Binary star

Binary stars are stellar systems composed of two stars that are gravitationally bound and orbit a shared centre of mass. They represent one of the most fundamental classes of multiple-star systems in astrophysics and are of great importance for studying stellar mass, evolution and dynamics. Some, such as Sirius—shown in a 2005 Hubble photograph with the bright Sirius A and the faint white dwarf Sirius B—are among the most famous objects in the night sky.

Discovery and Early Observations

Double stars—pairs of stars that appear close together—were first recorded when telescopes became available. Giovanni Battista Riccioli identified Mizar as double in 1650, and Father Fontenay observed Acrux to be double in 1685. Early astronomers initially assumed these were optical alignments rather than physical systems.
The first argument for true gravitational pairing appeared in 1767 when John Michell used statistical reasoning to show that star pairs occur far more frequently than random distribution would allow. His work suggested that gravitational attraction might bind such stars, foreshadowing the concept of binary systems.
William Herschel provided systematic observational evidence. Between 1779 and 1803, he catalogued hundreds of double stars and eventually concluded that many pairs were orbiting each other. By 1827, Félix Savary had computed the first orbital elements for the binary Xi Ursae Majoris. Over time, cataloguing expanded significantly; today, the Washington Double Star Catalog lists over 100,000 double-star pairs, though only a fraction are confirmed binaries with measured orbits.

Terminology and Classification

The term binary star was introduced by William Herschel in 1802 and refers to pairs of stars revolving around a common centre of mass. The broader term double star includes both physical binaries and optical doubles—stars that appear close together only because of line-of-sight coincidence.
Binary stars are classified according to the observational method used to detect them:

• Visual binaries are pairs whose separation is wide enough to be resolved by telescopes or interferometers.
• Spectroscopic binaries are detected using shifts in spectral lines caused by the Doppler effect as the stars move toward and away from an observer.
• Eclipsing binaries are detected when one star periodically passes in front of the other, causing characteristic changes in brightness.
• Astrometric binaries are inferred from observed deviations in the motion of a visible star, caused by the gravitational influence of an unseen companion.

A single binary star may fall under multiple observational classes depending on its orientation, separation and brightness.

Visual Binaries

Visual binaries require sufficient angular separation to appear as two distinct points through a telescope. Their detection depends heavily on telescope resolution and relative brightness, as a bright star can obscure a faint companion. Observers measure the position angle and angular separation over time; these measurements are plotted to determine the apparent orbit, an ellipse projected onto the sky. Using Kepler’s laws, astronomers can derive the true orbital elements, though physical scale requires knowledge of distance through parallax.

Spectroscopic Binaries

If the stars in a binary system are too close to resolve visually, their orbital motion may be detected through periodic Doppler shifts in their spectral lines. As one star approaches, its lines shift toward the blue; as it recedes, they shift toward the red. In double-lined spectroscopic binaries, lines from both stars are visible and alternate between single and double as the stars orbit. In single-lined systems, only one star’s spectrum is detectable.
Such systems often have very short orbital periods and high velocities, offering valuable insight into stellar masses.

Eclipsing and Photometric Binaries

When the orbital plane lies along our line of sight, the stars can eclipse each other. These eclipsing binaries show periodic drops in brightness, enabling precise determination of stellar radii, orbital inclination and other physical parameters. They form a major category of photometric binaries, which are identified through changes in observed light output.

Astrometric Binaries

Astrometric binaries reveal themselves through subtle irregularities in a star’s motion across the sky. The visible star appears to “wobble” due to the gravitational tug of a hidden companion, which may be too faint to detect directly. Astrometric methods have identified companions ranging from dim stars to compact objects such as white dwarfs or black holes.

Interaction and Evolution in Binary Systems

In many binary stars, the components are far enough apart to evolve independently. However, close binary systems can experience significant interaction:

• Tidal distortion can reshape stellar atmospheres.
• Mass transfer may occur if one star expands beyond its Roche lobe.
• Accretion phenomena often lead to unusual evolutionary pathways not possible in isolated stars.
• Compact-object binaries may emit X-rays, as seen in systems like Cygnus X-1, which contains a stellar-mass black hole.

Binary interactions are responsible for astrophysical phenomena such as novae, X-ray binaries and type Ia supernovae, the latter resulting from runaway thermonuclear ignition in a white dwarf accreting matter from a companion.

Astrophysical Importance and Occurrence

Binary stars are essential for determining stellar masses, the most fundamental parameter in stellar astrophysics. By analysing their orbital motions, astronomers can apply Newtonian dynamics to obtain mass measurements with high precision.
Binary and multiple-star systems are common across the Milky Way. They occur frequently at the centres of planetary nebulae and play a major role in stellar population studies. Systems like Sirius—a bright main-sequence star paired with a white dwarf—illustrate the evolutionary diversity of binaries.