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Q&A: Black Holes

Q:
Are there truly 100's of billions of suns locked up in just one black hole? Are there truly 300 million black holes (that number seems small)? How thinly spread would all this mass be if it were spread evenly throughout the universe? If a black hole is truly invisible does that mean that it really is not there at all, that it is temporarily somewhere else and only reemerges when some action takes place?

A:
The number of 100 billion suns locked up in a black hole is probably a little large. Let's assume that it is 3 billion. As you will soon see, it doesn't make much difference.

The mass of the Sun is 2 octillion metric tons (a metric ton is 1000 kilograms, and 1 octillion = 1,000,000,000,000,000,000,000, 000,000 or 1 followed by 27 zeros). If there are 300 billion black holes in the observable universe, each containing the mass of a 3 billion suns, their total mass would be 300 billion X 3 billion X 2 octillion metric tons = 18 billion billion octillion metric tons, or 1,800,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000 kilograms (or 18 followed by 47 zeros).

The observable universe is roughly 20 billion light years in diameter, so the volume of the observable universe is about 4000 octillion cubic light years (4 followed by 30 zeros). Dividing the mass of all the black holes by the volume of the observable universe, and taking into account that one light year is about ten trillion kilometers, you find that if all the matter in all the black holes in the observable universe were spread out uniformly, it would be 1/1,000,000,000,000,000,000,000,000,000,000 the density of air, or about a millionth as dense as interstellar space!

This shows just how big the universe is. The black hole is invisible, but it is definitely there, and makes its presence known by its gravitational force which causes stars and gas near it to swirl around the black hole at tremendous speeds.