Astronomers call a star that is fusing hydrogen to helium in its core a main sequence star; they make up around 90% of the universe's stars, and range from a tenth to 200 times the Sun's mass. About 90 percent of the true stars in our part of space are main-sequence stars, about 10 percent are white dwarfs, and fewer than 1 percent are giants or supergiants. Red dwarfs make up around 75% of the Milky Way's stars, because they are born in great numbers and live so long. Although massive stars have more fuel, they burn it so prodigiously that their lifetimes are much shorter than those of low-mass stars.

Binary stars are two stars that orbit each other, held together by gravity, and many stars belong to systems with two or more stars. About half of all stars are binary stars, and the masses of binary stars can be calculated from measurements of their orbits, just as the mass of the Sun is found from the planets' orbits. Stars come in different sizes, and our Sun is a medium-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. Brown dwarfs have between 13 and 80 times the mass of Jupiter, more than planets but not enough for the fusion of a main sequence star.

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 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. Annie Jump Cannon revised the classification of stellar spectra in the 1890s, rearranging the classes in order of decreasing temperature into the sequence O, B, A, F, G, K, M. Lines of hydrogen are not seen in the spectra of the hottest and coolest stars: in the hottest stars hydrogen atoms are completely ionized, and ionized hydrogen cannot produce absorption lines.

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. Astronomers use a color index, the difference between a star's brightness through two filters such as blue and visual (B-V). It runs from about -0.4 for the bluest stars at about 40,000 K to +2.0 for the reddest at about 2000 K. Stars come in different colors, and blue-white stars are much hotter than the Sun, while red stars are cooler.

Parallax is the apparent change in direction of a remote object when the observer's viewpoint changes; the smaller the parallax, the more distant the object. The first successful measurements of stellar parallax came in 1838, when Bessel, Henderson and Struve independently measured the parallaxes of the stars 61 Cygni, Alpha Centauri and Vega. A star with a parallax of one arcsecond is at a distance of one parsec, which is 3.26 light-years; a star with a parallax of 0.1 arcsecond is 10 parsecs away. As of November 2026, the record shows that ESA's Gaia spacecraft made more than three trillion observations of two billion stars and other objects between July 2014 and January 2025.

Polaris is not the brightest star in the sky. Sirius, the star with the brightest apparent magnitude, has a magnitude of -1.5, while Polaris is magnitude 2.0. Polaris stays in the same northerly direction as the Earth turns, while other stars seem to revolve around it. Cepheid variables are large, yellow, pulsating stars; most have periods of 3 to 50 days and are about 1000 to 10,000 times more luminous than the Sun. Polaris is a Cepheid whose surface slowly pulsates, changing its brightness by a few percent over a few days.

Words to know

• binary: two-star
• spectrograph: light splitter
• parallax: apparent shift
• Cepheid: pulsating star