How can you determine what elements makes up a star?
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answers:
Sanjay say: Absorption and Emission spectra
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Eric say: No one has ever been more than 8 or 9 miles down into the bowels of the Earth, yet they tell us that there's a core of iron and nickel where temperatures exceed 5,500K - nearly the temperature of the sun. If it's 5,500K, how is the core solid? How do they know how deep the crust goes? How do they know the depth of the mantle and the diameter of the core? How do they know what they're made of? Answer: they don't. They just make up numbers and theories and present them as facts. They pass off petrified wood as "moon rocks", they spend millions of dollars (out of a budget of billions), to tell us lies and give us photoshopped cartoon images. You think these people could possibly know what stars are made of? That's preposterous.
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Jeffrey K say: Use a spectrometer to break up its light into a spectrum. Different elements emit and absorb different wavelengths. This makes dark lines at certain places in the spectrum.
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Bill-M say: Spectroscopy.
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Clive say: Spectroscopy.
Split up the star's light into a rainbow, and look for the dark lines in it. Each different element absorbs light at very specific frequencies. Each one corresponds to an electron jumping between two of the "electron shells" in the atom, which are better called energy levels. Only light at the exact frequencies corresponding to these jumps is absorbed, so you get a rainbow spectrum with dark lines in it. (This is a consequence of quantum theory. You would expect that an atom can absorb or emit light at any frequency, but it turns out this isn't true, it can only do it at frequencies that match a jump between electron energy levels. So we get a pattern of lines.)
So now we have our spectrum of the star. We also know which lines each element makes because it's been done in laboratories - make a pure element give off light, heat it up maybe, and look at the spectrum of that. So we have a reference catalogue of what element makes what lines.
Now all you have to do is compare! Of course in the star's spectrum you've got all the lines from all the elements in the star all mixed up together, but you can identify which lines are made by which element and there's the answer. Just like fingerprinting.
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Ronald 7 say: By looking at its Spectrum
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Elaine M say: The color in the spectrum indicates what chemicals were burned.
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Raymond say: Long answer (spectroscopy)
An atom of hydrogen is very basic: one electron in orbit around one proton.
The orbit of the electron can change. Whichever one the electron is, at a particular moment, depends on the energy level that the electron carries. Call it "orbital energy".
(This is an analogy to help us understand -- reality is more complicated, but for simple problems, the analogy works well).
One thing that was found (on Earth, in labs) was that the energy levels come in fixed quantities. The lowest possible orbit is the same for all hydrogen atoms that are not heated. This level is called the "ground state". The electron has zero "excess energy". This level is also called orbit level 1.
If an electron is hit by a photon of light of EXACTLY the right amount of energy to bring it to level 2, it will absorb the energy of the photon and jump to orbital level number 2. It has to be exact (the electron does not give change). If the photon is just a bit too weak, the electron ignores it. If it is just a bit too strong, the electron ignores it...
unless the photon has enough energy to push it up to one of the higher levels (3, 4 and so on).
Thing is, for any specific jump, the photon must be of an EXACT energy level (which means, also, the exact wavelength corresponding to this amount of energy). If this atom of hydrogen is between you and the source of light (the star), it will absorb many of the photons of these exact wavelengths and they will be missing from the spectrum of the star.
If the hydrogen atom has absorbed energy, then the electrons (now in high orbiting levels) will eventually fall down to lower energy levels. In doing so, they will get rid of their excess energy by sending out a photon of the exact wavelength corresponding to the drop in energy.
This is true for all electrons in any element. Experiments done in labs (under strict controlled conditions) have allowed to determine many of the exact wavelengths that can affect each element (and some molecules).
By analyzing these wavelengths (for example, gaps in the spectrum of starlight), we can tell which elements caused which gaps.
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In addition, the lowest possible level for electron orbits is not always the ground state. The heat itself (the temperature of the atom itself) may keep the electrons energized so that it may be possible (for example) for electrons to drop from 5 to 4, or from 5 to 3, or from 4 to 3.... but never below 3 (because the atom is too hot for that).
Analyzing the gaps still allows us to identify the element but, as a bonus, we can even guess at an approximate temperature of the atoms just by looking at the spectrum (the surface temperature of the star is determined in a different manner, which is far more precise).
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Since the most common element in the universe is hydrogen, the frequencies of its transition energy levels are very well known. For example, the Lyman series (determined by a physicist named Lyman) shows all the possible transitions to level 1 (the ground state): 2 to 1, 3 to 1, 4 to 1, etc.
This pattern is called the "Lyman forest" (because, on the old graphic spectrographs, it looks like a clump of trees on a "horizon")
Not only are all wavelengths known, we also use the ratio between each level to identify the pattern, when observing a galaxy that is red-shifted (all the wavelengths have been shifted because of the movement of the distant galaxy).
It is therefore easy to identify this Lyman forest in a spectrographs. Once we have that, we can re-measure all the other wavelengths and identify the other elements according to their "forests" (even though the other "forests" are not as easy to find).
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david say: Do a very detailed of the spectra of light coming from that star. Compare to spectra of various elements to find which elements are present in the star.
==== Helium was 'discovered' in this way on the sun before it was found on Earth.
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Barry say: Analyse with a spectrometer.
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CarolOklaNola say: With spectroscopy.
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poldi2 say: Astronomers analyse the light from the star using a spectrometer. Each element either emits or absorbs specific frequencies of light (like a fingerprint). They spread the starlight out into a spectrum and measure the lines in the spectrum that are associated with specific elements.
https://courses.lumenlearning.com/astron...
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