How do astronomers determine the distance to distant galaxies?
I'm curious what methods scientists use to find out how far away those galaxies are that we can see through a telescope?
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I used to be really into astronomy and bought myself an amateur telescope, then switched to crafts, but I still remember the principle. Astronomers use several methods: for nearby galaxies they measure distances through variable stars (they shine with a certain brightness, and based on how they look from Earth, you can calculate the distance), and for really distant ones they rely on redshift - when a galaxy moves away, its light shifts toward the red end of the spectrum, and they calculate the distance from the magnitude of this shift. They also use supernova explosions as "standard candles" - they shine at roughly the same brightness, so you can judge the distance from how bright they appear.
Distances to galaxies aren't calculated just for fun - it's actually a pretty complex system of methods where one approach checks the other. There's no single magic tool that works for all distances at once.
For nearby galaxies, astronomers use variable stars as "standard candles" - for example, Cepheids, which pulse with a known pattern. If you know how intrinsically bright a star actually is, and you see how dim it appears from Earth, it's easy to calculate the distance. Further out, Type Ia supernovae come into play - they explode at roughly the same brightness and are visible from very far away. But here's what's important and often overlooked: for these methods to work accurately, you have to account for interstellar dust, which absorbs light and makes objects artificially dim. It's like looking at a car's headlight in fog - it shines brighter than it seems from a distance.
For the most distant galaxies, which we can't even resolve into individual stars, we use redshift - we analyze the light spectrum and see how it's been "stretched" by the expansion of the universe. This isn't a direct measurement of distance anymore, but a calculation based on the recession velocity of galaxies. Practical tip: if you want to understand the scale, check out photos from space telescopes like Hubble or modern observatories - they often show the redshift of galaxies, and that gives you a better sense of how far away they are than any numbers in parsecs.
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