Binary star

If components in binary star systems are close enough, they can gravitationally distort each other's outer stellar atmospheres.

In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain.

Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae.

Mizar, in the Big Dipper (Ursa Major), was observed to be double by Giovanni Battista Riccioli in 1650[1][2] (and probably earlier by Benedetto Castelli and Galileo).

He focused his investigation on the Pleiades cluster, and calculated that the likelihood of finding such a close grouping of stars was about one in half a million.

[10] The term binary was first used in this context by Sir William Herschel in 1802,[11] when he wrote:[12] If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other.

[16] Binary stars are classified into four types according to the way in which they are observed: visually, by observation; spectroscopically, by periodic changes in spectral lines; photometrically, by changes in brightness caused by an eclipse; or astrometrically, by measuring a deviation in a star's position caused by an unseen companion.

After a sufficient number of observations are recorded over a period of time, they are plotted in polar coordinates with the primary star at the origin, and the most probable ellipse is drawn through these points such that the Keplerian law of areas is satisfied.

From this projected ellipse the complete elements of the orbit may be computed, where the semi-major axis can only be expressed in angular units unless the stellar parallax, and hence the distance, of the system is known.

The orbit of a spectroscopic binary is determined by making a long series of observations of the radial velocity of one or both components of the system.

If the orbit is elliptical, the shape of the curve depends on the eccentricity of the ellipse and the orientation of the major axis with reference to the line of sight.

However, the product of the semi-major axis and the sine of the inclination (i.e. a sin i) may be determined directly in linear units (e.g. kilometres).

Algol, a triple star system in the constellation Perseus, contains the best-known example of an eclipsing binary.

This makes it feasible to use them to directly measure the distances to external galaxies, a process that is more accurate than using standard candles.

Eclipsing binaries offer a direct method to gauge the distance to galaxies to an improved 5% level of accuracy.

The companion could be very dim, so that it is currently undetectable or masked by the glare of its primary, or it could be an object that emits little or no electromagnetic radiation, for example a neutron star.

[25] The visible star's position is carefully measured and detected to vary, due to the gravitational influence from its counterpart.

Nearby stars often have a relatively high proper motion, so astrometric binaries will appear to follow a wobbly path across the sky.

However, the requirements to perform this measurement are very exacting, due to the great difference in the mass ratio, and the typically long period of the planet's orbit.

Examples: While it is not impossible that some binaries might be created through gravitational capture between two single stars, given the very low likelihood of such an event (three objects being actually required, as conservation of energy rules out a single gravitating body capturing another) and the high number of binaries currently in existence, this cannot be the primary formation process.

[49] The result is that matter will transfer from one star to another through a process known as Roche lobe overflow (RLOF), either being absorbed by direct impact or through an accretion disc.

These are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in.

Because this is a conserved quantity in physics, binaries give us important clues about the conditions under which the stars were formed.

According to theoretical simulations,[65] even widely separated binary stars often disrupt the discs of rocky grains from which protoplanets form.

[66] Detecting planets in multiple star systems introduces additional technical difficulties, which may be why they are only rarely found.

It is estimated that 50–60% of binary systems are capable of supporting habitable terrestrial planets within stable orbital ranges.

[66] The large distance between the components, as well as their difference in color, make Albireo one of the easiest observable visual binaries.

In 2005, using the Hubble Space Telescope, astronomers determined Sirius B to be 12,000 km (7,456 mi) in diameter, with a mass that is 98% of the Sun.

The name Algol means "demon star" (from Arabic: الغول al-ghūl), which was probably given due to its peculiar behavior.

Another visible ternary is Alpha Centauri, in the southern constellation of Centaurus, which contains the third-brightest star in the night sky, with an apparent visual magnitude of −0.01.