I’m not talking about something in our solar system , im talking about astronomers claiming that a star billions of light years away is made only out of hydrogen and helium. How do they know that? And I’ve seen some other papers about planet temperatures and other things how do they know that too?
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*Spectroscopy* and *quantum physics*
Quantum physics comes from the same root word as “quantity” and “quantize”, the point being that at really small scales, you start getting discrete amounts of stuff. In particular, electrons in atoms can only have certain amounts of energy. If an electron is hit by a photon with *just the right* amount of energy to move it to a different energy level, it can grab that photon and its energy. If an electron already has a lot of energy, it can drop to a lower energy level by releasing a photon. Every single element does this, and the specific wavelengths of light that get absorbed or emitted are unique to each element (since wavelength is related to energy).
*Spectroscopy* means “splitting light up into its component wavelengths”. Think about e.g. the cover art for the Dark Side of the Moon, that’s basically what astronomers do. The look at something dark and see which wavelengths are present, or they look at something bright and see which wavelengths are missing. That tells them exactly what elements are there.
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