I just can’t comprehend how we can tell that something is 500 light years away. Or how we know that a galaxy is 100 light years wide.
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We see an object by observing the light that it emits. Distant stars emit visible light, which is why we can see them with telescopes.
Everything in the universe is moving. Have you ever noticed how if an ambulance is driving towards you and playing its siren, the pitch of the siren gets higher as it gets closer to you, and then lower as it drives away? That’s because of the Doppler Effect, because the ambulance is moving fast enough relative to the sound it’s emitting, that it is “catching up” to its sound waves, causing them to be closer together, which makes them sound like a higher pitch.
Moving celestial bodies experience the same thing. The light coming off of a distant star is a lower frequency than the light coming off of a close star, like the Sun. That means its spectrum has less blue light, and more red light – which is referred to as “Redshift.” Hubble’s Law tells us that the further away something is, the more redshift we’ll see. So we figure out something is 500 light years away by measuring the amount of redshift and then doing the math for what distance it must be away from the Earth.
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