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Why are there small and large black holes? Also, why are some black holes invisible while others have white outlines? – Sedra and Humaid, Abu Dhabi, United Arab Emirates
Black holes are dense astronomical objects whose gravity is so strong that nothing, not even light, can escape. Everything that exceeds the limit of the gravitational effect of the black hole, called the event horizon, will fall into the black hole. Inside this deep, dense pit, it will never be seen again.
Black holes pollute the universe. Some small black holes are randomly sprinkled throughout galaxies such as the Milky Way. Other massive black holes, called “supermassive” black holes, are located at the centers of galaxies. These can weigh between a million and a billion times the mass of our Sun. So you may be wondering: How can astronomers see something so dark and so big?
I am an astronomer studying the first supermassive black holes to form in our universe. I want to understand how black holes form and what kind of astrophysical neighborhoods they grow in.
Types of black holes
Let’s talk about how black holes start their lives. The two famous scientists who first put forward the idea of a black hole were Albert Einstein and Karl Schwarzchild. They thought that when a massive star died, its core might shrink and shrink until it collapsed under its own weight. What we astronomers call a “stellar-mass black hole” is just another way of saying it’s relatively very small.
Stellar-mass black holes are only a few times larger than our Sun. But supermassive black holes are more mysterious. They are millions of times more massive than our sun and are packed into a small space about the size of our solar system. While some scientists think that supermassive black holes may form when many stars collide and collapse at the same time, others think they may have started growing several billion years ago.
Growing black holes
What do black holes look like? Most of the time they are not actively growing, so they are not visible. But we can tell they are there because the stars can still revolve around them, just like the Earth revolves around the Sun.
When something orbits an invisible object at high speeds, scientists know there must be a massive black hole in the middle. This also applies to our nearest supermassive black hole, located safely millions of kilometers away from you at the center of the Milky Way.
Meanwhile, when a hungry black hole consumes gas in a galaxy, it heats that gas until you see a glowing ring of X-rays, optical light, and infrared light around the black hole. When it uses up all the fuel near the event horizon, the light fades out again and becomes invisible.
Outlines around black holes
One of the most famous “white outlines” is the image of a black hole in the movie “Interstellar”. In that movie, they were trying to show a ring of white-hot, glowing gas falling into an actively growing black hole.
We cannot get such a close view in real life. The best view of the ring around a real black hole comes from the Event Horizon Telescope, which showed scientists the supermassive black hole at the center of a galaxy called M87. It may look blurry, but this donut is actually the sharpest image ever taken of something this far away.
There are many types of black holes in the universe. Some are small and invisible, while others grow to gigantic sizes by eating matter in the galaxy and glowing. But don’t worry, black holes can’t keep sucking in everything in the universe; Eventually there will be nothing close enough to fall into the black hole and the black hole will become invisible again. So you can keep asking questions about black holes.
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This article is republished from The Conversation, an independent, nonprofit news organization providing facts and analysis to help you understand our complex world.
Written by: Jaclyn Champagne, University of Arizona.
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Jaclyn Champagne receives funding from the National Science Foundation and the Space Telescope Science Institute.