How do we know what stars are made of?

How do you figure out what the chemicals inside a star are without being able to physically test them? - Question posed by Yeswin.

Even the nearest star to Earth (other than the Sun) is a bit more than four light years away. That's a very, very long way away as far as we're concerned, but peanuts to the rest of space. Actually getting to stars and finding out what they're made of is so difficult and time consuming a task as to be impossible with our current technology. So how do we know what they're made of?

The answer is in a technique called 'spectroscopy'. This uses the fact that everything we can see either gives out light or reflects it. An instrument called a 'spectrometer' is pointed at a distant star, galaxy, cloud of dust, planet, or whatever else a scientist might want to look at, and collects a snapshot of the light coming from it. The spectrometer spreads the light out a bit like a prism would so that we see a rainbow of colours rather than just white light.

Now, here's the clever bit: atoms of each of the different elements that make up a star (or anything else we can see) give off, reflect or absorb light at a different wavelength, which changes the colour that can be seen. This means that when the spectrometer spreads out the light from a distant star, the rainbow isn't complete; there are patches of colour and patches of darkness mixed up together. Because each element has its own unique rainbow, called an 'emission spectrum', we can tell what the star is made of based on what we can see in the rainbow, and what's missing.

For example, if we point a spectrometer at something and get this emission spectrum...
By Merikanto [Public domain], via Wikimedia Commons
... we know it's made of hydrogen. If, however, we get the following rainbow...
... we know it's composed of iron.

In reality, most things we look at are made up of more than one type of atom, so we get an emission spectrum that is a mixture of rainbows from different elements. We then have to undo the jigsaw to find out which elements would cause the individual spectrum that we see.

Just in case you're interested, the chart below shows some real emission spectra from different types of star. The letters and numbers down the left hand side describe the different types; the names of the stars used are listed down the right hand side.
By Robert Nemiroff (MTU) & Jerry Bonnell (USRA) (NASA), via Wikimedia Commons

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