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End of a stars life
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Lecture notes of 5 pages for the course Astro 101 at UofA (End of a stars life)
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Within this module, we have looked at the birth and life of stars. We have explored the stellarnurseries and discovered that stars, just like humans, walk di erent paths. Yes, we all eat, sleep,
and explore life, or in the case of a star, burn fuel and shine bright. However, just as there are
di erent paces of life for us humans so too can stars live and die in many di erent ways. Now that
we know the basics, let's explore the life of rock stars and that of average joes, in the stellar sense
that is. Let's see how di erent stars move towards the end of their lives. Here we will discover that
the life and subsequent death of a star are determined at birth by the star's mass. But how can this
be the case?
The story of a star's death begins at the point at which it leaves the safety and security of the main
sequence. The main sequence is the long main track observed in the Hertzsprung Russell diagram.
We have learned that stars seen in the upper left of this track burn hot, blue, and are massive,
weighing 10 to 100 times the mass of our sun, or possibly more. Stars at the lower right of the track
are cool by stellar standards. They are red and may only contain a tenth of the mass of our sun. We
have also learned that blue stars tend to be called high mass, while stars like our sun and smaller
are often called low mass stars. When a star leaves the main sequence of the Hertzsprung Russell
Diagram, the star is seen to move towards the right of this plot. This movement is the result of the
star becoming redder. But what does this change in color represent? What changes in the star's
interior are powering the shift? And what happens in the time between the departure from the main
sequence, and the star's demise?
This is what we are about to explore. When a star's core runs out of hydrogen fuel, the core can no
longer sustain the outward radiation force that balances the force of gravity, which is pulling
everything inwards. Therefore, the star will no longer be in hydrostatic equilibrium. This loss of
radiation pressure results in the stars core collapsing. The collapse of the core in turn causes the
temperature of the interior to increase. When a star burning primarily hydrogen su ers from such a
collapse, it will contract until the core reaches about 100 million degrees. At that temperature, the
star can begin to burn helium in its core. Helium will become the main source of energy for the star
at this point, as it fuses to create carbon and other trace elements. However, if we look just outside
the helium core, we can see that the core is surrounded by a shell of hydrogen that is also burning.
This hydrogen shell is slowly consuming more material as it moves outward through the star. As the
helium burns hotter than its predecessor, the hydrogen core, it burns more rapidly, therefore this
phase of the star's life is shorter. Once the helium core is exhausted, the core will once again
collapse. As it collapses, the temperature will once again increase. If the collapse leads to a
temperature of 600 million degrees, the star is able to burn carbon. Such a core would be
surrounded by both helium and hydrogen shells. Once the carbon burning is complete, the cycle
can again repeat, leading to neon, oxygen, and even silicon burning, with the core becoming
heavier and heavier. As each layer is hotter than the last, the star burns through the core more
rapidly. For example, a star could take billions of years to burn through its hydrogen whilst it may
only take hundreds of years to feed on a carbon core. By the time we reach silicon, it's possible for
a star to consume its core in about a day.
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, Hydrogen Burning shell
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The multiple layers of a star seen here have led to these stars sometimes gaining the nickname of
onion stars as onions have layers, just like ogres, or cakes, or parfaits. Everybody likes parfaits.
Where was I? Back to stars. Yes, they have layers. Okay, but it all stops with iron. When learning
about fusion and ssion we discovered that the most tightly bound atom is iron and that no more
energy can be gained by either breaking iron apart or by smashing two iron nuclei together. As a
result, there is no fuel left for the star to burn through.
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Fusion in the core must stop and the star will die. We will explore more of the death of stars in the
next two videos. So far we have been looking at what's going on inside the star, but what e ect
does that have on the outside of the star? Or rather, what e ect does that have on what we can
see? When stars run out of the current fuel in their core, we discovered that the cores collapse. This
continues until the star is able to burn a new type of fuel. So for example, when a star initially runs
out of hydrogen, it will collapse until the helium res begin to burn. What e ect does this have on
the envelope of the star? The envelope is the outer hydrogen rich region of the star, which is not
involved in nuclear fusion. It is the outer layer of the star.
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