Is Star Death Related to Black Hole Evaporation?


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X-Ray, Optical, and Infrared composite image of the remnant from the supernova observed by Kepler in 1604. Image Credit: NASA/ESA/JHU/R.Sankrit & W.Blair

A Decoded Science reader asked the question: Is there any known correlation between the death of a star and the dissipation of a black hole?

The quick answer is that black holes form from the deaths of massive stars, but these black holes do not dissipate in any reasonable length of time.

Answering this question more completely requires explaining the various background concepts first.

What is a Black Hole?

Stars, planets, and other astronomical objects have an escape velocity, which depends on the object’s total mass and radius. The escape velocity is the minimum speed needed to escape the object’s gravitational field.

According to Einstein’s special relativity theory, the speed of light is the ultimate speed limit in the universe.  Nothing can travel faster than the speed of light. A black hole is simply a star, or other object, that has collapsed to the point where its surface velocity exceeds the speed of light.

Nothing, not even light, can escape from a black hole. Anything that falls into a black hole is trapped forever.

How do Black Holes Form?

Ordinary stellar black holes form from the deaths of massive stars via type II supernova explosions. Caveat: Astronomers talk about star birth, star death, and stellar lives, but stars are not in any way alive in a biological sense.

The most massive stars end their lives very violently as type II supernova explosions During the supernova explosion, the massive star, in about a year, emits about as much total energy as the Sun emits in its entire 10 billion year hydrogen-burning lifetime.

The type II supernova blasts the star’s outer layers back into space, but a small core remains. If this core has more than 2 to 3 (The exact number is uncertain.) times the Sun’s mass, the stellar core will collapse into a black hole.

Black holes form from the death throes of the most massive stars and are therefore related to the deaths of stars.

Black Hole Evaporation Theories

To understand the strong gravitational fields of black holes, astrophysicists must use Einstein’s general relativity theory. Using only this theory of gravity tells astrophysicists that nothing can escape from black holes, and therefore that black holes cannot dissipate or evaporate.

Stephen Hawking giving a lecture for NASA in 2008. Image Credit: NASA/Paul Alers

Stephen Hawking, however, has studied the theory of black holes by combining the theories of both general relativity and quantum mechanics. Hawking concluded that the random nature of quantum mechanics allows the possibility that black holes can evaporate. The average time required for a black hole to evaporate increases as the mass of the black hole increases. For black holes having masses comparable to single stars, the average evaporation time is about 1067 years – that is a really long time! For comparison, the universe is only about 14 billion (14X109) years old.

Because the average evaporation time of a typical stellar black hole is so much longer than the age of the universe, it is extremely unlikely that any stellar black holes have evaporated. Hawking has however also theorized that less massive primordial black holes formed during the big bang, and could evaporate within the age of the universe.

Death of Stars and Black Hole Dissipation

To answer our reader’s question: although the formation of black holes does relate to the death of a star, dying stars are not related directly to black hole evaporation.

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