Stellar Evolution

Stars begin their lives
as a cloud of dust,
better known as a
Nebula.
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The star begins to form as the nebula condenses
through gravitational attraction.
Once the attraction has
made the body opaque to
electromagnetic radiation,
it is then classified as a
Protostar.

As this electromagnetic radiation is the same
radiation that the star is attempting to release.
Some of it stays in the Protostar and aids heating
the interior.

The Protostar can
not produce nuclear
reactions.
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When the core
temperatures and
pressures are high
enough – nuclear
reactions fusing Hydrogen
into Helium start.
The contraction then stops. Now the Protostar is
classified as a Main Sequence Star
Stellar Structure

The gravitational
attraction is balanced
by the pressure from
the energy released
during nuclear fusion.
When a star has
used about 60% of
the fusion is
completed it will
start it’s death!
 The temperature of
the star falls and the
core collapses under its gravity to stabilise.
 Temperature in the core rises causing more
nuclear fusion – Helium to Carbon.
 This causes the star to expand into a Red
Giant

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Once this relatively stable phase ends.
Each star will take a different path to
death depending on the mass.

For this example we are going to use a
star with a mass > 25 Solar Masses.
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In a Massive Star, the Red Giant phase is
split into stages of contraction and
expansion when huge quantities of
different elements are fused.
These form layers or shells inside the core
of the star.
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Eventually these shells collapse into each
other – causing a chain reaction.
This collapse causes a shockwave that
projects a large proportion of the mass of
the star outwards.
In a Massive star,
this shock wave
is classified as a
Supernova.

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The Supernova leaves behind a body, so massive,
its gravitational attraction is such that light can
not escape.
A Black Hole is born
To form a black hole the remnant must be
greater than 3 solar masses.

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How do we know that this happens and
something that we can’t see really exists?
The answer lies in
binary systems where
a star is seen to orbit
an object that is not
visible – such as
Cygnus X-1