Q&A: Black Holes

Q:
Does our galaxy contain a black hole and, if so, what do scientists know about it? Why does this phenomenon exist?

A:
Our Milky Way galaxy contains a supermassive black hole at its center. Astronomers estimate that it has a mass of a few million times the mass of the Sun. There are also many black holes in our galaxy that are much smaller, with masses a few times that of the Sun. You get a black hole whenever you have so much mass concentrated in one place that no object, not even light, can escape the incredibly strong gravity of this dense object. The smaller black holes form when massive stars run out of fuel and collapse down onto themselves.

Astronomers have now detected planets around more than 100 Sun-like stars, but these are planets that are mostly much bigger than the Earth. They are currently unable to detect solar systems like our own, but it's likely that many of these exist in our galaxy, besides our own (possibly many millions).