Have we found indirect evidence of the very first stars to shine in the Universe?
These first stars were whoppers, and when they blew up they did so… differently.
September 19, 2023 Issue #619
D’oh! In yesterday’s issue, BAN #618, when writing about the shadows on the Moon cast by Chandrayaan-3 and craters, I said the Sun was shining from the right, but it’s shining from the left to the right. I had a mild brain cloud there when I wrote that! That’s a frustrating mistake, since it may have confused people. My apologies, and mea culpa.
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It’s a big Universe. Here’s a thing about it.
Once upon a time, the very first stars shone in the Universe.
They were very different than stars today. Back then, the cosmos was so young that essentially only the lightest two elements — hydrogen and helium, created just minutes after the Big Bang — made up very nearly the entire budget of normal matter. Elements like iron, calcium, even oxygen, are created in the nuclear furnaces of stellar cores, but since these were the very first stars, those heavier elements didn’t yet exist.
This lack of heavy elements meant the stars could be huge — heavy elements absorb energy inside the star, making it hotter, and if they get hot enough they tear themselves apart. This puts an upper limit on how massive stars can be today, and the first stars, being made of simpler ingredients, would have been far, far more massive than stars today. It’s not clear what the exact upper limit was, but it was certainly many hundreds of times the Sun’s mass.
These huge beasts probably started forming about 100 million years after the Universe itself came into being. This is according to theory, at least; no one has any direct observations of these stars despite them being incredibly luminous. Because they existed and died out so long ago — well over 13 billion years in the past — we have to look to the furthest reaches of what can observe to see them. Light travels quickly, but at a finite speed, so the farther out we look the farther back in the past we see. But 13+ billion light-years is a very long way indeed, making it difficult to see them directly.
But, an idea goes, we might be able to see them indirectly. They would’ve had a profound effect on the environment around them and on the stars that formed after. And that’s just what a team of astronomers think they’ve found [link to paper].
This first generation of stars ended their lives as supernovae, exploding and scattering the heavy elements made in their cores out into space, where the next generation of stars could incorporate them into their own bodies as they formed out of enormous gas clouds. These explosions usually happen when iron builds up in a star’s core, which, via a complicated process, makes the star unstable and leads to supernova.
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But that’s not always the case. Stars generate energy by fusing lighter elements into heavier ones, like the Sun does by fusing hydrogen into helium. The kind of fusion that takes places depends on the pressure and temperature in the star’s core, and that depends on its mass; the higher the mass the more gravity squeezes the core, heating it up, allowing the star to fuse heavier elements.
Stars that have more than about 20 times the mass of the Sun can fuse elements all the way up to iron. But stars that are very massive — currently more than about 50 times the Sun’s mass, but back in that first generation of stars, due to their different chemical composition, it was more like ~140 – 260 solar masses — there’s a monkey in the wrench. Temperatures in the core get a lot higher, and when one of these monsters starts to fuse neon into oxygen it runs into severe trouble. The details are complex, but the fusion makes a lot of super-high-energy gamma rays, the most energetic form of light. These gamma rays have a lot of energy that helps hold the star up against its fierce gravity.
But these gamma rays also can convert into matter, creating matter-antimatters pairs of electrons and positrons. This steals energy from the core, causing it to collapse. This in turn creates a runaway reaction that essentially detonates the star. It explodes, in what’s called a pair-instability supernova.
If you want more details on this process I wrote a lot more about them in an article on The Old Blog™. I have a second article about them there as well.
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