Astrophysics (Index)About

binary star

(binary, binary system)
(system of two stars co-orbiting)

A binary star is a pair of stars that orbit each other. Double star means the same thing except that it also includes stars that are not orbiting and not close together but happen to be on the same line of sight from Earth, referred to as apparent binary or optical double stars. Binary (or more) star systems are said to be common: the most convincingly cited number I've found is that roughly third of all star systems have two or more stars. They are extremely useful in the study of stellar physics, both to use the orbital dynamics for stellar parameter determination, and for those close enough to interact further, giving additional situations to observe, to infer and test the physics of stellar structure.

A system can have three or more stars, termed a higher-order multiple star system, or sometimes the terms multiple star system and multiple star are used specifically for three or more.

A common classification of binary stars is based on the method by which they were determined to be binary:

Another classification is based upon how close they are and how much they interact:

A binary star's mass ratio (μ) is the ratio of the two masses, i.e., 1 for stars of equal mass. When the total mass can also be determined, e.g., from the orbital period and size, the mass of each star is evident.

Binary stars generally have similar composition (as shown by their spectra), as if they were formed together. Binaries formed together are known as primordial binaries, another sign being aligned rotation axes. A capture requires the coincidence of stars passing close to each other, plus something to change their velocity, such as a third star, or with just two, tidal forces between them (i.e., tidal-capture binaries). The "close pass" is more likely in areas with a very high density of stars such as the center of globular clusters or galaxies.

Given the large range of distances between the stars and the different sizes/spectral types of the individual stars, binary stars show a wide variety and interactions between them produce characteristics unseen in non-binary stars. For example:

The commonly-used system for designating the individual stars of a binary/multiple star system consist of adding letters to the star system's designator, with letter "A" for the brightest, "B" for the second brightest, then "C" and so on. For example, the two stars making up Sirius are termed "Sirius A" and "Sirius B". If two are very close and a third is distant, the two close stars might use lower-case suffixes, i.e., "Aa" and "Ab", with the further member called "B".

(star type,binary stars,double stars)
Further reading:

Referenced by pages:
accretion disk
Aitken Double Star Catalogue (ADS)
Alpha Centauri
Am star
astrometric binary
Black Widow Pulsar (B1957+20)
Burnham Double Star Catalogue (BDS)
black hole binary (BHB)
black hole merger
blended spectra
binary neutron star (BNS)
brown dwarf (BD)
carbon star (C)
candidate companion (CC)
methylidyne (CH)
circumbinary planet
common envelope
contact binary
core collapse
chemically peculiar star (CP star)
double-line spectroscopic binary (SB2)
double star
double star designation
dwarf nova (DN)
eclipsing binary (E)
EF Eridani
extra-solar planet
failed binary
galactic binary
GG Tau
globular cluster (GC)
gravitational lensing
GRO J1655-40
Guide Star Catalog (GSC)
gravitational wave (GW)
GW detection (GW)
HD 133131
HD 189733 b
Heggie-Hills law
helium planet
high-velocity star
instability region
Kepler Telescope
Kepler radius
Laplace-Lagrange secular theory
Luhman 16
Luyten 726-8
mass function
mass loss
mass ratio (μ)
gravitational microlensing
minimum mass (m sin i)
main-sequence lifetime (MS lifetime)
multiplicity fraction
neutron star (NS)
nova (N)
optical double
orbital inclination
orbit plot
post-common envelope binary (PCEB)
planetary system
probability mass function (PMF)
P-Pdot diagram
proper motion (PM)
Hulse-Taylor Binary (PSR B1913+16)
pulsar timing array (PTA)
pulsar (PSR)
radial velocity (RV)
redshift (z)
Roche lobe
Rossiter-McLaughlin effect (RM effect)
Catalogue of Southern Double Stars
radial velocity method
Scholz's Star
solar system
spectroscopic binary (SB)
spectrum binary
stellar population synthesis (SPS)
SS 433
star system
stellar-mass black hole
stellar distance determination
stellar evolution
stellar kinematics
stellar parameter determination
subdwarf (sd)
symbiotic binary (SS)
tidal capture
transiting planet
T Tauri
transit timing variations (TTV)
turn-off point (TO)
variable star
visual binary
Vogt-Russell theorem (VR theorem)
wide binaries (WB)
WR 140
X-ray burster (XRB)