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.
For distance: Trigonometry!
They use a method called stellar parallax. The look at the position of the star at one point in the year, and then view that same stars position at a different point in the year. By comparing position, we can triangulate the distance of the star from Earth. It isn’t dissimilar to how cartographers made maps centuries ago – if you have two angles of a triangle and the distance between them, you can calculate the other two sides of the triangle.
For mass, the calculations are complicated, but since we know the mass of the Earth, and the speed/distance of our orbit, we can look at the orbital radiuses and speed of other objects and plug those into our equations to determine their mass.
**Parallax**
First, we need a point of reference and the best reference point we have is the Sun. While ancient astronomers attempt to determine the distance to the Sun, they lacked the appropriate tools to make precise enough measurements, or knowledge of orbital mechanics to determine this with any reasonable degree of accuracy.
By the 17th century, we had a much better understanding of the orbits of planets, and were able to make connections about the orbits of the planets relative to each other. All we needed now was to know one of them.
The “one” was Venus. Occasionally, Venus will make a “transit.” That is, it’ll pass between the Earth and the Sun in a way that is visible. By measuring how long this transit lasts, we were able to, in the 18th century, calculate Venus’ orbit and, hence, Earth’s orbit, including our distance to the Sun.
Second, now that we know the distance to the Sun, we can use something called parallax to measure the distance from Earth to other stars. What you do is pick a star and measure the angle from Earth to that star. Then you wait 6 months and measure the angle to that star again. These two angles, plus the known distance between the position of the Earth (6 months apart, on opposite sides of the sun), uniquely defines a triangle whose apex is the chosen star. Once you know those three pieces of information, you can derive all the other information about that star, including the distance to it.
However, the further away something is, the more accurate you have to measure to get a good distance. Given our current tools, parallax only really works for things up to 100 light years away from Earth.
**Apparent Luminosity**
In the early 1900’s we discovered a type of star known as a Cepheid star. The cool thing about them is they dim and brighten in regular intervals. The interval depends only on the stars absolute brightness. A thing about luminosity is, the further away something is, the dimmer it appears. So if you look at one of these Cepheid stars and measure the period of its intervals, you can determine how bright it really is. If you compare how bright it really is with how bright it appears, you can determine how far away it is (since the rate at which things appear dimmer is mathematically related to how far away it is).
Cepheid stars are fairly common and fairly bright over all, allowing us to measure distances up to millions of light-years away.
We can use similar methods for other phenomenon with known brightnesses, such as supernovae, to measure distances even further away, up to a billion light years.
**Red Shift**
Lastly, is that the universe is expanding. The further two objects are away from each other, the faster they are moving away. When light is emitted from an object moving away from you, this “stretches out” the light which makes it appear “redder” than it really is^(*). So if you look at a kind of object whose brightness is known, you can measure how red-shifted its light is which tells you how fast it is moving away from you. Since the speed at which the object is moving away from you is related to its distance from you, you can then calculate its distance.
* – Not necessarily *literally* redder (though possibly so) but rather its wavelengths are longer (having been stretched out). This is in contrast to objects moving towards each other, whose wavelengths are squashed up and therefore smaller which makes it bluer. The terms red and blue refer to the fact that red and blue occupy the long and short wavelengths of our visible light spectrum, respectively.
Latest Answers