You cannot run a tape measure to a star. Astronomers instead use a chain of methods, the cosmic distance ladder, where each step calibrates the next, reaching from nearby planets to the edge of the observable universe.
Units
- Astronomical unit (AU): the average Earth-Sun distance, about 150 million km.
- Light-year: the distance light travels in a year, about 9.46 trillion km.
- Parsec: about 3.26 light-years, defined from parallax (see below).
Step 1: radar in the solar system
Bouncing radar off Venus and other planets and timing the echo pins down distances within the solar system, including the AU, which serves as the baseline for the next step.
Step 2: parallax
Hold a finger in front of your face and close each eye in turn: the finger seems to jump against the background. In the same way, a nearby star shifts slightly against distant stars as Earth moves around the Sun. Measuring that angle gives the distance with simple geometry. A star whose shift is one arcsecond is one parsec away.
Friedrich Bessel made the first stellar parallax measurement in 1838. Today, ESA’s Gaia spacecraft has measured parallaxes for more than a billion stars, mapping our part of the galaxy in 3D, see the Milky Way.
Step 3: standard candles
Beyond reliable parallax, astronomers use objects whose true brightness is known. Comparing true and apparent brightness gives the distance:
- Cepheid variable stars: in 1912, Henrietta Leavitt discovered that their pulsation period reveals their luminosity. Edwin Hubble used Cepheids to prove that Andromeda is a separate galaxy.
- Type Ia supernovae: exploding white dwarfs with a predictable peak brightness, visible across billions of light-years, see supernovae.
- Other methods: the tip of the red giant branch, and spectroscopic parallax using a star’s spectral type, see types of stars.
Step 4: redshift
For the most distant galaxies, the expansion of the universe takes over. The farther a galaxy is, the more its light is stretched toward red, according to Hubble’s law, see spectroscopy. Converting redshift into distance depends on the expansion rate, which is the subject of the “Hubble tension”, see the age of the universe.
Some distances
- The Moon: about 1.3 light-seconds.
- The Sun: about 8.3 light-minutes.
- Proxima Centauri, the nearest star: about 4.24 light-years.
- The center of the Milky Way: about 26,000 light-years.
- The Andromeda Galaxy: about 2.5 million light-years.
