How Are Black Holes Formed?
QUICK ANSWER
Most black holes form when massive stars (at least 8-20 times the Sun's mass) run out of fuel and their cores collapse under gravity. The core compresses into a tiny region where gravity overwhelms everything else, creating a black hole. Supermassive black holes form through different mechanisms over much longer timescales.
Black holes form in several different ways depending on their size. Stellar-mass black holes are made from collapsing massive stars in a relatively well-understood process. Supermassive black holes have a more mysterious origin and may form through a combination of mergers, gas accretion, and direct collapse over billions of years.
How do stellar-mass black holes form?
From dying massive stars. According to NASA, when a star at least 8 times more massive than the Sun runs out of nuclear fuel, its core can no longer support the weight of the outer layers and collapses. For stars massive enough (typically 20+ solar masses), the collapse produces a black hole. The outer layers blow off in a supernova explosion, leaving a stellar-mass black hole at the center. The process takes about a fraction of a second for the actual collapse, after millions of years of stellar evolution leading up to it.
How do supermassive black holes form?
More uncertainly. The early universe contained supermassive black holes billions of times the Sun's mass remarkably quickly after the Big Bang, faster than typical accretion can explain. Several theories compete: massive black holes may have formed directly from collapsing gas clouds, may have grown from primordial seeds in the early universe, or may have built up through mergers and rapid accretion. James Webb Space Telescope observations have complicated the picture by finding very large black holes in the very early universe.
Can black holes merge to make bigger ones?
Yes. When two galaxies merge (which happens often over billions of years), their central black holes can spiral together and eventually merge into a single larger black hole. The merger releases enormous amounts of energy as gravitational waves, ripples in spacetime detected by observatories like LIGO since 2015. Smaller mergers between stellar-mass black holes happen frequently. The first detected gravitational wave (GW150914 in September 2015) came from such a merger and confirmed Einstein's prediction of gravitational waves a century after he made it.
Are new black holes still forming today?
Yes, constantly. Stars massive enough to become black holes still exist and still die. Every time one runs out of fuel and goes supernova in the right way, a new stellar-mass black hole forms. Some recent supernova observations have caught the process in progress. Astronomers estimate that the Milky Way alone forms a new black hole every few decades through this process. Across the universe, with billions of galaxies, new black holes are forming somewhere essentially all the time.
Most black holes form when massive stars collapse at the end of their lives, producing stellar-mass black holes in a supernova explosion. Supermassive black holes form through more complex processes that scientists are still working out, possibly involving direct collapse of huge gas clouds or rapid growth from smaller seeds. New black holes form throughout the universe regularly, as long as there are massive stars dying.
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