View Single Post
Old 01-24-2003, 01:16 PM
Grey Grey is offline
Join Date: Sep 2002
Location: Ottawa, Canada
Posts: 6,430
For distances we used parallax initially. Stellar positions shift very slightly depending where the earth is in its orbit. From the angular shift we use some trig and we arrive at a distance. Then we discovered that certain stars had intrinsic luminosity that periodically varied. These Cepheid variable stars could then be used as a yardstick. You ID 1 star with a specific brightness at a specific distance, from there the next star with the same period but fainter is assumed to have the same surface brightness and is only dim because its further away. You can figure that distance based solely on how dim it is compared to your initial star. Now we use supernovas in a similar manner.

But as for understanding their make up, we work from the assumption that what we see on earth is applicable across the universe. If hydrogen, when heated, emits a specific spectrum here in the lab, then hydrogen 2 million light years away must emit the same spectrum. Now when you go and look, the spectrum is the same but shifted slightly to the red end of the spectrum. Why? Well turns about that cosmic expansion acts shifts light arriving at earth by a measure proportional to the distance it has traveled. And were back to how we estimate the initial distance.

Kind of rambling, sorry about that. Hope it helps.