Question: what is the biggest galaxy?

Jen Gupta answered on 13 Jun 2011:

This is a really good question that might seem really simple to start off with but actually isn’t so I hope I explain this well enough – please comment if you need anything explaining further!

So, first of all you’ve got to be a bit careful when you say “biggest”. This could either mean the largest or the most massive. The largest galaxy would be the one that has the longest distance between two opposite points. The most massive would be the one that is the heaviest. This means that the galaxy that is the largest, might not necessarily be the most massive. I guess that you probably mean the largest galaxy.

Secondly, we’ve got to decide what we mean by a galaxy. You probably think of a galaxy as a collection of stars but it’s a bit more complicated than that. For a start, galaxies also contain dust and gas that can extend out further than the stars. When we look at galaxies, we also find evidence for dark matter. This is matter that we can’t see but that has an effect on the way that stars in the galaxy move around and can extend out much further than any part of the galaxy that we can see. So when we say the “largest” and the “most massive” galaxy, do we take this dark matter into account?

As an example, our own galaxy, the Milky Way, is thought to contain about 200 billion stars and the disk of stars is about 100,000 light years across. The nearest spiral galaxy to us, Andromeda or M31, contains about a trillion stars and its disk is about 220,000 light years across. This makes M31 larger than the Milky Way. However recent observations suggest that the Milky Way contains more mass (because of dark matter) than M31 so the Milky Way is more massive. I guess if you take the dark matter into consideration, the Milky Way would also be larger than M31.

So far I’ve been talking about spiral galaxies (and getting a bit side-tracked!) but the largest galaxies in the universe are actually elliptical galaxies. These galaxies are thought to form when smaller galaxies collide together. The largest of these galaxies are found in the centres of galaxy clusters. Galaxy clusters are made up of lots of galaxies and are the largest things in the universe that are held together by gravity, just like the Earth is bound to the Sun by gravity. This means that there are lots of galaxies close to the huge central elliptical galaxy so it kind of absorbs these galaxies and grows. The largest one that I think we’ve seen is at the centre of the cluster Abell 2029 and is called IC 1101. It has a diameter of about 5.5 million light years. I can’t find much out about the most massive galaxy known but I would assume that IC 1101 is also the most massive galaxy as well – it contains around 100 trillion stars!