Welcome to Three Alpha! Since last time: In the Solar System, the Escapade orbiters successfully launched to begin their journey to Mars; in the Galaxy, the Pleiades contains 20 times more stars than previously thought; and in the Universe, there’s a new record for the brightest black hole flare ever seen.

Meanwhile, in this edition of the newsletter we’re focusing on why spectroscopy is such a useful technique. Read on for more…

One fantastically useful technique

Have you heard of 4MOST? That’s the 4-metre Multi-Object Spectroscopic Telescope, which is actually three spectrographs attached to the VISTA telescope in Chile. 4MOST has a wide field of view and its spectrographs are fed by 2,400 fibre optic cables, which can be individually adjusted to let the telescope observe 2,400 targets simultaneously. Last month it achieved first light, which means it is nearly ready to start making observations. But what actually are spectroscopic observations? Why are they useful?

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Spectroscopy is when you spread out a beam of light (like with a prism) and study how bright it is at each wavelength. It doesn’t produce beautiful images that we can admire, but rather a spectrum of each target that tells us how bright the target is at each wavelength. It may not be obvious, but this is actually one of the most important techniques in astronomy. Allow me to explain…

Picture a neon sign. These gas-filled electrified tubes are not just kitschy relics of the 20th century, they also showcase an important way that gasses interact with light. “Neon” sign is actually a misnomer, with several noble gasses being used to produce different colours (neon for red/orange, argon for lavender, helium for orange/pink, and so on). The gasses can be combined to produce a wider range of colours, but each gas emits specific colours when an electric current excites it.

It is not only these gasses which emit specific colours (i.e. wavelengths), of light when their atoms are excited. Every atom and molecule emits photons at specific wavelengths, as a result of the different gaps between their energy levels. That’s a quirk of quantum physics: each particle can possess only specific amounts of energy, and their energy levels jump up and down between levels. Often this change in energy level happens when a photon is emitted or absorbed, and the wavelength of that photon always matches the difference between energy levels.

The gasses used in neon signs happen to have energy levels that correspond to nice colours in the visible part of the spectrum. The gasses therefore emit light at those wavelengths, and they can also absorb light at the same wavelengths. Crucially, they can only emit or absorb light at those wavelengths and not at others. Usefully, the set of wavelengths which can be emitted or absorbed is unique for each different chemical.

When we make spectroscopic observations, we see narrow, dark lines in the spectrum caused by absorption (and, more rarely, bright lines caused by emission). We can read that like a barcode that tells you what the gas is made of. Point a spectrograph at an orange neon sign and it can tell you if it is neon or helium, or point it at a peach-coloured one and it will tell you it’s a mix of neon and argon.

This is how we know what the stars are made of. Helium is named after the Greek word for the Sun (helios) because it was discovered for the first time in the spectrum of the Sun. It’s also how we know which chemicals are in nebulae, what the composition is of the atmospheres of exoplanets, or the ingredients in a comet’s tail.

We can analyse the pattern of absorption/emission lines to learn many other things, because of the way those lines are affected by the various physical processes at work in the gas they’ve passed through. The most important of these is the Doppler effect, which you’ve probably heard of because of the way it alters sound from moving sources. The sound waves get shifted to shorter wavelengths if the source is moving towards you, and longer wavelengths if it is moving away. The same happens with light. You may have heard astronomers talking about the redshift of distant galaxies. That’s more or less the same as a Doppler shift; redder if the source is moving away, bluer if it is moving closer.

The way we measure redshift and blueshift is by looking at the spectral lines produced by the source. The pattern of the lines is the same (the lines are all shifted together) and there are certain common elements which we almost always see showing up in the spectrum (hydrogen, for example). What wavelength those lines appear at will depend on how much the light has been red- or blue shifted by the source’s motion. By looking at the overall pattern of the spacing between the lines, we can identify them and so measure the amount that they’ve been shifted.

That lets us measure how quickly the source is moving toward us or away from us (allowing us to measure things like the motion of galaxies, or the orbits of binary stars and planets), but also lets us measure things like temperature and rotation, where some molecules are moving towards us and others away. This causes the spectral lines to be spread out, and in extreme cases (like matter spiralling into a black hole) can even cause the line to split in two so we see a double peak.

Another interesting effect, which also causes lines to split, is called Zeeman splitting, which is where a strong magnetic field can cause the spectral lines to split into multiple lines and show up at different wavelengths (due to the quantum effects that the magnetic field has on the atoms’ energy levels). Measuring the Zeeman splitting lets us measure magnetic fields.

Spectroscopy is fantastically useful in astronomy, but it is also expensive (in terms of both time and money) because most spectrographs can only observe one target at a time. So if you want spectra of all the stars in a patch of sky, you need to take many observations (whereas with images you can take one observation that contains everything in the field of view). That’s what makes facilities like 4MOST so exciting. Being able to observe thousands of targets at once means we will get spectroscopic observations of far more targets than we could before. Who knows what we could learn?

Finally

You’ll be relieved to hear the doors of the Extremely Large Telescope work as intended!

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].