Topics Space and the universe

How do black holes form?

Most black holes form when a very massive star dies. Through its life, a star is held up by a balance: gravity pulls inward while the heat and pressure from nuclear fusion in its core push outward. When the fuel runs out, the push fails, the core collapses, and if enough mass is squeezed into a small enough space, nothing can escape its gravity, not even light. That boundary is the event horizon.

What makes this interesting is how much is well understood and how much is not. Black holes of a few times the Sun's mass, called stellar-mass black holes, fit this story well. Supermassive black holes, millions or billions of times the Sun's mass at the centers of galaxies, are harder. Scientists are still debating how they grew so large so early in the universe, and there are several competing ideas.

An episode on bre, whose hosts are AI and can be wrong, would walk through the life and death of a massive star, what collapse actually means, and where the open questions begin. You can press Talk at any point and ask something like what happens if the Sun does this, and a host will answer from what the episode covers.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. A star is a balancing actGravity pulls in, fusion pushes out. For most of a star's life the two are matched, and that balance is what keeps it stable.
  2. When the fuel runs outMassive stars fuse heavier and heavier elements until they reach iron, which does not release energy by fusing. Then the core can no longer hold itself up.
  3. Collapse and the event horizonThe core falls inward and, if enough mass remains, forms a region where escape would require more than the speed of light. That edge is the event horizon.
  4. Why the Sun will not become oneOnly stars well above the Sun's mass end this way. Lighter stars end up as white dwarfs, and some heavier ones leave neutron stars instead.
  5. Black holes without a dying starTwo black holes can merge into a bigger one, and gravitational waves from such mergers have been detected. Other formation routes are proposed but less certain.
  6. The giants at galaxy centersSupermassive black holes exist, but how they grew so fast is still debated. The episode lays out the leading ideas without picking a winner.
  7. How we know they are thereWe cannot see a black hole directly, but we can watch orbiting stars, glowing gas, and ripples in spacetime, and in 2019 a telescope network imaged the shadow of one.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, let's start with a picture. A huge star, many times heavier than our Sun, sitting in space looking calm. Cal, what is actually holding it up?

  2. CalAI host

    Fire. Well, fusion. The core is crushing itself with its own weight, and the energy from fusing atoms pushes right back. It is like a crowd leaning on a door that is leaning back.

  3. breAI host

    And that standoff lasts for millions of years.

  4. CalAI host

    Right, and then the fuel runs low, and the door stops pushing. I keep thinking about how fast the end is compared to the whole life. Millions of years of balance, then the core gives way in a moment.

  5. breAI host

    Okay, here's the part nobody tells you: not every collapse makes a black hole. So what decides it?

  6. CalAI host

    Mostly how much mass is left in the core. Too little and it stops as something dense but stable. Enough, and nothing known can stop it.

  7. breAI host

    And scientists are sure about that cutoff?

  8. CalAI host

    Not to an exact number, honestly. The broad story is solid, the fine edges are still being worked out.

Questions people also ask

Can the Sun become a black hole?
No. The Sun does not have enough mass to collapse into one. In several billion years it is expected to swell into a red giant, then shed its outer layers and leave behind a white dwarf, a small, dense, slowly cooling core.
What is inside a black hole?
Nobody knows for certain. Our best theory of gravity predicts a point of extreme density at the center, but that theory is expected to break down there. Nothing inside can be observed from outside, so the question remains open.
How do supermassive black holes form?
This is still debated. They may grow from smaller black holes by swallowing gas and stars and by merging, or they may start from much heavier seeds, such as collapsing gas clouds. Observations of very early, very large ones keep the question lively.
Can we see a black hole?
Not directly, since light cannot escape. We detect them by the motion of nearby stars, hot glowing gas falling in, and gravitational waves from mergers. In 2019, the Event Horizon Telescope produced an image of the shadow of a black hole in a distant galaxy.

Related topics

More: all 300 topics, space and the universe, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.