How We Measure the Stars

The ingenious astronomical methods used to calculate cosmic distances without ever leaving Earth.

Stellar Parallax

For relatively close stars (up to a few thousand light years), astronomers use parallax. By observing a star's position in the sky in January, and then again in July when Earth is on the opposite side of the Sun, the star appears to shift slightly against the distant background stars. Using simple trigonometry, this shift reveals the star's exact distance.

Standard Candles

For distances too great for parallax, astronomers rely on "standard candles"—celestial objects with known, predictable luminosity. Cepheid variable stars, for example, pulse at a rate directly related to their intrinsic brightness. By comparing how bright they actually are to how dim they appear from Earth, we can calculate the distance using the inverse-square law of light.

Cosmological Redshift

For the most distant galaxies on the edge of the observable universe, we look at the redshift of their light. Because the universe is expanding, light from distant objects is stretched into longer, redder wavelengths as it travels to us. The greater the redshift, the faster the object is receding, and the farther away it is, following Hubble's Law.