Raj Jeganathan asked:
Hi Dr. Chris
Your Podcast, naked astronomy, ask the naked scientist are all very interesting and excellent for science hungry people like us.
Great companion during my daily walk.
My question is on astronomy and very simple. When you look at distant object like stars, the brightness will be controlled by two factors one the luminosity, and the other is the distance. So how can you determine the distance?
Dr. Raj Jeganathan
We posed this question to Author, Physicist and Campaigner, Simon Singh...
The way you do this is by using something called parallax. Now that means that you measure the angle to a star using a telescope and then you move your telescope to a different position and you look for a shift in the angle to the star. The problem is, the stars are so very, very far away, you need to move your telescope a long way in order to get a perceptible shift in angle. A few meters, a few kilometres, a few hundred kilometres just isnít enough. It wasnít until the 19th century that an astronomer, Friedrich Bessel, moved his telescope to the other side of the Sun. He took a measurement in July from the Earth and then he waited six months for the Earth to go right around the other side of the Sun, he took another measurement. Even though the Earth and his telescope had moved such a vast distance, the shifting angles of the star was just 1/6000th of a degree, a tiny shift, but that tiny shift was enough for him to work out the distance to the local stars around us. Now to give you an idea how far away those stars are, it takes about 8 minutes for light from the Sun to reach us on Earth. It takes over 4 years for light from our closest star to reach us. That's how you measure the distances to the stars.
The distance to 'close' stars can be measured by the paralax method versus the very far distant background stars. Put simply, measure the angle of the star to a baseline in, say, April. Wait 6 months until the Earth has travelled 93,000,000 x2 miles and is now on the other side of the sun and measure the angle again. Use triganometry and that is the distance. That's OK for quite a few light years. After that we need to use the cephid technique, there pulsating stars whose brighness is related to the frequency of the pulsations. Some assumtion has to made about intervening dust absorbing the light. Then after that there are the type 1a supernovas that blow up with about the same brightness as each other. Then for the very far distant, we can use doppler shift. Astrogazer, Thu, 2nd Dec 2010
Very nice explanation. yor_on, Fri, 3rd Dec 2010
There are other factors that allow the distance to most stars to be estimated quite accurately. Most stars are members of what is known as the "main sequence" which means their size and luminosity is determined by their colour and spectrum. even most stars that are for various reasons off this sequence can be identified by their spectrum. Soul Surfer, Sat, 4th Dec 2010
Unfortunately, Astrogazer, the cephids you mention are more variable than we thought, meaning many estimates of stellar distance may be innaccurate. This was mentioned in a recent Jodrell Bank Jodcast. rhade, Wed, 16th Mar 2011
Can't you just use a tape measure ? sheesh ! neilep, Tue, 22nd Mar 2011