Science, Maths & Technology

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An introduction to exoplanets

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# 1  Spectra: how we learned what stars are made of

Back in Week 3 you learned about a technique called spectroscopy. This is the way that astronomers use light to determine the presence of chemicals in the atmosphere of an object. In Week 3, you learned about doing this for a star – but it works just as well for planets.

Remember, this is what the spectrum of the Sun looks like:

Figure _unit8.1.1 Figure 1  Solar spectrum, in which the light has been spread out to a very high degree over many horizontal strips

When the light from the Sun is split up into its constituent colours, some of those colours are missing because gases in the outer regions of the Sun absorb those specific wavelengths of light, stopping them from making their way to us.

In Week 3, you learned about using the precise positions of the dark absorption lines in a star’s spectrum to measure the ‘wobble’ of a star with a planet in orbit. But long before astronomers searched for exoplanets, these lines were used to learn about what stars are made of. In the early 1800s, scientist Joseph Fraunhofer made very careful measurements of the positions of the black lines in the Sun’s spectrum. Later that century, Sir William Huggins matched measurements of dark lines in other stars with the known absorption features of substances that had been studied on Earth: modern astronomical spectroscopy was born. As you learned in Week 3, this was how helium was discovered: after all the lines had been matched up with the chemicals known on Earth, a few lines seen in the spectrum of the Sun and in the spectra of other stars remained. These lines were due to helium.

You studies different types of star in Week 4. These different types of star are distinguished from each other by their spectra. Stars of different temperatures have absorption lines due to different gases. Stars were first categorised in this way in the early twentieth century, with most of the work being undertaken by Annie Jump Cannon, one of the earliest recognised female astronomers.

Figure _unit8.1.2 Figure 2  Spectra of different star types

The dark absorption lines in the spectra in Figure 2 can be identified as belonging to various gases. As the stars get cooler, you can see that the number of lines changes, and also the overall colour of the star changes. Hot O stars are much bluer and have fewer lines, whereas the coolest M stars are redder and have lots of lines. In fact, a great deal of the light that M stars emit is so red that we can’t actually see it at all.