A Platypus Star

The composition of the mysterious star announced last week. (Figure from Astronomy PIcture of the Day; click for more information.)

Gather ’round, children, and I’ll tell you a story of how you came to be.

Well, a short version of the story. One without any biology in it at all, or mommies and daddies, or bad synthesizer-based music. But it’s still part of the story of how you came to be. Actually, you’re not going to come into it much at all. Never mind. It’s really a story about stars, and chemical elements, and things that look like they shouldn’t be.

The Big Bang was hot, and during the first three minutes was hot and dense enough for nuclear fusion to occur. However, to make atoms, there had to be the raw ingredients first: protons and electrons. (Neutrons too, though you can make those out of protons and electrons, if you have enough spare energy lying around.) So in the very early universe, there was a tiny window where the entire cosmos was hot enough and dense enough to produce atoms and fuse them together to make heavier atoms. This process has the fancy name of Big Bang nucleosynthesis (BBN): when the entire universe was like the core of a star.

The first three minutes sufficed to produce nearly all the hydrogen (the lightest element), helium (second-lightest), and lithium (third-lightest) the modern universe contains…but nearly nothing else. The rest of the elements come from stars. According to astronomers’ best models, the first stars — known as Population III stars — were huge: very massive, very bright, very hot, and as a result, very short-lived. The large size had to do with their environment and available raw materials: when they formed, the universe was still relatively hot and dense, and of course only hydrogen and helium were available for star formation. Without at least trace amounts of heavier elements such as carbon, it’s hard to condense clouds of gas small enough to make low-mass stars.

High-mass stars have correspondingly high rates of fusion, so they burn through their nuclear fuel quickly and go supernova, leaving behind black holes. The supernova explosions from the Population III provided the seeds of the next generation of stars, known as Population II stars. These now contain traces of heavier elements around all the heavier elements (perversely known as “metals” to astronomers), and have a wider range of masses. Many Population II stars are still around today, but the more massive ones also had short lives and went supernova, continuing the process of spreading heavier elements around. The third generation of stars, including our Sun, are known as Population I, and these have the highest concentration of heavier elements, although they are still mostly hydrogen and helium. In every generation of stars, supernovae of the most massive representatives are responsible for the spread of heavier elements such as the calcium in our bones, the oxygen we breathe, the nitrogen that makes up most of our atmosphere, and the iron that comprises most of the Earth’s interior. To quote Carl Sagan, we are star stuff.

Now we reach the mystery: because early Population III stars were predicted to be so massive and short-lived, astronomers didn’t expect any of them to stick around until today. Note I use the past tense; last week, spectral analysis of an image from the Sloan Digital Sky Survey (SDSS) turned up a star in the Milky Way whose profile fits the chemical composition of a Population III star, but which is far too small. Low-mass stars are very long-lived: the Milky Way and other galaxies are full of Population II and I stars, many of which formed a long time ago. (Our Sun is about 5 billion years old, but there are stars dating back much much much farther!) Analyzing the spectra of these stars tells astronomers what chemicals are in them, which in turn tells us something about how and when they formed.

The newly-discovered star is a platypus: a bit of this and that, glued together by history to make a mystery for us. The chemistry is closest to Population III: mostly hydrogen and helium, as with most stars, but no heavier elements. Yet it’s not a perfect fit there, either: there’s too little lithium, which is saying something, since there isn’t a lot of lithium around to begin with! (Read the links in the ESO article for more information on this; I’ll leave it to actual stellar astronomers to talk about that.) And of course it has too little mass, which would be more appropriate for a Population II or I star: it’s actually about 80% of our Sun’s mass, which decidedly puts it into the “low-mass” category. How such a low-mass star can form at any point in the universe’s history with no heavy elements is a big question.

Like the platypus, however, I’m confident the answer will be forthcoming. We know now that mammal ancestors split off from the reptiles a long time ago, and there were several branching points, one of which produced the ancestors of today’s monotremes (platypuses and echidnas). The newly-discovered star will help us understand star formation better, however we resolve the mystery, and bring us closer to the true story of our full history, from the Big Bang to modern stars to funny bipedal creatures who look at starlight and think about where they came from.

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