Q&A: Black Holes
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?
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
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.