Question: how big do you think the galaxy is

Jen Gupta answered on 22 Jun 2011:

I assume by “the galaxy” you mean our galaxy as in the Milky Way. And I assume by “big” you mean how long/wide it is. I’ve written about the possible meanings of the term “big” in another question (/cobaltj11-zone/2011/06/what-is-the-biggest-galaxy) One of the things you’ll probably have figured out by talking to us through this is that being a scientist teaches you that you need to be specific when asking questions 😉

Well we think that the disk of the Milky Way is about 100,000 lightyears across. This means that if a light is shone at one end of the disk it will be 100,000 years before that light is seen at the other end! The disk is the bit that contains the spiral arms of the galaxy and most of the stuff that we can see. But then there’s also a halo of more diffuse (spread out) stuff that probably extends further out than the disk and we also think there’s an even bigger halo of dark matter surrounding our galaxy. I seem to remember that this dark matter halo could extend out 300,000 lightyears from the centre of the galaxy which would mean that it’s about 600,000 lightyears end to end!

One of the main reasons why we really struggle to understand the details of the Milky Way is because we’re inside it! When talking about the sizes of galaxies it’s also a bit hard because you have to define where a galaxy ends! And obviously there’s all this dark matter that we don’t really understand. So my question to you is – do you believe my answer?!