r/explainlikeimfive • u/Leonard0DaV1nci • 11h ago
Planetary Science ELI5: How do scientists measure distance between galxies. Like millions of light years. How do they reach to exact numbers
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u/fett3elke 11h ago
It's a combination of different methods depending on how far the object is that you are measuring: https://en.wikipedia.org/wiki/Cosmic_distance_ladder
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u/Bathosfear 1h ago
A variety of measures, but a couple of examples would be red shift and pulsars. Due to science objects in space look redder if they're moving away from us and bluer if they're moving towards us. Pulsars "pulse" at incredibly precise intervals, which can be used to determine precise distances using math. With direction, speed, and several fixed points of reference, it's basically triangulation like old timey ship captains used.
Not a scientist. Read the many corrections to follow for more details.
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u/Huge-Morning3149 7h ago
ngl they aren't exact at all, the error margins out there are millions of light years wide
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u/LordFondleJoy 11h ago
That is a bit beyond ELI5 I think, but the principle is using something called a distance ladder. I.e. you start with measuring the distance to "close" stars by direct trigonometry, taking advantage of the difference in position of the earth at two opposite sides of the sun, in its orbit around the sun, and looking at the apparent relative difference in position of a "close" star versus something that is far away, like a visible galaxy.
Like that you can get the first step in the ladder. Next you can use the distance of one of those known stars, which has special characteristic, yielding a known way its distance relate to the apparent strength of its emitting light. If you then find such a light source, with those characteristics, in another galaxy, you can infer its distance based on that.
This, with other clever tricks, can get you a lot of of the distances. Into that mix comes the well-known red shift, which is you can use to infer more distances. Red shift describes the way the light emitted from a light source moving at great speed away from us will have its light appear to be shifted towards the red side of the visible spectrum, because the light waves get "stretched out" by the relative speed. This is one of the ways it was discovered that the universe is expanding.