6.3 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Variable star | 9/10 | https://en.wikipedia.org/wiki/Variable_star | reference | science, encyclopedia | 2026-05-05T13:33:52.261350+00:00 | kb-cron |
DQ Herculis systems are interacting binaries in which a low-mass star transfers mass to a highly magnetic white dwarf. The white dwarf spin period is significantly shorter than the binary orbital period and can sometimes be detected as a photometric periodicity. An accretion disk usually forms around the white dwarf, but its innermost regions are magnetically truncated by the white dwarf. Once captured by the white dwarf's magnetic field, the material from the inner disk travels along the magnetic field lines until it accretes. In extreme cases, the white dwarf's magnetism prevents the formation of an accretion disk.
==== AM Herculis variables ====
In these cataclysmic variables, the white dwarf's magnetic field is so strong that it synchronizes the white dwarf's spin period with the binary orbital period. Instead of forming an accretion disk, the accretion flow is channeled along the white dwarf's magnetic field lines until it impacts the white dwarf near a magnetic pole. Cyclotron radiation beamed from the accretion region can cause orbital variations of several magnitudes. BY Cam-type systems are known as asynchronous polars due to a slight (1–2%) difference between the rotation period and the orbital period. This asynchronity is believed to be caused by flare activity on the accreting white dwarf.
==== X-ray binaries ====
High mass X-ray binaries consist of a Be star or a supergiant in a relatively close orbit with a neutron star companion. Mass is being transferred to the accreting compact object from the donor star, which results in X-ray emission. In the case of a Be star, a gaseous disk orbiting the star at the equator is responsible for the optical variability, while the interaction of the companion is truncating the disk.
== Extrinsic variable stars == There are two main groups of extrinsic variables: rotating stars and eclipsing stars.
=== Rotating variable stars === Stars with sizeable sunspots may show significant variations in brightness as they rotate, and brighter areas of the surface are brought into view. Bright spots also occur at the magnetic poles of magnetic stars. Stars with ellipsoidal shapes may also show changes in brightness as they present varying areas of their surfaces to the observer.
==== Non-spherical stars ==== Rotating stars can vary in brightness due to their shape.
Ellipsoidal variables
These are very close binaries, the components of which are non-spherical due to their tidal interaction. As the stars rotate the area of their surface presented towards the observer changes and this in turn affects their brightness as seen from Earth. In 1920, Pi5 Orionis became the first system where this effect was clearly detected. The light curves of these systems are often quasi-sinusoidal in shape. A comparison with the radial velocity curve will often suffice to determine if the system is ellipsoidal. A further clue to an ellipsoidal variable is a rounding of the light curve at maximum. The reflection-effect, where each component illuminates the facing hemisphere of the nearby companion, is a further complication to orbit and shape analysis.
==== Stellar spots ====
For a magnetically active star, the surface is not uniformly bright, but has darker and brighter areas (like the sun's solar spots). The star's chromosphere too may vary in brightness. As the star rotates, brightness variations of a few tenths of magnitudes are observed.
FK Comae Berenices variables
These stars rotate extremely rapidly for an evolved star (~100 km/s at the equator); hence they are ellipsoidal in shape. They are (apparently) single giant stars with spectral types G and K and show strong chromospheric emission lines. Examples of this rare class are FK Com, V1794 Cygni and YY Mensae. A possible explanation for the rapid rotation of FK Comae stars is that they are the result of the merger of a (contact) binary.
BY Draconis variable stars
BY Draconis stars are a common type of variable with spectral class F, G, K or M that vary by less than 0.5 magnitudes (70% change in luminosity) with periods of a few days. The stellar magnetic field creates an inhomogeneous pattern of dark star spots and bright faculae across the surface, which are carried into and out of the line of sight by the star's rotation. These stars are typically young and rapidly rotating; they show strong emission lines in their spectrum. As the star ages, the interaction of the magnetic field with the stellar wind drags down the rotation rate, lowering the activity level. Many stars in the spectral range of F to M-class, including the Sun, display various levels of surface activity that is driven by their magnetic dynamo. The activity can be concentrated in latitude ranges and the amplitude can vary over time based on one or more stellar cycles. For example, the Sun has a single activity cycle lasting about 11 years, with the Sun turning slightly more blue during peak activity. A long period of chromospheric inactivity on an otherwise active star is termed a Maunder minimum. This rare event happened to the Sun during the 17th century, but, as of 2012, has not been definitely identified on another star.
==== Magnetic fields ==== These variables have a magnetic field but lack significant chromospheric activity.
Alpha2 Canum Venaticorum variables
Alpha2 Canum Venaticorum (α2 CVn or ACV) variables are magnetic chemically peculiar stars of spectral class B0–F0 that show fluctuations of 0.01 to 0.1 magnitudes (1% to 10%) due to differences of elemental abundances as inhomogeneously distributed across the stellar surface. I.e. they have "chemical spots". They also display spectral and magnetic field variability as they slowly rotate. The cause of the inhomogeneous energy distribution on these stars is thought to be enhanced line blanketing as chemical spectral lines are made more intense by the Zeeman effect. This results in added heating of some layers of the atmosphere as the UV flux is converted to the visual band through backwarming. This class of variable is named after the first Ap star to show rotationally-modulated photometric variability, Alpha2 Canum Venaticorum.
SX Arietis variables