Beta Lyrae variable
Encyclopedia
Beta Lyrae variables are a class of close binary star
s. Their total brightness is variable
because the two component stars orbit each other, and in this orbit one component periodically passes in front of the other one, thereby blocking its light. The two component stars of Beta Lyrae systems are quite heavy (several solar
masses each) and extended (giants
or supergiant
s). They are so close, that their shapes are heavily distorted by mutual gravitation
forces: the stars have ellipsoidal shapes, and there are extensive mass flows from one component to the other.
, has become a giant or supergiant. Such extended stars easily lose mass, just because they are so large: gravitation at their surface is weak, so gas easily escapes (the so-called stellar wind
). In close binary systems such as beta Lyrae systems, a second effect reinforces this mass loss: when a giant star swells, it may reach its Roche limit
, that is, a mathematical surface surrounding the two components of a binary star where matter may freely flow from one component to the other.
In binary stars the heaviest star generally is the first to evolve into a giant or supergiant. Calculations show that its mass loss then will become so large that in a comparatively very short time (less than half a million years) this star, that was once the heaviest, now becomes the lighter of the two components. Part of its mass is transferred to the companion star, the rest is lost in space.
s of beta Lyrae variables are quite smooth: eclipses start and end so gradually that the exact moments are impossible to tell. This is because the flow of mass between the components is so large that it envelopes the whole system in a common atmosphere. The amplitude
of the brightness variations is in most cases less than one magnitude
; the largest amplitude known is 2.3 magnitudes (V480 Lyrae).
The period of the brightness variations is very regular. It is determined by the revolution period of the binary, the time it takes for the two components to once orbit around each other. These periods are short, typically one or a few days. The shortest known period is 0.29 days (QY Hydrae); the longest is 198.5 days (W Crucis). In beta Lyrae systems with periods longer than 100 days one of the components generally is a supergiant
.
Beta Lyrae systems are sometimes considered to be a subtype of the Algol variable
s; however, their light curves are different (the eclipse
s of Algol variables are much more sharply defined). On the other hand, beta Lyrae variables look a bit like W Ursae Majoris variable
s; however, the latter are in general yet closer binaries (so-called contact binaries), and their component stars are mostly lighter than the beta Lyrae system components (about one solar mass).
s is β Lyrae, also called Sheliak. Its variability was discovered in 1784 by John Goodricke
.
Nearly a thousand β Lyrae binaries are known: the latest edition of the General Catalogue of Variable Stars (2003) lists 835 of them (2.2% of all variable stars). Data for the ten brightest β Lyrae variables are given below. (See also the list of known variable stars.)
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
s. Their total brightness is variable
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
because the two component stars orbit each other, and in this orbit one component periodically passes in front of the other one, thereby blocking its light. The two component stars of Beta Lyrae systems are quite heavy (several solar
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
masses each) and extended (giants
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
or supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...
s). They are so close, that their shapes are heavily distorted by mutual gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
forces: the stars have ellipsoidal shapes, and there are extensive mass flows from one component to the other.
Mass flows
These mass flows occur because one of the stars, in the course of its evolutionStellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
, has become a giant or supergiant. Such extended stars easily lose mass, just because they are so large: gravitation at their surface is weak, so gas easily escapes (the so-called stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
). In close binary systems such as beta Lyrae systems, a second effect reinforces this mass loss: when a giant star swells, it may reach its Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...
, that is, a mathematical surface surrounding the two components of a binary star where matter may freely flow from one component to the other.
In binary stars the heaviest star generally is the first to evolve into a giant or supergiant. Calculations show that its mass loss then will become so large that in a comparatively very short time (less than half a million years) this star, that was once the heaviest, now becomes the lighter of the two components. Part of its mass is transferred to the companion star, the rest is lost in space.
Light curves
The light curveLight curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
s of beta Lyrae variables are quite smooth: eclipses start and end so gradually that the exact moments are impossible to tell. This is because the flow of mass between the components is so large that it envelopes the whole system in a common atmosphere. The amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
of the brightness variations is in most cases less than one magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
; the largest amplitude known is 2.3 magnitudes (V480 Lyrae).
The period of the brightness variations is very regular. It is determined by the revolution period of the binary, the time it takes for the two components to once orbit around each other. These periods are short, typically one or a few days. The shortest known period is 0.29 days (QY Hydrae); the longest is 198.5 days (W Crucis). In beta Lyrae systems with periods longer than 100 days one of the components generally is a supergiant
Supergiant
Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...
.
Beta Lyrae systems are sometimes considered to be a subtype of the Algol variable
Algol variable
Algol variables or Algol-type binaries are a class of eclipsing binary stars where the orbital plane of the stars are coincident with the line of sight from Earth. When the cooler component passes in front of the hotter one, part of the latter's light is blocked, and the total brightness of the...
s; however, their light curves are different (the eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
s of Algol variables are much more sharply defined). On the other hand, beta Lyrae variables look a bit like W Ursae Majoris variable
W Ursae Majoris variable
A W Ursae Majoris variable is a type of eclipsing binary variable star. These stars are close binaries, whose surfaces are in contact with one another. They are termed contact binaries because the two stars touch and they essentially share material in their outer layers. Through the neck between...
s; however, the latter are in general yet closer binaries (so-called contact binaries), and their component stars are mostly lighter than the beta Lyrae system components (about one solar mass).
Examples of β Lyrae stars
The prototype of the β Lyrae type variable starVariable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
s is β Lyrae, also called Sheliak. Its variability was discovered in 1784 by John Goodricke
John Goodricke
John Goodricke FRS was an eminent and profoundly deaf amateur astronomer. He is best known for his observations of the variable star Algol in 1782.- Life and work :...
.
Nearly a thousand β Lyrae binaries are known: the latest edition of the General Catalogue of Variable Stars (2003) lists 835 of them (2.2% of all variable stars). Data for the ten brightest β Lyrae variables are given below. (See also the list of known variable stars.)
| star | type* | period (days) | apparent visualmagnitude Apparent magnitude The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere... (max, min) |
spectrum Stellar classification In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure... |
distance(lightyears) |
|---|---|---|---|---|---|
| ζ And Zeta Andromedae Zeta Andromedae is a star system in the constellation Andromeda. It is approximately 181 light years from Earth.... |
EB/GS/RS | 17.7695 | 3.92-4.14 | K1II-III | 181 |
| UW CMa | ~EB/KE | 4.393407 | 4.84-5.33 | O7Ia:fp+OB | ~3000 |
| τ CMa Tau Canis Majoris Tau Canis Majoris is an eclipsing spectroscopic binary in the constellation Canis Major and the brightest star of an open cluster designated NGC 2362... |
EB | 1.28 | 4.32-4.37 | O9Ib | ~3000 |
| β Lyr Beta Lyrae Beta Lyrae is a binary star system approximately 882 light-years away in the constellation Lyra. Beta Lyrae has the traditional name Sheliak , from الشلياق šiliyāq, the Arabic name of the constellation Lyra.Beta Lyrae is an eclipsing semi-detached binary system made up of a B7II primary star and... (prototype) |
EB | 12.913834 | 3.25-4.36 | B8II-IIIep | 880 |
| δ Pic | ~EB/D | 1.672541 | 4.65-4.90 | B3III+O9V | 1700 |
| V Pup | EB/SD | 1.4544859 | 4.35-4.92 | B1Vp+B3: | 1200 |
| PU Pup | EB | 2.57895 | 4.69-4.75 | B9 | 550 |
| υ Sgr Upsilon Sagittarii Upsilon Sagittarii is a spectroscopic binary star system in the constellation Sagittarius. Upsilon Sagittarii is the prototypical hydrogen-deficient binary , and one of only four such systems known... |
EB/GS | 137.939 | 4.53-4.61 | B8pI:+O9V ? (or F2p?) | ~1700 |
| μ1 Sco | EB/SD | 1.44626907 | 2.94-3.22 | B1.5V+B6.5V | 800 |
| π Sco Pi Scorpii Pi Scorpii is a triple star system in the constellation Scorpius. It is approximately 459 light years from Earth.Pi Scorpii A is a contact binary of the Beta Lyrae type. Both its members are hot, blue-white B-type dwarfs. Together they have a mean apparent magnitude of +2.89. Their rotation... |
EB | 1.57 | 2.82-2.85 | B1V+B2V | 460 |
| *) EB = Beta Lyrae variable; for other codes see: General Catalogue of Variable Stars | |||||