My understanding is that when an object moves away from you it increases the wavelength of the light presenting with a colour that is on the red side of the colourscale. Scientists also are able to determine what elements are present on planets based on the light it emits. How can they tell the difference?
I’m sorry for possibly using the incorrect terminology! Thanks in advance folks!
Edited the post because previously I had suggested that shorter wavelengths tended to the red side when in fact longer wavelengths tended to the red side of the colour spectrum.
In: 17
Basically different elements emit(or absorb) a specific pattern of wavelengths, this is due to the possible energy levels of electrons. So every element basically has a signature that we can look for, and if that signal is shifted towards red we can calculate very precisely how much the light has been redshifted, and the signal can also tell us about the chemical/atomic make up of whatever we are looking at.
Imagine your name was ABC and everyone knew it. Your sister’s name is LMN and everyone knew it.
Suddenly you start signing your name as BCD, and your sister MNO. It’s pretty clear that something weird is happening and for some reasons you’ve both just started writing your names exactly one letter off.
While it might take you a while to figure out *why* your writing names this way, it’ll still be clear that you are you and your sister is your sister.
That’s what’s happening with red-shifting only instead of names is the exact colors of the light the atoms give off. If we know that all the colors should be a certain way and we find a set of colors that are different, but consistently different in an ‘everything get’s a +1’ sort of way.
A star emits all frequencies of visible light, so you should see a pure spectrum. Elements within the star will absorb certain frequencies, so you actually see a spectrum with lots of black dots in it.
We can measure how far away the star is, based on parallax or proximity to standard candles (objects that we know have a certain brightness).
Only distant galaxies, where individual stars are too small to be seen, have a significant redshift.
<cough the wavelength *increases* with red shift. The *frequency* reduces.>
1. A beam of light leaves a star.
2. It arrives at our measurement device.
3. We can measure that beam’s wavelength to determine how far away it originated.
A) Elements in the star’s corona absorb some of the light as it leaves the star.
B) Those absorptions leave gaps in the beam’s spectrum.
C) We can identify the elements in the corona based on the position of those gaps in the beam’s spectrum.
Because of the structure of each atoms, the way they absorb electromagnetic radiation is unique. You could think of it as a unique finger print of the atom. If we send light through let’s say a cloud of oxygen, and then we analyze the spectrum of that light, we will see dark lines where specific wavelength of light were absorbed by the oxygen. That’s the ”finger print” of the oxygen.
Now like you said when an object moves away from us it reduce the wavelength of the light toward the red. But this is a regular shift so the pattern remain the same.
Let’s use number to give an example. Let’s say that the wavelength absorbed by oxygen is 3 and the wavelength absorbed by hydrogen is 7. So if let light go through a cloud of water, on the other side you would see black line at 3 and 7.
But what if this light had a red shift, well then maybe you see black lines at 6 and 10. You can still recognize the pattern here, there is 4 of difference between the two. And to be sure, you can calculate the red shift is indeed 4 and now you know for sure that the light passed through oxygen and hydrogen.
Of course in reality spectroscopy is much more complex, but you get the idea.
Red shift tends to be noticeable from the light from *galaxies*, not distant planets. Even the furthest planets we can see are within our own galaxy, so moving with us. It is distant galaxies that are moving away rapidly.
But yes, it is a fun question. The answer is that different elements have very specific “signature” patterns of light. For example, something with hydrogen in it, that is glowing, [looks like this](https://en.wikipedia.org/wiki/File:Emission_spectrum-H.svg). You get very narrow spikes of light in the purple, blue, light blue and red part of the spectrum, and nothing anywhere else. Going beyond just visible light [you get something a bit more complicated](https://en.wikipedia.org/wiki/File:Hydrogen_spectrum.svg), but the same sort of thing – very specific wavelengths of light.
You also get the same sort of thing in reverse, with absorption spectra; when light travels through materials specific wavelengths of light will be absorbed (e.g. [this for hydrogen](https://en.wikipedia.org/wiki/File:Spectral-lines-absorption.svg)).
So what astronomers do is look at the light coming from or through something, look for the gaps (if the light is passing through the thing they are interested in) and spikes (if the light is being emitted by the thing), and do some complicated analysis to figure out what combination of elements could give those patterns.
For red shift, you have some idea of what to expect (you know that galaxies are mostly hydrogen) so you look the distinctive hydrogen lines, but find them in the wrong place. How far off they are tells you how much the light has been red-shifted. For example, going back to our [hydrogen emission spectrum](https://en.wikipedia.org/wiki/File:Emission_spectrum-H.svg), the gaps between the bright spots have very specific widths (a narrow one, a wider one and a huge one – and this pattern is repeated across the whole spectrum). So if you get a bunch of light that has come from a galaxy, and you find there are spikes and wavelengths with the same gaps but in a different part of the spectrum (say the first spike is in blue, then the close one is in light blue, the next in green, and the one after is infrared) that’s a good hint that you’re seeing light from hydrogen that has been shifted a bit.
To use a (bad) analogy from your comment history, think about different formations used by rugby players, for different set pieces; a line-out looks largely the same wherever in the pitch it is happening. Someone who knows the game would be able to tell a line-out from a scrum (or equivalent – rugby isn’t my speciality). Looking at the positioning of the players an observer could tell whether they’re about to do a line-out or a scrum (different elements), and whether it is taking place at one end of the pitch or the other (how red-shifted).
We’re looking at the light of distant stars, not planets. Stars are bright, planets are not.
Every element releases a specific set of frequencies when it’s excited. This is actually how we discovered helium. We looked at the sun and saw peaks of light that didn’t match uo with any known element, so it was named “helium” after the Greek word for sun, Helios, because the sun is the only place we knew this element existed. Later, we were able to isolate it here on Earth.
When we look at a distant star, we can see a set of peaks, usually hydrogen, since the universe is about 73% hydrogen today, and the further back in time you go, the more hydrogen there is. We compare the peaks of hydrogen to the peaks we observe, and they’re all a little closer to the red end of the spectrum, so we say they’re red shifted, which means they’re moving away from us.
We can also look at type 1a supernovae. A type 1a supernova occurs when a white dwarf and a living star are in the same system. The white dwarf starts stealing hydrogen from the parent star until eventually it stole enough to reignite fusion. This always happens with the same amount of mass, so the resulting flash is always the same. When we see them happen close, there’s no redshift, and when they happen far away, there is.
The type 1a supernova method is applied to stars that are close enough that we can see individual stars. After that, we start relying on redshift because we can’t see individual stars, but the glow of a galaxy as a whole.
(To condense/simplify the answer from grumblingduke)
All atoms absorb and emit light slightly differently which means that different atoms end up having different [spectral line](https://en.wikipedia.org/wiki/Spectral_line) patterns. These patterns essentially act like unique fingerprints.
When relativistic speeds get involved, the bright lines in these patterns might become more stretched out (redshift) or scrunched up (blueshift) but the proportional distances between lines keeps the fingerprint recognizable.
Furthermore, because multiple different elements from the same star/galaxy/etc. will be moving at the same relativistic speed that means that comparing the stretch/scrunch of multiple different fingerprints means we can double-check our work. Either they’re all stretched/scrunched by the same amount (because they’re going the same speed) or somebody made a mistake and we have to reevaluate what fingerprint(s) may have been misidentified.
ELI5: Every element absorbs (takes in) and emits (like a bulb) light at VERY specific wavelengths. Hydrogen, for example, emits/absorbs at 656 nanometers, 486 nanometers, 434 nanometers, and 410 nanometers (in the visible light range.) So we know, since hydrogen is… pretty much everywhere, that we should ALWAYS see those 4 lines exactly that far apart from each other. There are similar numbers for EVERY single element.
So what we do is we look for lines that are EXACTLY that far apart from each other. ALL of them will be redshifted (toward higher wavelengths), but the distance between them will remain the same. And that’s the important part.
They can tell what elements are present in a distant planet simply by limiting the view of the telescope (with a light detector attached to it, called a “spectrometer”) to that planet. If they can ONLY see light from that planet, then ALL of the wavelengths detected will be from the elements on that planet. (It’s worth noting that you will ALWAYS see the light from the star near that planet. And it’ll look like something called “blackbody radiation”. The emissions from the elements will be on top of this curve.) Look at this image of the sun, you can see the “dips” in the spectrum. These are due to specific molecules. The “dips” due to atoms (not molecules) are far, FAR smaller, you’d have to zoom in a ton to see them.
https://www.e-education.psu.edu/meteo300/sites/www.e-education.psu.edu.meteo300/files/images/lesson6/Solar_spectrum_en.svg.png
These absorptions and emissions are called the “atomic spectral series” or just the “atomic spectra” of any particular element.
Every element has it’s own “series” of these emissions. And that series never changes, it just shifts to higher wavelengths. That “series” is related to how far apart the energy levels of the atoms are (hence why they never change, as the energy levels are solutions to a particular equation, and math never changes), but this is slightly beyond a ELI5 explanation.
Exoplanet scientist here! This is exactly what I do every day.
Different chemicals absorb unique and VERY SPECIFIC wavelengths of light.
For example, the element calcium absorbs three lines at 849.8nm, 854.2nm and 866.2nm. Calcium is the only element with this feature.
No two chemicals share the same absorption lines, and we have huge databases of spectral lines absorbed by different chemicals.
This is like how every human has unique fingerprints, no two people have identical fingerprints, so it would be straightforward to compare a random fingerprint you find at a crime scene to a database of human fingerprints. This is how criminals are often caught.
This tells us about the chemicals in an exoplanet’s atmosphere.
You can compare the spectra you measure with big telescopes, to lists of absorption lines from databases, and look for matches.
Planets are always moving however, and movement will red- or blue-shift the light, meaning the absorbed lines measured by the telescope will be in the wrong place.
Thankfully, you can just factor in the additional info that your comparison to those line lists will have some constant offset due to the planet’s velocity.
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