Stellar atmospheres and their dietary requirements

Declaring that I’m an astronomer at social events never fails to cause a sensation.

Sat next to a starry-eyed historian at a College dinner last night, he asked: “how do we learn about stars?” as he picked through the salad vegetables on his plate.

Starlight which we observe using telescopes, I said, encodes a wealth of information about the star’s temperature, gravity, and chemical makeup. Telescopes collect parcels of light, called photons, like a bucket collects rain. In1610, Galileo used a simple tube with lenses which he called a spyglass to observe the sky and collect more light than his eyes could. Telescopes have developed a lot since then. Now they can be huge, costing up to hundreds of millions, even billions of dollars. The bigger the telescope, the more light it collects, thus the fainter the celestial objects it can detect such as planets, stars, and galaxies.

Starlight is made of photons with a spectrum of colours and wavelengths. An instrument attached to the telescope can sort incoming starlight by wavelength into a spectrum that we can study to decipher the information it encodes, using what is called spectroscopic analysis. Historically, the most prominent scientists in this field were women. This is because in the 1800s it was considered “inappropriate” for a woman to be using a telescope, so female astronomers were delegated indoor positions, like laboratories. Annie J. Cannon, alongside other female astronomers, spent almost an entire lifetime accurately analysing hundreds of thousands of stellar spectra at the observatory in Harvard. She manually classified a record of more than 200,000 stars.

How is a spectrum produced?

The star is encased in an atmosphere rich in elements that were either present when the star was born or formed later in its core and then surfaced. When starlight penetrates this atmosphere, the intensity of some of its components drops, and dark bands form in the spectrum.

Pointing to the plate of my historian friend, I said “So you have picked only the tomatoes from your salad”. “Yes I don’t eat leafy greens but I like tomatoes”, he replied.

I said “well then, next time if I have my back to the person dining next to me and I glance over and I see a plate with tomatoes missing and green leaves all piled up untouched, I suppose I would guess it’s highly likely that it’s you sitting next to me. Wouldn’t you agree?” He nodded expectantly as he munched the last tomato.

Similarly, the gas in the star’s atmosphere is relatively cool, thus hungry for light. It is ready to snatch a photon when it encounters one. It has a very particular taste, too. Hydrogen or sodium in the atmosphere, for example, would pick out specific photons from the spectrum. Hydrogen has a taste for photons in the red part of the spectrum, while sodium fancies the yellow type. Because we know the elements’ tastes from laboratory experiments, just like me knowing you fancy tomatoes, we can identify what’s in the stellar atmosphere simply from the missing photons in its spectrum, or its dark bands.

The chemical forensic evidence

This taste is as unique to each element as a fingerprint, there is no way we can get mixed up. Each element in the star’s atmosphere has atoms composed of nuclei orbited by electrons. An electron is like a highwayman. When it encounters a traveling photon with the exact energy it needs to promote itself to higher energy, it steals the photon and, understandably, gets excited. Thus the photon of that particular energy goes missing. This causes a dip in the light intensity or a dark band in the spectrum. The darker the band at a particular wavelength, the larger the concentration of the element “stealing it”. Thus this forensic evidence not only identifies the element but also its quantity.

The missing culprit

Now sometimes a problem arises. We observe a certain star and find things we do not expect.  For example, we observe a star that we know is not mature enough to have possibly made any barium. But we notice that the photons that barium likes are missing in large amounts from its spectrum! Where did the barium come from then? We suspect that it may be a contamination case, that the star has acquired it from a nearby mature star that produced it. This, in other words, indicates the presence of a hidden companion sharing its matter with the star we are studying.

At other times we find puzzling amounts of nitrogen for example. This makes us revisit the theory our models are based on and refine it for higher accuracy. Often we realise that we had oversimplified matters. For instance, we find that the fact that stars spin complicates the picture in ways we didn’t anticipate. This comparison between our model predictions and observations is a powerful tool to improve our understanding of how stars evolve. Still in some cases we would observe a chemical enrichment or depletion that we cannot yet explain. We call those “peculiar stars”.

“You are interested in the past human history”, I told my friend, “and I am interested in the past star history. By studying the chemical makeup of different populations of stars in the Milky Way, we construct the history of star formation across the entire Galaxy.”

My tale rested just as cheese and fruit were being served, with port of course, which is never to be put down during formal Cambridge college dinners, but passed along just like the hard-earned knowledge that we build up on every day.

“But wait, why only mature stars, as you called them, can make certain elements like barium?” he was curious to know. “That, my friend, is a tale for another long dinner”.