Dulwich Hill Bicycle Club

this is what i imagine Pat does when he's not trying to break his bike in half sprinting on the track.

http://vimeo.com/41434123

How do they know which stars are 'standard candles'?

All along I thought Parallax was the bad guy in the green latern

How do they know which stars are 'standard candles'?

As the video states, a commonly used "standard candle" is a Cepheid variable star. These stars pulsate regularly, and the relationship between the frequency of the pulsation and the luminoscity of these stars is generally well known. So you simply have to find a Cepheid variable star, observe its pulsation frequency, from which you can look up its known luminoscity, and from there, you calculate the distance by the loss of brightness at your observation point. As the inverse square law for brightness states, when the distance from a light source doubles, its brightness is reduced by a factor of four. Since the only unknown is the distance, you put all the other inputs into the equation, and voila: the distance is now known!

Another standard candle often used in astronomy is a Type 1a supernova, which is essentially a white dwarf star exploding. This happens when a white dwarf attracts matter, usually from an adjacent star. It grows until it reaches a critical size, when it can no longer support its own mass, and it blows up. These explosions are all very similar in size and brightness, wherever they occur in the universe, so they are great for calculating distance to the explosion. Not entirely common though...