# How do we measure distances across the universe?

22 February 2009

## Question

How do we measure distances across the universe?

Chris Smith shed some light on this question...

Chris - This is a very difficult question to answer, or at least it was.

The problem is that, if you're looking at stars in the night sky, if a star is at a certain distance from you, its brightness can't really be used as a measure of how far away it is because a bigger star will be brighter to begin with; and because light gets dimmer the farther it is from you, a big star can be a lot farther away than a small star and yet they'll both appear exactly the same brightness. How do you solve that one?

This kept astronomers guessing for a very long time until about the turn of last century. A woman in contact with Hubble, after whom the Hubble Space Telescope is named, solved the problem. Her name was Henrietta Levitt and she was looking at star charts. She noticed that some stars appeared to get bigger and brighter and then dimmer and weaker. They did it with a regular period. These have now become known as the "stellar yardsticks". They're called Cepheid variables. They're stars that swell up and shrink down.

Because the period at which they do that varies with the size of the star, you therefore know, if you look at how often a star like that is blinking on and off, how big it is. Therefore you know how bright it is. Because light follows an inverse square law you can work backwards to work out how bright that star must be and therefore how far away it is.

Scientists now use these Cepheid variables when they look at a distant star structure they can use the period of any Cepheid variables that are there to work out how far away those particular entities are. That's a stellar yardstick and it was solved by a lady at Harvard a hundred years ago.