Physics Question #138

Kay, a 14 year old female from the Internet asks on February 23, 2000,

How can a star become small and dense enough to become a black hole? My understanding is that a neutron star forms when a star collapses and everything is destroyed except its protons and electrons which combine to form neutrons. If a star is big enough, it will crush the neutrons as well, and collapse into nothingness. This is the only way a star can become smaller than a neutron star. If this is so, then how is it possible for a star to become small and dense enough to be a black hole?

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The answer

Jess Brewer answered on February 23, 2000

I think the confusion is in your interpretation of the phrase, "collapse into nothingness." A sufficiently massive neutron star does not stop collapsing when the neutrons get close enough together to see the strong close-range repulsive potential of the neutrons, but just keeps on shrinking to a point; that part is right, but it is wrong to think that because it becomes infinitely small it loses some of its gravitational "oomph". Quite the opposite: in fact, the collapsed object (once called a "collapsar") is just what we mean by a "black hole" -- although of course there are lots of possible complications associated with its spin and electrical charge.

So just remember, a black hole is a collapsed neutron star (or something even heavier). There may be smaller and lighter black holes around too, but they would probably have to be left over from the Big Bang.

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