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Lesson 7 - Stellar Evolution (Part 2) - Star Death

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Scott Harnath

on 18 December 2014

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Transcript of Lesson 7 - Stellar Evolution (Part 2) - Star Death

We're made of star-stuff
Last Lesson
Stellar Evolution (part 2)- Star Death
In our last lesson we discussed how stars are born from massive clouds of gas, called Nebulae.
Within Nebulae, pockets of the gas begin to collapse under gravity until they reach a point where the pressure is so great, fusion occurs.
Once this occurs, a star is officially born.
Types of stars
There were 3 main types of stars that we discussed, Red Dwarfs, Yellow Stars and Blue Giant Stars
But why do stars die?
Fusion takes many smaller atomic nuclei and combines them to make larger nuclei.
A by-product of this process is a massive amount of energy that is released.
Unfortunately though, the bigger the nuclei you fuse together, the less energy you get out.
So as a star runs out of Hydrogen, it starts to produce less energy... this is when it starts to die
Star Death
How a star dies depends directly on how big it is.

Big stars (blue) burn up their fuel really quickly and so die sooner

Small stars (red) take longer to burn their fuel and so take longer to die
All that gas makes a star bloated
As the star attempts to fuse higher and higher order elements, fusion layers are pushed further outwards causing the star to swell up.
Because the energy from the star is then spread out over a larger surface, the surface cools, turning the star into a red giant.
Lesson 4.1
The differences in colour are due to the amount of gas that collapsed to form the star in the first place. The more gas, the bigger a star is and the bluer it becomes...
... but this is not all that is dependent on its size
If the star is large enough, that is it is about 8 times larger than our own sun, it will die in a spectacular explosion known as a supernova.

When this happens, the star, in a last ditch effort to keep on fusing elements together, collapses upon itself and in the process creates all of the higher order elements present in our Universe. But this also creates a massive explosion of energy which then propels the star's guts out all over the Universe
So do all stars explode?
No. Smaller stars like our sun don't have enough mass to undergo this final collapse and so don't (or won't) go supernova.
Instead what happens is that they swell up into a red giant but then the outer layers simply drift off into space creating a new nebula whilst the core of the star is left behind.
This star core is known as a white dwarf, a small but bright little star that will keep shining for a little while before it eventually stops fusing things together

A black hole is an object so gravitationally strong, not even light can escape it's pull (hence why it's black).
When this happens, gravity is so strong that it collapses all the matter into a single point, also known as a singularity
Black Holes and Neutron Stars
Of course some stars are so massive that even after they go supernova they leave something behind.
If a star is truly massive, then after it goes supernova, it might leave behind a incredibly dense core known as a neutron star. These stars have such intense gravitational pressure on them that the space between atoms has actually been filled in with more atom.
A neutron star is effectively a massive ball of neutrons
But of course we all know that it doesn't stop there. Add a bit more mass to a neutron star and the whole thing collapses into what we call a black hole.

Carl Sagan
Full transcript