“Some galaxies seem to be actively forming new stars in them, and it turns out that the fuel from which stars form is actually gas—cold diffuse gas in the galaxy that's made up of molecules. Just like air is a gas that's made of molecules, there will be hydrogen, carbon and oxygen and other kinds of gas floating around inside a galaxy. If it gets dense enough it can collapse and make stars,” explains Hickox.
“But then there are other galaxies that don't have any star formation at all. All we see in them are very old stars that were formed even billions of years ago. And so the big conundrum is, why does this happen?”
Listen to the full story, broadcast 3/27/15 by NHPR.