(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
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:
- visual binary - separate stars can be seen or photographed.
- eclipsing binary - a periodic variation in luminosity suggesting a star passing in front of its companion.
- spectrum binary - a star whose spectrum looks like the combined spectrum of two stars.
- astrometric binary - measurement of the star's position in the sky over time suggests an orbit-like movement around an unseen companion.
- spectroscopic binary - measurement of the spectrum over time shows an apparent Doppler shift signifying the type of radial motion that orbiting a companion would produce.
Another set of classes is based upon how close
they are and how much they interact:
- compact binary - small orbits: one criteria is that they rotate in five days or less.
- close binary star - similar meaning: close enough to consider their cross-influence and interaction.
- detached binary - sufficiently distant that the stars are (basically) spherical.
- semidetached binary - sufficiently close that one of the stars transfers mass to the other, but not touching.
- contact binary - touching; at minimum, both stars "stretched" by gravity to a contact point.
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:
- barium star - a star showing unexpected barium absorption lines, often apparently a red giant that previously received mass from a companion carbon star.
- X-ray binary (XRB) or binary X-ray system - includes an accreting neutron star or black hole, whose high gravitational potential energy provides the energy to produce X-rays (through shock). With a strong magnetic field, it might be an X-ray pulsar. These may be low-mass X-ray binaries (LMXB), intermediate-mass X-ray binaries (IMXB), or high-mass X-ray binaries (HMXB or massive X-ray binaries, MXRB) according to the mass of the companion star to the BH/neutron-star. Some are X-ray bursters, with bursts thought to stem from changing levels of accretion, such a transient termed an X-ray burst or X-ray nova. A Be X-ray binary (BeXRB) is a such a binary including a Be star.
- VV Cephei systems - (like VV Cephei) consisting of a giant star and a hot star forming a long-period, interacting eclipsing binary.
- ζ Aurigae systems - (or Zeta Aurigae systems, like the star ζ Aurigae) are analogous to VV Cephei systems but with a different class of giant.
- post-common envelope binary (PCEB).
(star type,binary stars,double stars)
Referenced by pages:
Aitken Double Star Catalogue (ADS)
Alpha Centauri (α Centauri)
Black Widow Pulsar (B1957+20)
barium star (Ba star)
Burnham Double Star Catalogue (BDS)
black hole binary (BHB)
black hole merger
binary neutron star (BNS)
brown dwarf (BD)
carbon star (C)
candidate companion (CC)
chemically peculiar star (CP star)
double-line spectroscopic binary (SB2)
double star designation
dwarf nova (DN)
eclipsing binary (E)
globular cluster (GC)
Guide Star Catalog (GSC)
gravitational wave (GW)
GW detection (GW)
HD 189733 b
Laplace-Lagrange secular theory
mass ratio (μ)
M-type star (M)
minimum mass (m sin i)
main-sequence lifetime (MS lifetime)
neutron star (NS)
post-common envelope binary (PCEB)
probability mass function (PMF)
proper motion (PM)
Hulse-Taylor Binary (PSR B1913+16)
pulsar timing array (PTA)
radial velocity (RV)
Rossiter-McLaughlin effect (RM effect)
Catalogue of Southern Double Stars
radial velocity method
spectroscopic binary (SB)
stellar population synthesis (SPS)
stellar-mass black hole
stellar distance determination
stellar parameter determination
surface gravity (g)
symbiotic binary (SS)
transit timing variations (TTV)
turn-off point (TO)
Type Ia supernova
Vogt-Russell theorem (VR theorem)
wide binaries (WB)
X-ray burster (XRB)