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. A higher-order multiple star system has three or more stars and the terms multiple star system and multiple star are sometimes used with that meaning, but often meant to include binary stars as well.

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 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.

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".

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. Simulations have suggested that tidal capture binaries are often the result of an encounter between a single star and an existing binary that exchanges a star in the binary.

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

Another set of classes is based upon how close they are and how much they interact:

Given these possibilities as well as 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:

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

Referenced by pages:
accretion disk
Aitken Double Star Catalogue (ADS)
Alpha Centauri (α Centauri)
Am star
astrometric binary
Black Widow Pulsar (B1957+20)
barium star (Ba star)
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
heartbeat star
Heggie-Hills law
helium burning
helium planet
helium star
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)
mass transfer
M-type star (M)
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)
surface gravity (g)
symbiotic binary (SS)
tidal capture
tidal migration
transiting planet
T Tauri
transit timing variations (TTV)
turn-off point (TO)
Type Ia supernova
variable star
visual binary
Vogt-Russell theorem (VR theorem)
wide binaries (WB)
WR 140
X-ray burster (XRB)
ZTF J1539+5027