|MadSci Network: Astronomy|
Low to medium mass stars (about 8 times the mass of our Sun or less) do not have the gravity to generate core temperatures high enough fuse carbon into even heavier elements like oxygen, neon, etc. So, their cores will be made primarily of carbon. Fusion never happens in the outer layers of the star (it isn't hot enough), so the while the core has become carbon, the outer layers are still mostly hydrogen and helium.
Since there is no more fusion, the carbon core of the star will collapse into a very dense ball about the size of the Earth (a white dwarf star), and the star's outer layers will be ejected to space (forming a planetary nebula).
The collapsed core will be very hot, millions of degrees. So, the carbon in the core won't crystallize into a diamond right away. Over time, the dead stellar core will cool off, and eventually, the carbon will start to crystallize into a diamond-like configuration. This cooling off takes a few billion years to happen! Also, it won't be like a diamond here on Earth, it will be very dense: a teaspoon-sized diamond from the stellar core will weigh about five tons.
One "diamond star" has been confirmed by astronomers. Its name is BPM 37093, and it lies about 50 light years from Earth. Astronomers estimate the dead stellar core is a 10 billion trillion trillion carat diamond (thats a "1" followed by 34 zeros!). For comparison, the largest diamond on Earth is the 530-carat Star of Africa (part of the British Crown Jewels).
Finally, stars larger than 8 times the mass of our Sun can fuse carbon into heavier elements, so they won't end up as "balls of carbon." They end their lives as neutron stars (incredibly dense balls of neutrons) or black holes. [In the process of forming a neutron star, they explode in a supernova. Some of the carbon formed during their life is then ejected into space. All of the carbon on the Earth, including in all of our diamonds, was once forged in some now long dead star! Moderator]
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