# How can you measure the temperature of a distant star?

24 October 2011

## Question

How can you measure the temperature of a distant star?

Dominic - First of all, to measure the temperature of a star, it's really about the colour of the star. If you take a piece of coal and set fire to it, as it gets hotter, it will start to glow red hot and then perhaps if it's very hot, it will start to glow yellow hot and then an intense fire will start to appear white or maybe even blue. So, by applying that logic back to a star, a blue star is a hot star and a red star is a cooler star. Chris - What about what's in the star? Dominic - Now what's in the star is more difficult, but different elements emit a very specific wavelength and produce what we called spectral lines. If you have a spectroscope that splits up the light of that star into different colours, you can see that certain colours are either not present or there's a huge amount of light at that particular wavelength, and that tells you there must atoms of a particular element emitting light in that star so you can tell what that star must be made of.

Chris:: Terrific, Dominic. In fact, Robert Bunsen was the forefather of the science of spectroscopy, so Bunsen of burner fame also discovered that you could work out what things were by looking at the certain absorption pattern they make, and what wavelengths of light they soak up. And ironically, after working for many, many years in his laboratory on identifying some rare compounds, he unfortunately left a big sheaf of notes with 10 years work in the sun in his lab, went off to the pub - well I don't know he went off to the pub, but I suspect he went out for lunch - came back and the sun set fire to it. He lost a lot and had to start all over again. But because a very good scientist and he did the whole lot again from scratch.