How many galaxies are there?

(Nasa.gov may be dark right now, along with all of its twitter accounts, but Astroquizzical is still going! Remember to submit your questions via the ask page, the sidebar, Facebook or twitter. Apologies in advance for broken links throughout the blog during the US government shutdown.)

To borrow a phrase from Carl Sagan - there are billions upon billions of galaxies out there.

The image above is the Hubble Ultra Deep Field, and I encourage you to look at the high resolution version and pan around. Every single bright pixel in that image is a galaxy (with the exceptions of the spiky ones - those are stars in our galaxy that got in the way). This image alone contains 15,500 galaxies, and it’s a tiny, tiny fraction of the night sky. It’s about a 1200 times smaller than the area blocked out by the tip of your little finger held at arm’s length.

The patch of sky imaged by the Ultra Deep Field was chosen because it was particularly dark. If you want to look back towards the dawn of the universe, at the most distant galaxies possible, you need to avoid looking in a region of the sky with very bright nearby objects in the sky. For us, our biggest problem is avoiding our own galaxy. Certain parts of the sky are completely unusable for studies of distant galaxies; the gas and dust in our own galaxy is simply too bright, and blocks out the light from more distant things. We also have to avoid other nearby galaxies for the same reason - we want to be looking at the blackest, emptiest part of the night sky we can find.

When looking at very dark parts of the sky, an extraordinary amount of time had to be dedicated to acquiring this image. The total amount of time Hubble spent staring at this patch of sky was one million seconds - if this had been observed in a single session, it would have lasted for a little over 11 and a half days.

Since the universe is roughly the same in every direction we can look, we expect the universe to roughly look like the Hubble Ultra-Deep Field in every direction, if we had the ability and the time to look in every direction in the sky. To start to get a handle on how many galaxies this is; a thousand of these images fit under your little fingertip. In any direction you care to face in the night sky, raise your smallest fingertip - you’ve just blocked out the light from over 18 million galaxies.

Have your own question? Something here unclear? Feel free to ask!