Submitted 1 hour ago by hokuspokus@piefed.social to [deleted]
I’ve always wondered how astronomers can actually observe galaxies that are millions or even billions of light-years away. What exactly are we seeing when we look at them?
And when scientists say there are hundreds of billions, or possibly trillions, of galaxies in the observable universe, how do they know that if we obviously haven’t seen and counted every single one?
It turns out that when light travels through the mostly emptiness of space, it can travel amazingly far without being significantly diminished. So we see galaxies the same way we see stars - you can see some with the naked eye, in fact, but to see detail you need a telescope. They come in somewhat different configurations, but generally look like this:
We don’t know exactly how many there are, we have to estimate it, and the range of estimates is huge: between 100 billion and 200 billion!
How do we estimate it? By counting the galaxies in a teeny tiny area of the sky, and then multiplying by that fraction. There are an amazing number of galaxies that are viewable in an area of the sky the size of the head of a pin held at arm’s length. It looks like this;
By the way, the galaxy at the top of this comment is eighty million light years away. That means the light were seeing from it now left eighty million years ago; we’re seeing back in time.
have you seen the Hubble Deep Field image?
it’s a picture of a section of the sky “about the size of a tennis ball at a hundred meters” taken by pointing the telescope at a point that appeared completely dark over a period of 10 days. there are over 3000 galaxies in that image.
I remember the first time I saw that image. The dazzling scope of it. The feeling that Carl Sagan described about the Pale Blue Dot photograph is the closest thing I’ve seen to describing the incredible sense of smallness the Deep Field image gives me. I still get a shadow of it, especially with the JWST images, but that was the first time I really felt how huge the universe is. It’s… words fail me. It’s so big, and there’s so much stuff in it. It boggles the mind.
I can’t fucking wait for the Roman data to start rolling in. We’re going to find so many cool things we’ve never seen before.
Here’s a very zoomable image of the sky. If you zoom in, you can see galaxies. If you then zoom out a bit you get a sense of how many there are.
Just look at this picture: esahubble.org/images/heic0406a/
This is my go to when my problems start to feel too big :D
By pointing a telescope at them and looking. You can see some of the larger ones through a cheap home telescope for a couple hundred dollars.
crapwittyname@feddit.uk 27 minutes ago
We make the following assumptions:
1a. Therefore the universe should look the same from where we are as from almost anywhere else.
2a. Therefore we can accurately tell how far away a supernova is by its light signature.
Therefore, we can map out and count the galaxies in our local area, and, using assumption 1, we know that the number of galaxies in our volume of space is the same as in every volume of space. Multiply the one by the other and you’ve got your answer.