A light-year is the distance that light travels in one year. The Sun is the closest star to Earth. Proxima Centauri is the nearest star to the Sun. As of November 2026, it is about 4.24 light-years away. Except for the Sun, stars are so far away that they look like pinpoints of light. Only a few thousand stars are visible from Earth without a telescope, and most of the brightest stars are relatively nearby, within a few hundred light-years. The farther away an object in space is, the farther in the past we see it, because light takes time to travel.

Hipparchus did not have a telescope to measure brightness. He estimated by eye and sorted stars into six brightness categories he called magnitudes, with first magnitude the brightest. On the modern magnitude scale, a difference of five magnitudes is exactly a brightness ratio of 100 to 1, so smaller numbers mean brighter stars. Sirius is the brightest star in the night sky. As the star with the brightest apparent magnitude, it has a magnitude of -1.5, while Polaris is magnitude 2.0.

Stars come in different colors, such as red, orange, yellow, white and blue. Blue-white stars are much hotter than the Sun, and red stars are cooler. Stars come in different sizes, and there are bigger stars and smaller stars than the Sun, which is an average-sized star. Astronomers have found stars that are 1,000 times bigger in diameter than the Sun, and others that are one tenth its size.

Starlight passes through a spectrograph to make a spectrum, and in 1814 Joseph Fraunhofer saw dark lines crossing the Sun's spectrum. A star's color shows its surface temperature: blue stars are about 25,000 K, white about 10,000 K, yellow like the Sun about 6000 K, orange about 4000 K and red about 3000 K. A star's temperature determines which absorption lines are in its spectrum, so astronomers sort stars into spectral classes O, B, A, F, G, K and M from hottest to coldest. In the 1860s William and Margaret Huggins identified lines in stellar spectra as those of known elements on Earth. This showed that the same chemical elements found in the Sun and planets exist in the stars, and hydrogen is by far the most abundant element in most stars.

Words to know

• magnitudes: brightness ranks
• spectrograph: light splitter
• spectrum: spread of light
• elements: basic substances