Curiosities: How many galaxies have humans discovered?

Aug. 3, 2009

“We don’t know,” says Ed Churchwell, professor of astronomy. “We know it’s a very large number.”

It’s in the hundreds of billions, Churchwell says. In contrast, there are but 4 billion stars in our own galaxy, the Milky Way — and the number will keep growing for some time before we run out of galaxies to count.

“To count them all, you have to be able to look far enough back in time or deep enough in space to see when galaxies were formed,” Churchwell says. “We haven’t reached that point yet. It’s not a well-determined number, but at some point we’re going to reach it.”

For the time being, those hundreds of billions in the tally are extrapolated from a picture taken by the Hubble Space Telescope in 2003 and 2004. Pointed at a single piece of space for several months — a spot covering less than one-tenth of one-millionth of the sky — Hubble returned an image of galaxies 13 billion light years away.

“You look at that and say, ‘How many galaxies can I see?’” Churchwell explains. “And that turns out to be a very large number.”

In fact, there are about 10,000 galaxies in the picture, called the Hubble Ultra Deep Field.

“Then you take that number of galaxies from that postage-stamp-sized piece of the sky and multiply it by the number of postage-stamp-sized pieces of sky,” Churchwell says. “And that turns out to be a much larger number.”

Download Hubble Ultra Deep Field images.