If a star was close enough (within a few hundred light years), they could measure it with parallax. But what if it’s farther? Here’s the problem. If you see a bright star, is it truly bright, or is it just close? (A ten watt light bulb will look bright if it’s right in your face, and a hundred watt one will be dim if it’s down the hall.)
For a long time, everyone was stuck, and the universe, while huge, seemed a lot smaller than it actually was. If you know how bright a bulb – or star – is supposed to be, and then compare it with how bright it looks, then you can figure out the distance. But, trouble is – stars don’t have a little number saying “Ten billion watts, fire hazard if used in the ‘fridge”, do they?
Way back in 1912, Henrietta Leavitt realized that a Cephid variable star (named after the first one discovered in the constellation Cepheus) did something really cool. She measured the distances of the several nearby Cepheids with parallax and realized the brighter ones varied slower than the dimmer ones.
In other words, the “higher wattage” stars blinked slower than the “lower wattage” ones. Now astronomers could identify the true brightness (luminosity)of a star, and calculate it’s distance by how bright it appeared on earth. If a “100 watt” star in one part of the sky is dimmer than another “100 watt” star in another, the dimmer one is farther away, because we know it’s actually bright, and the distance is the cause of the faintness.
I know this sounds a bit strange, so here’s a great video on the topic. Edwin Hubble used this yardstick in 1924 to look at Cepheid variable stars in the “Andromeda Nebula” and figured out…it was another galaxy, 2.5 million light years away. (Our own galaxy is a bit over a hundred thousand light years across.)