One of the oldest stars in the Universe spotted in our cosmic back yard

It may have been a second generation star, and it’s in a Milky Way satellite galaxy

June 3, 2024 Issue #729

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Astro Tidbit

A brief synopsis of some interesting astronomy/science news

Astronomers have found what may very well be a star that was born in the extremely early Universe, and in fact may have been in only the second generations of stars that ever existed! 

This is pretty cool. When the Universe was extremely young, before stars even formed, it was almost entirely hydrogen and helium (and a wee soupçon of lithium). That means the first stars that ever formed, sometimes called Population III stars for historical reasons, were made of only those two elements.

These stars were huge, hundreds of times the mass of the Sun, and lived very short lives. They exploded as supernovae, creating heavier elements like calcium and iron, and scattering them into space. The next generation of stars born were also mostly hydrogen and helium, but also “polluted” with some heavier elements. Many of those stars exploded, making more heavy elements, and this cycle continued and continues today.

Stars that are young have lots of heavy elements in them, and stars that are old lack them. This means we have a way of figuring out hold old stars are! These elements emit and absorb light at very specific wavelengths, so by breaking the light from stars into spectra (like a rainbow, but with thousands of individual colors) we can ID these elements, determine their abundances, and get an age for the star. 

The astronomers in this case targeted the Large Magellanic Cloud (or LMC), a galaxy that is one of the closest to the Milky Way, under 200,000 light-years from us [link to journal paper]. They first dug into the database from the ESA Gaia spacecraft, which measures the positions, motions, distances, and colors of over a billion stars in the sky. They identified several that were low in heavy elements, then took spectra of them using an instrument on the mammoth Gemini South Telescope.

A fuzzy galaxy with a faintly seen bluish spiral arm, dotted with red gas clouds and thousands and thousands of stars in a black background.

The Large Magellanic Cloud. Credit: CTIO/NOIRLab/NSF/AURA/SMASH/D. Nidever (Montana State University) Acknowledgment: Image processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani & Davide de Martin

 

Astronomers usually measure these abundances (which we call “metallicity” for silly (also historical) reasons but it’s the lingo) relative to the Sun, since we have a decent idea of how much of all these elements are in our own star. Most of the stars they looked at have a very low ratio of iron to hydrogen (measuring it relative to hydrogen makes it easy to standardize things, so we can compare stars to each other). But one, which they designated LMC-119, is incredibly anemic, with only 0.007% as much iron as the Sun has!

That’s far lower than even the other low-metallicity stars in their sample—it’s the lowest ever seen in a star in another galaxy—which makes the astronomers think this much be only a second-generation star. Had it been born much later it would have more iron in it, so it must be from the very earliest of all stars born, only barely polluted by the first generation of superstars born before it. 

There was another surprise, too. Another standard way of looking at abundances is to compare an element with iron. Why? Because this can tell you more about how the star formed; different environments make different kinds of stars, so they may not only be low in iron, but low in other elements compared to iron. And in LMC-119 the astronomers found it was low in carbon when measured this way compared to other low-metallicity stars in the Milky Way. This means the star formed in a very different environment than stars did here, and it also implies that the first generation of stars may have affected their surroundings differently in the LMC than they did in the Milky Way as well.

All in all this is pretty amazing. That first generation of stars has been hypothesized and modeled in a zillion ways, but we still haven’t been able to find one (the big bright ones exploded billions of years ago, and even ones less massive than the Sun would’ve turned into red giants and died long ago; only the very faintest and most difficult stars to find from back then might still be around). By studying the oldest stars we can, though, we can learn about these mysterious stars second-hand. Indirect knowledge is still knowledge, so the more stars like these we can find, the more we’ll learn about our own origins.

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I’m not saying my newsletter isn’t enough to keep you informed about everything in the Universe, but that is a mighty big beat, so you might want to read others. When I was at the Boston Museum of Science recently to give a talk about the solar eclipse, I met Talia Sepersky, an energetic and enthusiastic science communicator. She runs the BMoS newsletter Spaced Out, which gives a rundown of what’s what over your head. It’s great, and you should sign up for it.

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