Binary Stars: The Sky's Twins
Stars that come in pairs. Sirius, Alpha Centauri and Algol are really pairs of stars locked in orbit. Meet the sky's famous binaries and the double world of Pluto and Charon.
Sirius A and its tiny white dwarf partner, Sirius B (artist's impression): NASA, ESA and G. Bacon (STScI), Public domain
Two suns, one dance
A binary star is a pair of stars held together by gravity, each circling a shared balance point called the center of mass. The astronomer William Herschel coined the term in 1802 after watching double stars over many years and seeing that some were not just lined up by chance but actually orbiting each other. Binaries are common in our galaxy, and they are also extremely useful: by timing an orbit, astronomers can weigh the stars inside it.
Famous pairs you can find
Sirius, the brightest star in the night sky, is a binary about 8.6 light-years away. Its bright bluish white star, Sirius A, has a faint companion, Sirius B, a white dwarf roughly the size of Earth that packs in about as much mass as the Sun. In 1844 Friedrich Bessel predicted an unseen partner from the way Sirius wobbled, and in 1862 Alvan Graham Clark finally spotted it. The two orbit every 50 years or so.
Alpha Centauri A and B, about 4.3 light-years away, are the nearest binary to the Sun. Both are Sun-like stars, and their stretched orbit takes about 79 years, carrying them from roughly Saturn's distance from the Sun to about Pluto's. A small red dwarf, Proxima Centauri, hangs nearby as a third member.
Albireo in the constellation Cygnus is a favorite for backyard telescopes, showing a golden star beside a blue one. It is a reminder that not every pretty double is a true pair: astronomers still have not settled whether Albireo's two stars are bound together or simply line up from our point of view.
Algol, the winking star
Some binaries are tilted so that the stars pass in front of each other as seen from Earth. These eclipsing binaries dim on a schedule. The most famous is Algol in Perseus, whose name comes from an Arabic phrase meaning head of the ghoul. Every 2.87 days, its fainter partner crosses the brighter star and Algol drops noticeably in brightness for about 10 hours. Geminiano Montanari noted the change in 1667, and in 1783 a young English astronomer, John Goodricke, suggested a dark body passing in front of the star was the cause. Eclipsing binaries are now used to measure the sizes and masses of stars and even the distances to nearby galaxies.
A double world at the edge
Pairs are not limited to stars. Pluto's largest moon, Charon, is just over half Pluto's width, so big that the two bodies orbit a balance point in empty space between them rather than inside Pluto. They are locked face to face, each always showing the other the same side, and they complete a turn every 6.4 days. Charon was discovered in 1978 by James Christy, and NASA's New Horizons spacecraft flew past both worlds in July 2015. Some scientists call them a double dwarf planet, though officially Charon is still Pluto's moon.
- Sirius A and its tiny white dwarf partner, Sirius B (artist's impression): NASA, ESA and G. Bacon (STScI), Public domain
- Albireo's gold and blue stars through an 8-inch telescope: Topoignaz, CC BY-SA 4.0
- Pluto and Charon, a double world, seen by New Horizons: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute, Public domain
Text written by Actual Twin.
