Richard, Australia asked:
How do we measure distances across the universe?
Chris - This is a very difficult question o 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 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 star 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, you know 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.
Richard Nirui asked the Naked Scientists: I am addicted to your show and absolutely love it. Thank you all for such a wonderful and unique programme. I have three questions: I)How can astronomers tell how far the light from a star had to travel to reach us? In other words how do they determine how far a star is from us? And how accurate is this measurement,a few light years or thousands of light years? II)where did the big bang took place relative to the present universe? Did it happen in the center of the universe as we know it, exploding matter and energy into the space and time symmetrically? III) Big bang happened some 13.7 billion years ago. The light and energy from the big bang should have been travelling faster than the matter that has given rise to stars, planets, etc. In other words the light from the big bang is moving away from us not towards us. Does that mean no matter how strong a telescope we ever make, we will never be able to see what happened in the beginning of the universe? What do you think? Richard Nirui, Sun, 27th Jul 2008
Just to add to answers to your question 1, the Wikipedia summary really only covers extra-galactic bodies. There are other methods that are used within our galaxy which are not mentioned. One such method is able to estimate the distances of pulsars to quite good accuracy, although does depend on an assumed knowledge about the density of interstellar material and how this changes within the galaxy. This is based on the knowledge that electromagnetic radiation is emitted from Pulsars at all wavelengths simultaneously (Bremstrahlung radiation) and then "disperses" in time as a result of the small but significant insterstellar medium which slows the light down (depending on wavelegth) to below its speed in a vacuum. The amount of dispersion (for example how long blue light arrives after red light, although this feature extends well into the radio spectrum) can be correlated well with distance. The whole method has been correlated quite well with other methods of distance measurement, which also helps to confirm assumptions about the galactic model. graham.d, Mon, 28th Jul 2008
Yes - using Cepheid Variable stars is also a very smart method, I think. Some stars oscillate in brightness (they are very large and can't decide whether to blow up or collapse and keep oscillating for most of their lives).
"I dont see any reasonable natural reasons why speeds is constant."