What exactly does intrinsic brightness mean in technical terms?

Question

I am somewhat confused on how this was used to determine that the universe is expanding at an increasing rate.

Answer 1

Imagine that you are at a standard distance from the Sun.  Maybe it's a light year.  How bright is it?  Now imagine that you do the same thing for Vega.  Is Vega (from 1 light year) brighter or dimmer than the Sun (from 1 light year).  If you know the distance to a star, and you know how bright it seems to be, you can compute it's intrinsic brightness.  That allows you to compare one star to another.

You can also compare stars to each other by the study of the spectra of the light they produced.  And, you can tell if one star is very much like another.  So if you have a nearby star, whose intrinsic brightness you know, and another star, that matches it, you can work out how far away it is.  You know it's intrinsic brightness, and you know how bright it appears to be.

For extremely distant stars, in galaxies billions of light years away, you need a star that is bright enough so you can see it.  That would be an exploding star.  Such stars can be a billion or ten billion times brighter than normal.  Their light can be separated from other stars in their host galaxy.  Now certain exploding stars, type 1a supernova, have known intrinsic brightness.  So if you measure how bright it appears to be, you can work out how far away it is.  You can also look at it's red shift - an artifact of the expansion of the Universe similar to the Doppler shift, you can find out how fast it was moving away from us at the time.  Now, the speed of light is finite. So the farther away a star is, the farther back in time you are looking.  So you can figure out the expansion speed at various points back in time.

I'm leaving out a few details, but that's it in a nutshell.

I highly recommend Alex Filippenko's What's new in Astronomy, 2003 video series from The Teaching Company.  Opps.  I don't see it there.  Well, he's great, so perhaps his new course: Understanding the Universe will do it for you.

He also goes over this in an October 2006 public lecture that is available as an audio podcast.  He does refer to slides, and i've seen them in his video series, but i think you'll get the idea from the audio alone.  He's quite descriptive.  And there are some great questions he answers at the end.