I’ve been watching videos on YT about astronomy for years now. It’s fascinating. I never really ask questions and just believe what the experts have to say. But today I have decided to ask this question… A question that I’ve often ignored before.
So yesterday, I was watching this video and it mentioned a star/some celestial body. I don’t remember exactly, sorry. It said that the astronomers have calculated that this celestial body is about *13 billion lightyears* away from us and this many billions-something-huge.
It’s just so baffling to me. How do they do it? They’re calculating masses and distances of bodies that are supposedly soo sooo huge and far away, when we haven’t even actually managed to step on mars, yet. My point is, how are we capable of determining all this accurately when on a universal scale, we’re capable of pretty much nothing. How do these calculations work?
In: Physics
Size:
For planets that are really far away, we look at the movement of the star they’re orbiting. When one body orbits another, although it seems like the star is stationary and the planet goes around it, they’re actaully orbiting around one another. This means that the star will “wiggle” ever so slightly. You can see how this works in [this](https://spaceplace.nasa.gov/review/barycenter/doppspec-above.en.gif) and [this](https://spaceplace.nasa.gov/barycenter/en/dopspec-inline.en.gif) gif. We can detect this wiggling by looking at tiny yet periodic changes in a star’s light. The wavelength will vary ever so slightly due to the movement of the star. This technique, called doppler spectroscopy, can give us an astounding amount of information when combined with a few other techniques and measurements, even letting us determine the mass and orbital period of the planet, and sometimes even the temperature on that planet.
For stars we can usually determine the size by looking at the type of light they emit. The mass of the star is partly determined by the temperature of the star, and the light emitted by the star can tell us the temperature of the star. So we can use this to get an idea of the size and mass of a star.
Distance:
We use parallax. Hold your finger out in front of you at arms length. Look at the background just behind the finger as you move your head from side to side. Try to avoid moving your finger. You should notice that the background moves too, which is to be expected. Now, do the same trick but the first time hold your finger up in a way that the background is quite close to you. You should notice that when you move your head the background moves more than if the background is far away. This phenomenon is also noticable when you look out of one of the passenger windows of a car while driving. If you look at trees right next to the road, you’ll see them whizz by, but if you look at a tree that’s a long way away it seems to move slower. This effect is called parallax and we use it to measure how far away stars are. In astronomy we use stars that are far away as a reference and we make the star in question the “finger”. [Here](https://javalab.org/en/stellar_parallax_en/) is a good example of how this works. Instead of moving our heads we let the Earth orbit the sun, and then we use basic trigonometry to figure out what the distance is.
For planets this doesn’t really work because they don’t emit light, so what we usually do is look at the star it’s orbiting, and see how far away that is. This then gives us a decent idea of the distance to that planet.
The reason we’re so good at this is because the physics involved is really well understood. The laws describing how planets orbit stars are really well known, and this means that we can extract a shitload of information out of what few things we can measure. I always see it as milking every single drop of information we get for everything it can tell us about the star it came from. One of the people in my calculus classes always used to call it “interrogation of data” and he honestly wasn’t that far off, because we get so much data out of so little data.
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