Double and Multiple Stars

105 double and multiple stars by constellation

Sort any column, or type a name or constellation; the buttons above the map filter by how far apart the stars are. Separation is in arcseconds (″), 1/3,600 of a degree; the angle gives the direction of the fainter star from the brighter, measured from north through east. Where a row lists several separations, they are for the third and fourth stars of a multiple system. Pair says whether the two stars are known to orbit each other, share a motion through space, or only line up by chance.

25 of these pairs split in binoculars; 11 need a 4-inch telescope and steady air

How close a pair a telescope can split depends on its aperture: the Dawes limit is about 4.6″ divided by the aperture in inches, so 1.2″ for a 4-inch and 0.6″ for an 8-inch. In practice the air sets the limit on most nights, and an unequal pair is harder than an equal one because the bright star's glare hides its companion. The closest pair here is Alpha Piscium (Alrescha), 1.9″ apart; the widest is Alpha Capricorni (Algedi), 381.2″.

Color contrast makes Beta Cygni (Albireo), Gamma Andromedae (Almach), Iota Cancri, Epsilon Bootis (Izar) and Alpha Herculis (Rasalgethi) favorites

Paired stars of different temperature show their colors side by side, and the contrast makes each look more vivid than it would alone. Albireo's gold and blue are the best known; Almach in Andromeda is orange and blue, Izar in Boötes orange and blue-green. Low power and a slight defocus bring the colors out.

Double stars proved that gravity works beyond the solar system

• Zeta Ursae Majoris (Mizar). Benedetto Castelli saw Mizar split in a telescope in 1617, the first double star seen through a telescope. With Alcor beside it, the pair is a naked-eye test of eyesight.
• Alpha Geminorum (Castor). William Herschel measured Castor's two stars over 25 years and showed in 1803 that they orbit each other: the first proof that gravity holds among the stars. Castor is now known to be six stars.
• Xi Ursae Majoris (Alula Australis). Félix Savary computed its orbit in 1828, the first orbit worked out for a binary star.
• 61 Cygni. Friedrich Bessel measured its parallax in 1838, the first distance to a star: about 11 light-years.
• Epsilon Lyrae (Double Double). Binoculars split it into two stars, and a telescope at high power splits each of those again: the Double Double.
• Theta 1 Orionis (Trapezium). Four young stars at the heart of the Orion Nebula, born together within the last million years.

16 of the pairs here are known to orbit each other, 16 more move together through space, and 2 only line up by chance. 13 are triple or quadruple systems.

Half of these pairs are closer than 287 light-years; the nearest is Alpha Centauri (Rigil Kentaurus) at 4.3

Each dot is one system, on a scale where each step is ten times farther.