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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When some material burns, it emits a light that has a “pattern” that can identify what elements that material is. This is a science called spectroscopy. By analyzing light from the stars, it is possible to figure out what elements that star is made of.
Stars are also classified by brightness, size etc. These form a fairly well defined pattern which allows scientists to estimate a range of distances a planet needs to be from that star in order for that planet to have a temperature similar to earth’s. This is not an exact prediction, of course, a lot of things have to be estimated.
Many things in astronomy work that way. By collecting a lot of observation from different stars, etc scientists build a model of how stars work and how the age over time. As they get learn how to build better telescopes and measuring equipment, they can measure things more precisely and refine their models of the cosmos.
As different elements burn or undergo fusion, they put out different combinations of light frequencies. Looking at these ‘spectral signatures’, will tell observers what elements are being burned.
It also goes the other way. If light passes through an atmosphere around a planet, the elements in that atmosphere will absorb certain combinations of frequencies. That can also be analyzed to see what elements are present.
Do a search on “emission spectrum” and “absorption spectrum” and you’ll find all sorts of information about this.
*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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