Why are some black holes bigger than others? An astronomer explains how these celestial vacuums grow

By | December 18, 2023

Curious Kids is a series for children of all ages. If you have a question that you would like an expert to answer, please send it to Curiouskidsus@theconversation.com.


Why are there small and large black holes? And why are some black holes invisible and others have white outlines? – Sedra and Humaid, Abu Dhabi, United Arab Emirates


Black holes are compact astronomical objects with a gravity so strong that nothing, not even light, can escape. Anything that exceeds the limit of a black hole’s gravitational influence, the so-called event horizon, will fall into the black hole. In this deep, closed pit it can never be seen again.

Black holes pollute the universe. Some smaller black holes are randomly distributed throughout galaxies such as our Milky Way. Other giant black holes, called “supermassive” black holes, reside at the centers of galaxies. They can weigh between a million and a billion times the mass of our sun. So you might be wondering: How can astronomers see something so dark and so big?

I’m an astronomer who studies the very first supermassive black holes that formed in our universe. I want to understand how black holes form and what kind of astrophysical neighborhoods they grow up in.

Types of black holes

Let’s talk about how black holes start their lives. Two famous scientists, Albert Einstein and Karl Schwarzchild, first presented the idea of ​​a black hole. They thought that when a large star dies, its core might shrink and shrink until it collapses under its own weight. This is what we astronomers call a “stellar mass black hole,” which is just another way of saying that it is relatively very small.

Stellar-mass black holes are only a few times larger than our Sun. Supermassive black holes, however, are more of a mystery. They are many millions of times more massive than our sun, and they are packed into a small area about the size of our solar system. Some scientists think supermassive black holes may be formed by many stars colliding and collapsing at once, while others think they may have started growing several billion years ago.

Growing black holes

What do black holes look like? Usually they do not grow actively and are therefore invisible. But we can tell they are there because stars can still revolve around them, just like the Earth revolves around the sun.

When something orbits an invisible object at high speed, scientists know that there must be a huge black hole at its center. This is the case for the nearest supermassive black hole, which is located at the center of the Milky Way – safely millions of kilometers away from you.

Meanwhile, when a hungry black hole eats gas in a galaxy, it heats that gas until you can see a glowing ring of X-rays, optical light and infrared light around the black hole. Once all the fuel near the event horizon is exhausted, the light goes out again and becomes invisible.

Contours around black holes

One of the best-known ‘white contours’ is the image of a black hole from the film ‘Interstellar’. In that movie they tried to show the white hot glowing ring of gases falling into the actively growing black hole.

In real life we ​​don’t get such a close-up view. The best view of the ring around a real black hole comes from the Event Horizon Telescope, which shows scientists the supermassive black hole at the center of a galaxy called M87. It may seem blurry, but this donut is actually the sharpest photo ever taken of something so far away.

De allereerste afbeelding van een zwart gat werd in 2019 gemaakt door de Event Horizon Telescope. Je kunt het licht zien terwijl het rond de intense zwaartekracht van het zwarte gat in het centrum van een sterrenstelsel genaamd M87 buigt.  Het ziet er misschien wazig uit, maar dit is het equivalent van het kunnen lezen van een krant op een tafel in Parijs als je in New York staat.  <a href=Event Horizon telescope” data-src=”https://s.yimg.com/ny/api/res/1.2/R5.0UkImsesWz82Sf406zg–/YXBwaWQ9aGlnaGxhbmRlcjt3PTk2MDtoPTU1OQ–/https://media.zenfs.com/en/the_conversation_us_articles_815/29a715538 13b7ec910d8fc4a2059eac8″ />

There are many types of black holes in the universe. Some are small and invisible, and some grow to gigantic proportions by eating things in a galaxy and shining brightly. But don’t worry, black holes can’t just keep sucking in everything in the universe. Eventually, nothing will be close enough to the black hole to fall into it, and it will become invisible again. So you can safely continue asking questions about black holes.


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And since curiosity knows no age limit – adults, let us know what you’re wondering too. We won’t be able to answer all questions, but we will do our best.

This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.

It was 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.

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