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The Question

(Submitted February 10, 1999)

How can a star be compressed to form a black hole?

The Answer

Gravity does the work. When you have enough material together, gravity can be very strong. And the more mass you have, the lower the density needs to be in order to make a black hole.

If all nuclear burning in a star greater than about 1.4 solar masses were to stop, and the star allowed to cool and solidify, the solidified material would not be strong enough to support its own weight, and it would collapse as the electrons were pushed into the protons to make neutrons. This neutron star material is stronger, but the star would be only about 20 km in diameter. If you piled on more material, you would eventually get to the point where there is so much gravity that not even the neutrons could hold it up, and the star would collapse into a black hole.

David Palmer and Samar Safi-Harb
for Ask an Astrophysicist

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