Stellar Black Holes
A stellar black hole is what is left behind when a giant star runs out of fuel and dies in a huge explosion. The star collapses in on itself so hard that gravity pulls everything (even light) into a tiny point. There is no shining surface, no flames, no edge you could touch. Just a region of space so dense that nothing can escape it.
Stellar black holes are the most common kind of black hole in the universe. Our own Milky Way galaxy probably contains about a hundred million of them, hidden in the dark.
- How heavy?5 to 100 times our SunA medium one weighs about as much as 10 Suns
- How are they made?A giant star diesOnly stars 20+ times bigger than the Sun become black holes
- How big is one?About the size of a cityA 10-Sun black hole is roughly 30 km across
- How many in our galaxy?approx. 100 millionMost are invisible because nothing is falling in
- Nearest one to Earth1,560 light-yearsCalled Gaia BH1, found in 2022
- First one ever seen mergeGW150914Two black holes crashed together in 2015
How big are stellar black holes?
Compared to other kinds of black hole. Bigger bar = heavier black hole.
Stellar black holes (purple bars) are very small compared to the giant supermassive black holes at the centres of galaxies (red bar). The biggest stellar black hole still weighs 40,000 times less than the one at the centre of our own Milky Way.
What is a stellar black hole?
A stellar black hole is a black hole made when a very heavy star dies. Most stars are not heavy enough to make one. Our own Sun, for example, will end its life as a small glowing object called a white dwarf, not a black hole. Only stars at least 20 times heavier than the Sun finish up as black holes.
The word "stellar" just means "of a star". So a stellar black hole is one that comes from a star. There are other kinds of black holes, like the giant supermassive ones at the centres of galaxies, but those are made in a different way.
How does a star turn into a black hole?
Every star is in a tug-of-war between two forces. Gravity is always pulling the star's gas inward, trying to crush it down. At the same time, the nuclear reactions in the centre of the star push outward, like the explosions inside a rocket engine. The two forces balance, and the star sits there shining for millions or billions of years.
When a giant star finally runs out of fuel, the outward push stops, but gravity keeps on pulling. The star collapses in a fraction of a second. The outer layers of the star are flung off in a huge explosion called a supernova. For a few weeks, that single dying star shines brighter than a whole galaxy. The very middle of the star, called the core, gets squashed so hard that it becomes a black hole.
How do we see something invisible?
You cannot actually see a black hole. No light comes out of one, so it looks like nothing at all against the dark of space. But we can spot them in two clever ways.
The first way is watching another star. A lot of stellar black holes have a companion star orbiting close by. The black hole steals gas off the companion. The gas spirals down towards the black hole, heating up to millions of degrees on the way, and gives off bright X-rays. We detect the X-rays with telescopes in space and know there must be a black hole there.
The second way is even cleverer. When two black holes crash into each other, they make ripples in space itself, called gravitational waves. We have a giant detector called LIGO that can pick up these ripples even though they are tiny by the time they reach Earth. LIGO's first detection in 2015 was two stellar black holes spinning together and merging.
Where are the nearest ones?
The closest known stellar black hole to Earth is called Gaia BH1. It is approx. 1,560 light-years away. A light-year is how far light travels in one year, which is approx. 9.5 trillion kilometres. So even the nearest black hole is very, very far away. There is absolutely no danger of one ever reaching Earth.
Deeper dive: the science of stellar collapse
Inside a stable star, the energy that pushes outwards comes from nuclear fusion. Hydrogen atoms fuse together to make helium, then helium fuses to make carbon, then carbon fuses to make oxygen, and so on up the periodic table. Each step releases energy. But once the star starts trying to fuse iron, the reaction stops releasing energy and starts soaking it up instead. Iron is the dead end of stellar fusion. As soon as a star's core becomes mostly iron, the outward push collapses almost instantly.
The result depends on the mass of the dying star. A star up to approx. 8 times the mass of the Sun ends its life as a slowly cooling white dwarf. Between 8 and 20 solar masses (a solar mass is the mass of our Sun), the collapse is stopped at the very last moment by neutron pressure, and a neutron star forms instead. Above 20 solar masses, nothing can resist the collapse, and a stellar black hole is the result.
There is a strange gap in the numbers, sometimes called the mass gap. Between roughly 2.5 and 5 solar masses, we have not reliably found either neutron stars or black holes. It may be that nature simply does not produce many objects in that range, or it may be that we have not detected them yet. A 2019 event called GW190814 may have been a black hole right inside this gap, but it could also have been the heaviest neutron star ever seen. We are still not certain.
The discovery of gravitational waves in 2015 has changed our understanding of stellar black holes. We have now spotted over 90 mergers between them, and many of the black holes we have detected are bigger than anyone expected (often 30, 50 or even 100 solar masses). Either heavy stars are producing bigger black holes than the old theories predicted, or some of these black holes are themselves the result of earlier mergers, building up bigger and bigger over time.
To learn what happens at the very edge of a black hole, read about the event horizon. To meet the giant ones at the centres of galaxies, see the page on supermassive black holes.