How do we calculate the light years between the galaxies?
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We know the universe is expanding, when a light source travels away from you the light it emits gets redshifted. We know a galaxy will emit certain frequencies of light therefore we can estimate the distance by seeing how red shifted the objects light is.
1) Generally, the further away stuff is from us, the more redshifted it is. Thus, if you can figure out how redshifted it is, you can make an educated guess as to how far it is from you.
2) There are certain light-emitting events that produce a very consistent and repeatable profile of light no matter where they happen in the universe. Thus, if you see (and can recognize) one of these events, you can almost immediately figure out how far away the event was from you.
There’s 3 different ways.
1. Paralax. Hold your thumb out in front of you and close one eye. Remember where your thumb is relative to the background. Now, without moving your thumb, close that eye and open the other. Your thumb should have appeared to move relative to the background. That’s because your thumb is closer than the background. Now scale this up to looking at the sky in January vs July (ignoring the fact that some stars are seasonal depending on where you are) we can do the same thing except instead of using the distance between your eyes as a baseline, we use the diameter of the Earth’s orbit.
At a certain point, things are too far for parallax to work, bring us to…
2. Cephied variable stars. Some stars vary in brightness, notable the namesake of this method, Delta Cephei. Well as it turns out, stars that vary like this (there are other variable stars, but we’re just focusing on Cephieds) vary very predictably based on their average brightness. By measuring their period, we can calculate their absolute luminosity (how bright it would look if it were 10 parsecs away) and then compare that to its observed luminosity (how bright it actually looks) as use that to calculate how are away it is. We figure this out by looking at how far away Cephied stars are that within range to use paralax.
At a certain point, we can’t make out individual stars anymore, so this method no longer works, so we move on to…
3. Redshift. When things move aways from us, the light they give off gets redshifted, the wavelength gets longer. Everything very far away from us is actually moving away from us due to the expansion of the universe, causing redshift, and the further away something is, the faster its moving, causing more redshift. Whenever we see something that we know what wavelength it’s supposed to be giving off, we can tell how fast it’s going by the amount it’s redshifted, and by extention, how far away it is. There is one thing we know the wavelength of very well, and that is a type 1a supernova (a white dwarf in a binary system takes material from the other star until hydrogen fusion reignites, which always happens at the same mass) so by looking at a distant galaxy for that very specific peak of light, and by seeing how much it was redshifted, we can tell how far away a distant galaxy is.
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