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What is a Black Hole?

A black hole in space is an object that has so much material packed into a small region that not even light can escape its gravitational pull! Nothing can escape a black hole.

Cool fact: You would have to squash the mass of Earth into the size of a marble to create a black hole!

How are we able to observe a black hole if it’s so dark? A black hole pulls material from a nearby star towards it. This material forms a disk that rotates around the black hole, which astronomers can observe. See the artist’s rendering below. The material in the disk will eventually either fall into the black hole or be expelled away in powerful jets of high-energy radiation. 

Image credit: chandra.harvard.edu

The Most Famous Black Hole: Cygnus X-1

Straight overhead in the night sky, in the constellation of Cygnus the Swan, hovers the most famous black hole: Cygnus X-1. An intense stream of x-rays radiates from that spot in the sky. This black hole boasts around 15 times the mass of our Sun. It’s spinning 800 times per second! That’s close to the fastest that it is physically possible for it to spin!

Cygnus X-1 is called a normal “stellar-type black hole” which was formed by the collapse of a star. Believe it or not, 15 times the mass of our Sun is on the small side of black holes. Turns out, black holes essentially come in two sizes, small and large. Sorry, we are completely out of medium.

A large-size hole is easy to find, too…

The Black Hole in the Center of our Galaxy

There’s a supermassive black hole that marks the exact center of our Milky Way Galaxy, which goes by the odd name of Sagittarius A* (and is pronounced Sagittarius “A star”).

• The supermassive black hole at the center of the Milky Way, Sagittarius A*, is 4.3 million solar masses.
• This is the only black hole whose mass has been measured directly by observing the full orbit of a circling star. Black holes grow by accreting surrounding matter and by merging with other black holes
• This black hole has the same mass as four million Suns. (So this is the large, economy-size model.)
• Every galaxy probably has a supermassive black hole at its core. Many of them are violent places that emit ultra-fast jets of material, lethal x-rays, and other unpleasantness. Ours does not. Sagittarius A star is not presently gobbling up star stuff. Nothing is falling into its event horizon. 

Spotting a Black Hole

Sagittarius A* will be particularly easy to spot the night of Sunday, October 14 and Monday, October 15.

On the 14th, you’ll see the Moon hovering right next to a bright star which is actually the planet Saturn. If it’s cloudy, look again the 15th when the Moon will be left of Saturn but they’ll both still be in Sagittarius.

Just to the lower right of the Moon and Saturn, both nights, is the supermassive black hole Sagittarius A*, marking the exact center of our Milky Way galaxy!  

We know that this supermassive black hole is there because of the way the stars orbit the hole, which has a mass of 4 million times the mass of the Sun. 

Check out this time-lapse video by the European Southern Observatory which includes 16 years of stars sped up by a factor of 32 million. Watch them dance around a mysterious blank center.

What Would Happen If You Fell Into a Black Hole?

It depends. With small black hole Cygnus X-1 straight overhead and supermassive Sagittarius A star low in the southwest these nights, you might wonder which is more dangerous?

You’d think the heavier one would pose the bigger problem. But it’s quite the opposite. Deadly tidal forces would actually be gentler if you approached the heavier black hole.

• As you approached smaller Cygnus X-1, being pulled by gravity, your head would be pulled towards the black hole first. Then your body would be stretched like a strand of spaghetti. For small black holes, this stretching is so strong that your body is completely torn apart into pieces of dog food before you reached the black hole itself.
• If you fall into a supermassive black hole such as Sagittarrius A *, your body remains intact, even as you cross into the black hole. But then you reach the center and you get squashed into a single point of super-high density. You have become one with the black hole.

No matter. You won’t be falling into any black holes. But at least you can tell your friends where to find it! Don’t jump!

Read more about black holes in my short article, ”Where in Space Are We Headed?”