Thursday, September 24, 2020
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The First Stars


Source: The Guardian
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500 million years after the Big Bang, the pristine mixture of hydrogen and helium that made up the universe back then collapsed into large balls of gas and started to give off heat and light. These were the first ever stars. They were very important, beginning the ionization of the free-floating gas in the universe, giving us the crisp, clear view of space we see today. They started the production of ‘metals’—elements heavier than helium, which were necessary for planets, and life, to evolve. These stars also could be the source of the supermassive black holes lurking in the center of every galaxy. Observing these stars would be instrumental in finding out about the beginning stages of the universe. The only problem? There don’t seem to be any of these stars left.

Scientists classify spectra of stars using ‘populations’ I, II, and III. Population I stars have .5% or higher metallicity, Population II stars have less than .5% metallicity, and Population III stars are the elusive first stars, with no trace of any metals in their spectra, being created from the completely pure hydrogen-helium mixture before any stars were created. The metallicity of the stars could play a very important part in why there are no more of them.

For a nebula to form stars, its gravity needs to overcome the thermal pressure of the cloud, so the cloud needs to cool down. In the modern universe, such a nebula would generally have a sprinkling of metals. These metals absorb heat and emit it as photons, greatly speeding up the cooling process. Once the cloud cools down enough, it starts a rapid compression. This compression is so intense that slightly denser pockets of gas start collapsing as well, fragmenting the cloud into many normal-sized stars. However, in the absence of metals, like it was 13 billion years ago, the gas clouds would not cool down as much, and the compression would be far more gradual, which means that the denser pockets in the clouds would not collapse, forming far fewer stars from the same amount of gas. Obviously, this created stars that were scales of magnitude larger than any stars we see today, and only would have lasted a few hundred thousand years—a single grain in the sands of time of the universe, before consuming all of their fuel and going supernova.

So is that it? There’s no hope of studying these amazing stars? Of course not! Light has a finite speed— around 300 million meters per second. This means that any photons emitted by distant objects will take a long time to reach this earth. This distance can be measured by light years, 6 trillion miles, or the space that light travels in a year. By aiming humanity’s most powerful telescopes at distant parts of the universe, billions of light years away, we are able to see galaxies from distant time periods, and several of the spectra from these galaxies have been very promising. In fact, the primary objective of the James Webb Space Telescope, a new telescope designed to replace Hubble, will be to find these Population III stars in ancient galaxies, and will be far better equipped for the job than Hubble, with powerful infrared sensors. .

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