10,000 Galaxies

In our last issue, we weighed the Milky Way. And that got us thinking about all the other galaxies out there. So today, let's tally them up, with help from the Hubble Space Telescope.

No telescope has ever captured more cosmic beauty than the Hubble--or produced an image more humbling than the Hubble Ultra Deep Field. From the ground, the spot of space that Hubble's scientists studied for the image seems empty and desolate. But when Hubble focused its sensitive eyes there for a million seconds, 10,000 galaxies came into view--some more than 13 billion light years away. It's the farthest (and furthest back in time) that human eyes have ever seen.

One image, one spot of sky, 10,000 galaxies. That's humbling enough. But if a single image can reveal 10,000 galaxies, just how many galaxies are there? How many "billions and billions" of stars light the night sky?

Sucking the Universe through a Straw

Hubble scientists say their mind-boggling picture is like a "core sample" of the universe, made of light instead of Arctic ice. Imagine, the scientists say, that you're looking through an 8-foot-long soda straw, where each galaxy you see is a different distance from the Earth.

Remember, too, that you're looking back in time, as light from a galaxy 13 billion light years away takes 13 billion years to get here. Those 10,000 galaxies are spread both through space and 13 billion years of cosmic history--all packed into a slice of sky just one-tenth of the diameter of the full moon.

So, how many 8-foot-long straws would Hubble have to suck the universe through to taste the entire sky? According to the experts, about 12.7 million. They say the plucky telescope would need about a million years of uninterrupted time to make the images, too.

Cosmic Math

If each of those 12.7 million straws sucked in another 10,000 galaxies, we'd have about 127 billion galactic neighbors. Some estimates count on only 100 billion galaxies showing up in the census. Others think we'd find five times that.

It's best not to even think about how many stars that is. On a dark night in the country, you can see a few thousand. Yet astronomers guess there are anywhere from 100 billion to 1 trillion stars in our Milky Way galaxy alone. Multiply the low number by 100 billion galaxies and you get at least 10 sextillion stars. That's 10,000,000,000,000,000,000,000.

The real number could be ten times that. Or a hundred times that. Some NASA experts put the figure at a "zillion." No one knows. Astronomers are used to working with scary numbers, but when it comes to counting the stars in the firmament, most say that counting all the grains of sand from all the deserts and beaches of the world would be easier.