Measuring distances to stars and galaxies is a tough problem because of the vast scale of the universe. Even the closest star, for example, is 4.2 light-years away. That means that a beam of light from the star, traveling at 186,000 miles (300,000 km) per second, needs 4.2 years to reach Earth. The nearest large galaxy, M31, is about 2.5 million light-years away, while the most distant galaxies yet observed are thought to be about 12 billion light-years away.
To measure these distances, astronomers use a variety of techniques. Each technique is like a step on a ladder, so you need all the steps to safely reach the top — in this case, the farthest reaches of the universe.
The most accurate technique, called parallax, works only for the closest stars. Astronomers measure a tiny back-and-forth “wiggle” in the star’s position compared to other stars as they view it from different points in Earth’s orbit around the Sun.
The next technique uses Cepheid variables. These stars pulse in and out, growing brighter and fainter as they do so. There is a relationship between the length of the pulsations and the star’s true brightness. Astronomers use parallax to determine the distances to nearby Cepheids, revealing their true brightnesses, then use that scale to determine the distances to Cepheids in other galaxies. So far, this technique has worked for galaxies as far as 60 million light-years away.
Some of those galaxies also have the types of stars that form the next step in the cosmic distance ladder: exploding stars known as Type Ia supernovae. Such stars all peak at the same brightness (brighter than billions of normal stars), so measuring their apparent brightness allows astronomers to calculate their true brightness, which in turn reveals their distance. This technique works for galaxies that are up to a few billion light-years away.
The final technique measures how fast galaxies are moving away from us, which is known as redshift. Because the universe is expanding as a result of the Big Bang, more-distant galaxies show a greater redshift, which means they’re moving away from us at a faster speed. Astronomers measure the redshifts of galaxies with Type Ia supernovae, and use that baseline to determine the distances to other galaxies. Redshift measurements reveal the distances to galaxies near the edge of the observable universe — about 12 billion light-years away.