realise that if you only have
10 times more fuel, but are using it up a million times quicker, you will
run out of steam 100,000 times sooner). These stars then quickly turn to
Helium to satisfy their energy needs and become Red Super Giant stars, expanding
out to the orbit of Mars in the process. The Helium soon runs out, but these
stars are able to continue to process additional elements as atomic fuel.
After Helium, these massive stars convert their Carbon to Oxygen, and then
into successively heavier elements. Each conversion gives less and less
energy, and support the star's appetite for fuel for less and less time.
The last few days of the star's 10 million year life are spent burning Silicon
into Iron.
Although iron can be fused into heavier elements such as Cobalt and Nickel,
these reactions do not act as a nuclear furnace, but rather as a nuclear
refrigerator. The star, which was held up by the pressure caused by all
the heat generated (Hot air in a balloon has more pressure causing the air
to expand making the air lighter than the cooler outside air), suddenly
begins to fall onto itself, with the inner part of the star collapsing to
a neutron star, and the outer parts of the star being thrown off and forming
a.....
A Type II supernova to
be precise.
The picture shows supernova 1987A before and after as caught by David Malin
and the Anglo Australian Observatory. SN 1987A was discovered on February
23, 1987, and although its light took 170,000 years to reach Earth, it became
easily visible to the naked eye. It was the closest and brightest Supernova
as seen from the Earth in the past 350 years.
So where is the next supernova
going to be in our galaxy. Well, we are not sure. One good guess is the
the Eta Carinae, one of the most massive stars in our galaxy. This star
will explode at some point, maybe tomorrow, maybe in 10,000 years, but it
will only visible to the inhabitants of the southern hemisphere. (Picture
from Mike Bessell, MSSSO)
My favourite candidate
for the next supernova is Betelgeuse in the constellation Orion. It could
also explode tomorrow or in 10,000 years. When it does go, it will be bright,
about like the moon when it is 1/2 full, But it will be a point of light
rather than a disk, and will be a spectacular sight in the sky for many
months. (Picture courtesy of Mike Bessell, MSSSO>
One of the most spectacular objects in the sky must have been the Supernovae
of 1054 observed by the Chinese. Today this explosion is seen as the expanding
remains of the star (we call it the Crab Nebula), and inside is a pulsar
which is the rotating neutron star shining a beacon of light towards the
Earth many times a second.
Type Ia Supernovae
Type Ia Supernova are the explosions of white dwarfs. This is a pinnacle
that only a few stars like our sun are able to achieve. Unfortunately we
are not sure exactly how these events occur. We think they are related to
white dwarf stars which are near another star in a binary system. Chandrasekhar,
as part of his Nobel Prize in Physics demonstrated that white dwarf stars,
if they become more massive than 1.4 times our sun can explode. They do
this because at this point, the forces (electrons repelling electrons) which
keep the star from collapsing against the force of gravity, lose their battle,
and the white dwarf begins to collapse. As you may recall, white dwarf stars
are not made of Iron, (instead they are composed of Carbon and Oxygen) and
there is still substantial amounts of nuclear energy left in their atoms.
As the white dwarf begins to collapse against the weight of gravity, this
material is ignited, and rather than collapsing further, this nuclear blast
wave consumes the star in a second, creating an explosion 10 to 100 times
brighter than a Type II supernova.
SN 1994D observed
with the Hubble Space Telescope. The SN is the bright star in the lower
left corner.
Type Ia supernovae
in the nearby universe are observed to all have a similar brightness,
and this makes them very powerful objects for measuring distances. In
addition, because they are so bright, they can be seen at great distances,
and these two things make them currently unique objects for measuring
the vast distances of the Universe. Unfortunately, they are very rare.
The last one seen in our galaxy was in 1006, and it must have been incredibly
bright - easily visible in the daytime.
The idea to measure the Universe with Supernovae is not new, it has long
been contemplated, but it is only in the past decade that it has become
feasible. The first distant SN Ia was discovered in 1988 by a Danish team,
but it wasn't until 1994 that they were discovered in large numbers. Since
1995 two teams have been discovering these objects: Our team, the High-Z
SN Search, and the Supernova Cosmology Project.
To measure the fate of the Universe, we need both distant and nearby objects,
as it is only through the comparison of nearby and distant objects that
the Universe's behaviour uncovered. Amazingly enough, the first good nearby
sample was only completed in 1996 by a group at Cerro Tololo Inter-American
Observatory (CTIO), and these objects are what enable us to use supernovae
to measure the ultimate fate of the Universe.
Type Ia Supernovae are not all exactly the same brightness. They vary
by as much as a factor of two. But Mark Phillips, Mario Hamuy and collaborators
at Cerro Tololo Inter- American Observatory in Chile showed that faint
supernovae rise and fall very quickly, whereas bright supernova brighten
and fade much more slowly, By looking at how much the objects faded in
the first 15 days following maximum light, their work showed that type
Ia supernovae can give distances which are good to about 7% - equal to
the best of astronomical distance indicators.
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