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This variation may be caused by a change in emitted light or by
something partly blocking the light, so variable stars are classified as
either:
- Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
- Extrinsic variables, whose apparent changes in brightness are due to
changes in the amount of their light that can reach Earth; for example,
because the star has an orbiting companion that sometimes eclipses it.
Many, possibly most, stars have at least some variation in luminosity: the energy output of our
Sun, for example, varies by about 0.1% over an 11-year
solar cycle.
Discovery
An
ancient Egyptian calendar of lucky and unlucky days composed some 3,200
years ago may be the oldest preserved historical document of the
discovery of a variable star,
the eclipsing binary
Algol.
Of the modern astronomers, the first variable star was identified in 1638 when
Johannes Holwarda noticed that
Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by
David Fabricius in 1596. This discovery, combined with
supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as
Aristotle
and other ancient philosophers had taught. In this way, the discovery
of variable stars contributed to the astronomical revolution of the
sixteenth and early seventeenth centuries.
The second variable star to be described was the eclipsing variable Algol, by
Geminiano Montanari in 1669;
John Goodricke gave the correct explanation of its variability in 1784.
Chi Cygni was identified in 1686 by
G. Kirch, then
R Hydrae in 1704 by
G. D. Maraldi. By 1786 ten variable stars were known. John Goodricke himself discovered
Delta Cephei and
Beta Lyrae.
Since 1850 the number of known variable stars has increased rapidly,
especially after 1890 when it became possible to identify variable stars
by means of photography.
The latest edition of the
General Catalogue of Variable Stars
(2008) lists more than 46,000 variable stars in the Milky Way, as well
as 10,000 in other galaxies, and over 10,000 'suspected' variables.
Detecting variability
The
most common kinds of variability involve changes in brightness, but
other types of variability also occur, in particular changes in the
spectrum. By combining
light curve data with observed spectral changes, astronomers are often able to explain why a particular star is variable.
Variable star observations
Variable stars are generally analysed using
photometry,
spectrophotometry and
spectroscopy. Measurements of their changes in brightness can be plotted to produce
light curves. For regular variables, the
period of variation and its
amplitude
can be very well established; for many variable stars, though, these
quantities may vary slowly over time, or even from one period to the
next. Peak brightnesses in the light curve are known as
maxima, while troughs are known as
minima.
Amateur astronomers can do useful scientific study of variable stars by visually comparing the star with other stars within the same
telescopic
field of view of which the magnitudes are known and constant. By
estimating the variable's magnitude and noting the time of observation a
visual lightcurve can be constructed. The
American Association of Variable Star Observers collects such observations from participants around the world and shares the data with the scientific community.
From the light curve the following data are derived:
- are the brightness variations periodical, semiperiodical, irregular, or unique?
- what is the period of the brightness fluctuations?
- what is the shape of the light curve (symmetrical or not,
angular or smoothly varying, does each cycle have only one or more than
one minima, etcetera)?
From the spectrum the following data are derived:
- what kind of star is it: what is its temperature, its luminosity class (dwarf star, giant star, supergiant, etc.)?
- is it a single star, or a binary? (the combined spectrum of a binary
star may show elements from the spectra of each of the member stars)
- does the spectrum change with time? (for example, the star may turn hotter and cooler periodically)
- changes in brightness may depend strongly on the part of the
spectrum that is observed (for example, large variations in visible
light but hardly any changes in the infrared)
- if the wavelengths of spectral lines are shifted this points to
movements (for example, a periodical swelling and shrinking of the star,
or its rotation, or an expanding gas shell) (Doppler effect)
- strong magnetic fields on the star betray themselves in the spectrum
- abnormal emission or absorption lines may be indication of a hot stellar atmosphere, or gas clouds surrounding the star.
In very few cases it is possible to make pictures of a stellar disk. These may show darker spots on its surface.
Interpretation of observations
Combining
light curves with spectral data often gives a clue as to the changes
that occur in a variable star. For example, evidence for a pulsating
star is found in its shifting spectrum because its surface periodically
moves toward and away from us, with the same frequency as its changing
brightness.
About two-thirds of all variable stars appear to be pulsating. In the 1930s astronomer
Arthur Stanley Eddington
showed that the mathematical equations that describe the interior of a
star may lead to instabilities that cause a star to pulsate. The most
common type of instability is related to oscillations in the degree of
ionization in outer, convective layers of the star.
Suppose the star is in the swelling phase. Its outer layers
expand, causing them to cool. Because of the decreasing temperature the
degree of ionization also decreases. This makes the gas more
transparent, and thus makes it easier for the star to radiate its
energy. This in turn will make the star start to contract. As the gas is
thereby compressed, it is heated and the degree of ionization again
increases. This makes the gas more opaque, and radiation temporarily
becomes captured in the gas. This heats the gas further, leading it to
expand once again. Thus a cycle of expansion and compression (swelling
and shrinking) is maintained.
The pulsation of cepheids is known to be driven by oscillations in the ionization of
helium (from He
++ to He
+ and back to He
++).
Nomenclature
In a given constellation, the first variable stars discovered were designated with letters R through Z, e.g.
R Andromedae. This system of
nomenclature was developed by
Friedrich W. Argelander, who gave the first previously unnamed variable in a constellation the letter R, the first letter not used by
Bayer. Letters RR through RZ, SS through SZ, up to ZZ are used for the next discoveries, e.g.
RR Lyrae.
Later discoveries used letters AA through AZ, BB through BZ, and up to
QQ through QZ (with J omitted). Once those 334 combinations are
exhausted, variables are numbered in order of discovery, starting with
the prefixed V335 onwards.
Classification
Variable stars may be either intrinsic or extrinsic.
- Intrinsic variable stars: stars where the variability is
being caused by changes in the physical properties of the stars
themselves. This category can be divided into three subgroups.
- Pulsating variables, stars whose radius alternately expands and
contracts as part of their natural evolutionary ageing processes.
- Eruptive variables, stars who experience eruptions on their surfaces like flares or mass ejections.
- Cataclysmic or explosive variables, stars that undergo a cataclysmic change in their properties like novae and supernovae.
- Extrinsic variable stars: stars where the variability is caused by external properties like rotation or eclipses. There are two main subgroups.
- Eclipsing binaries, double stars where, as seen from Earth's vantage point the stars occasionally eclipse one another as they orbit.
- Rotating variables, stars whose variability is caused by phenomena
related to their rotation. Examples are stars with extreme "sunspots"
which affect the apparent brightness or stars that have fast rotation
speeds causing them to become ellipsoidal in shape.
These subgroups themselves are further divided into specific types of
variable stars that are usually named after their prototype. For
example, dwarf novae are designated U Geminorum stars after the first recognized star in the class, U Geminorum.
Intrinsic variable stars
Pulsating variable stars
The pulsating stars swell and shrink, affecting their brightness and spectrum. Pulsations are generally split into:
radial, where the entire star expands and shrinks as a whole; and
non-radial, where one part of the star expand while another part shrinks.
Depending on the type of pulsation and its location within the star, there is a natural or
fundamental frequency which determines the period of the star. Stars may also pulsate in a
harmonic or
overtone
which is a higher frequency, corresponding to a shorter period.
Pulsating variable stars sometimes have a single well-defined period,
but often they pulsate simultaneously with multiple frequencies and
complex analysis is required to determine the separate
interfering periods. In some cases, the pulsations do not have a defined frequency, causing a random variation, referred to as
stochastic. The study of stellar interiors using their pulsations is known as
asteroseismology.
The expansion phase of a pulsation is caused by the blocking of
the internal energy flow by material with a high opacity, but this must
occur at a particular depth of the star to create visible pulsations. If
the expansion occurs below a convective zone then no variation will be
visible at the surface. If the expansion occurs too close to the surface
the restoring force will be too weak to create a pulsation. The
restoring force to create the contraction phase of a pulsation can be
pressure if the pulsation occurs in a non-degenerate layer deep inside a
star, and this is called an
acoustic or
pressure mode of pulsation, abbreviated to
p-mode. In other cases, the restoring force is
gravity and this is called a
g-mode. Pulsating variable stars typically pulsate in only one of these modes.
Cepheids and cepheid-like variables
This group consists of several kinds of pulsating stars, all found on the
instability strip, that swell and shrink very regularly caused by the star's own mass
resonance, generally by the
fundamental frequency. Generally the
Eddington valve
mechanism for pulsating variables is believed to account for
cepheid-like pulsations. Each of the subgroups on the instability strip
has a fixed relationship between period and absolute magnitude, as well
as a relation between period and mean density of the star. The
period-luminosity relationship was first established for Delta Cepheids
by
Henrietta Leavitt, and makes these high luminosity Cepheids very useful for determining distances to galaxies within the
Local Group and beyond.
Edwin Hubble used this method to prove that the so-called spiral nebulae are in fact distant galaxies.
Note that the Cepheids are named only for
Delta Cephei, while a completely separate class of variables is named after
Beta Cephei.
Classical Cepheid variables
Classical Cepheids (or Delta Cephei variables) are population I
(young, massive, and luminous) yellow supergiants which undergo
pulsations with very regular periods on the order of days to months. On
September 10, 1784,
Edward Pigott detected the variability of
Eta Aquilae, the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids is the star
Delta Cephei, discovered to be variable by
John Goodricke a few months later.
Type II Cepheids
Type II Cepheids (historically termed W Virginis stars) have
extremely regular light pulsations and a luminosity relation much like
the δ Cephei variables, so initially they were confused with the latter
category. Type II Cepheids stars belong to older
Population II stars, than do the type I Cepheids. The Type II have somewhat lower
metallicity,
much lower mass, somewhat lower luminosity, and a slightly offset
period verses luminosity relationship, so it is always important to know
which type of star is being observed.
RR Lyrae variables
These stars are somewhat similar to Cepheids, but are not as luminous
and have shorter periods. They are older than type I Cepheids,
belonging to
Population II, but of lower mass than type II Cepheids. Due to their common occurrence in
globular clusters, they are occasionally referred to as
cluster Cepheids.
They also have a well established period-luminosity relationship, and
so are also useful as distance indicators. These A-type stars vary by
about 0.2–2 magnitudes (20% to over 500% change in luminosity) over a
period of several hours to a day or more.
Delta Scuti variables
Delta Scuti (δ Sct) variables are similar to Cepheids but much fainter and with much shorter periods. They were once known as
Dwarf Cepheids.
They often show many superimposed periods, which combine to form an
extremely complex light curve. The typical δ Scuti star has an amplitude
of 0.003–0.9 magnitudes (0.3% to about 130% change in luminosity) and a
period of 0.01–0.2 days. Their
spectral type is usually between A0 and F5.
SX Phoenicis variables
These stars of spectral type A2 to F5, similar to δ Scuti variables,
are found mainly in globular clusters. They exhibit fluctuations in
their brightness in the order of 0.7 magnitude (about 100% change in
luminosity) or so every 1 to 2 hours.
Rapidly oscillating Ap variables
These stars of spectral type A or occasionally F0, a sub-class of δ
Scuti variables found on the main sequence. They have extremely rapid
variations with periods of a few minutes and amplitudes of a few
thousandths of a magnitude.
Long period variables
The long period variables are cool evolved stars that pulsate with periods in the range of weeks to several years.
Mira variables
Mira variables are AGB red giants. Over periods of many months they fade and brighten by between 2.5 and 11
magnitudes, a sixfold to 30 thousandfold change in luminosity.
Mira
itself, also known as Omicron Ceti (ο Cet), varies in brightness from
almost 2nd magnitude to as faint as 10th magnitude with a period of
roughly 332 days. The very large visual amplitudes are mainly due to the
shifting of energy output between visual and infra-red as the
temperature of the star changes. In a few cases, Mira variables show
dramatic period changes over a period of decades, thought to be related
to the thermal pulsing cycle of the most advanced AGB stars.
Semiregular variables
These are
red giants or
supergiants.
Semiregular variables may show a definite period on occasion, but more
often show less well-defined variations that can sometimes be resolved
into multiple periods. A well-known example of a semiregular variable is
Betelgeuse,
which varies from about magnitudes +0.2 to +1.2 (a factor 2.5 change in
luminosity). At least some of the semi-regular variables are very
closely related to Mira variables, possibly the only difference being
pulsating in a different harmonic.
Slow irregular variables
These are
red giants or
supergiants
with little or no detectable periodicity. Some are poorly studied
semiregular variables, often with multiple periods, but others may
simply be chaotic.
Long secondary period variables
Many variable red giants and supergiants show variations over several
hundred to several thousand days. The brightness may change by several
magnitudes although it is often much smaller, with the more rapid
primary variations are superimposed. The reasons for this type of
variation are not clearly understood, being variously ascribed to
pulsations, binarity, and stellar rotation.
Beta Cephei variables
Beta Cephei (β Cep) variables (sometimes called
Beta Canis Majoris variables, especially in Europe)
undergo short period pulsations in the order of 0.1–0.6 days with an
amplitude of 0.01–0.3 magnitudes (1% to 30% change in luminosity). They
are at their brightest during minimum contraction. Many stars of this
kind exhibits multiple pulsation periods.
Slowly pulsating B-type stars
Slowly pulsating B (SPB) stars are hot main-sequence stars slightly
less luminous than the Beta Cephei stars, with longer periods and larger
amplitudes.
Very rapidly pulsating hot (subdwarf B) stars
The prototype of this rare class is
V361 Hydrae, a 15th magnitude
subdwarf B star.
They pulsate with periods of a few minutes and may simultaneous pulsate
with multiple periods. They have amplitudes of a few hundredths of a
magnitude and are given the GCVS acronym RPHS. They are
p-mode pulsators.
PV Telescopii variables
Stars in this class are type Bp supergiants with a period of 0.1–1
day and an amplitude of 0.1 magnitude on average. Their spectra are
peculiar by having weak
hydrogen while on the other hand
carbon and
helium lines are extra strong, a type of
Extreme helium star.
RV Tauri variables
These are yellow supergiant stars (actually low mass post-AGB stars
at the most luminous stage of their lives) which have alternating deep
and shallow minima. This double-peaked variation typically has periods
of 30–100 days and amplitudes of 3–4 magnitudes. Superimposed on this
variation, there may be long-term variations over periods of several
years. Their spectra are of type F or G at maximum light and type K or M
at minimum brightness. They lie near the instability strip, cooler than
type I Cepheids more luminous than type II Cepheids. Their pulsations
are caused by the same basic mechanisms related to helium opacity, but
they are at a very different stage of their lives.
Alpha Cygni variables
Alpha Cygni (α Cyg) variables are nonradially pulsating supergiants of
spectral classes B
ep to A
epIa.
Their periods range from several days to several weeks, and their
amplitudes of variation are typically of the order of 0.1 magnitudes.
The light changes, which often seem irregular, are caused by the
superposition of many oscillations with close periods.
Deneb, in the constellation of
Cygnus is the prototype of this class.
Gamma Doradus variables
Gamma Doradus (γ Dor) variables are non-radially pulsating main-sequence stars of
spectral classes F to late A. Their periods are around one day and their amplitudes typically of the order of 0.1 magnitudes.
Pulsating white dwarfs
These non-radially pulsating stars have short periods of hundreds to
thousands of seconds with tiny fluctuations of 0.001 to 0.2 magnitudes.
Known types of pulsating white dwarf (or pre-white dwarf) include the DAV, or ZZ Ceti, stars, with hydrogen-dominated atmospheres and the spectral type DA; DBV, or V777 Her, stars, with helium-dominated atmospheres and the spectral type DB; and GW Vir stars, with atmospheres dominated by helium, carbon, and oxygen. GW Vir stars may be subdivided into DOV and PNNV stars.
Solar-like oscillations
The
Sun
oscillates with very low amplitude in a large number of modes having
periods around 5 minutes. The study of these oscillations is known as
helioseismology. Oscillations in the Sun are driven stochastically by
convection in its outer layers. The term
solar-like oscillations
is used to describe oscillations in other stars that are excited in the
same way and the study of these oscillations is one of the main areas
of active research in the field of
asteroseismology.
BLAP variables
A Blue Large-Amplitude Pulsator (BLAP) is a pulsating star
characterized by changes of 0.2 to 0.4 magnitudes with typical periods
of 20 to 40 minutes.
Eruptive variable stars
Eruptive
variable stars show irregular or semi-regular brightness variations
caused by material being lost from the star, or in some cases being
accreted to it. Despite the name these are not explosive events, those
are the cataclysmic variables.
Protostars
Protostars are young objects that have not yet completed the process
of contraction from a gas nebula to a veritable star. Most protostars
exhibit irregular brightness variations.
Herbig Ae/Be stars
Variability of more massive (2–8
solar mass)
Herbig Ae/Be stars is thought to be due to gas-dust clumps, orbiting in the circumstellar disks.
Orion variables
Orion variables are young, hot
pre–main-sequence stars
usually embedded in nebulosity. They have irregular periods with
amplitudes of several magnitudes. A well-known subtype of Orion
variables are the
T Tauri variables. Variability of
T Tauri stars is due to spots on the stellar surface and gas-dust clumps, orbiting in the circumstellar disks.
FU Orionis variables
These stars reside in reflection nebulae and show gradual increases
in their luminosity in the order of 6 magnitudes followed by a lengthy
phase of constant brightness. They then dim by 2 magnitudes (six times
dimmer) or so over a period of many years.
V1057 Cygni for
example dimmed by 2.5 magnitude (ten times dimmer) during an eleven-year
period. FU Orionis variables are of spectral type A through G and are
possibly an evolutionary phase in the life of
T Tauri stars.
Giants and supergiants
Large
stars lose their matter relatively easily. For this reason variability
due to eruptions and mass loss is fairly common among giants and
supergiants.
Luminous blue variables
Also known as the
S Doradus variables, the most luminous stars known belong to this class. Examples include the
hypergiants η Carinae and
P Cygni.
They have permanent high mass loss, but at intervals of years internal
pulsations cause the star to exceed its Eddington limit and the mass
loss increases hugely. Visual brightness increases although the overall
luminosity is largely unchanged. Giant eruptions observed in a few LBVs
do increase the luminosity, so much so that they have been tagged
supernova impostors, and may be a different type of event.
Yellow hypergiants
These massive evolved stars are unstable due to their high luminosity
and position above the instability strip, and they exhibit slow but
sometimes large photometric and spectroscopic changes due to high mass
loss and occasional larger eruptions, combined with secular variation on
an observable timescale. The best known example is
Rho Cassiopeiae.
R Coronae Borealis variables
While classed as eruptive variables, these stars do not undergo
periodic increases in brightness. Instead they spend most of their time
at maximum brightness, but at irregular intervals they suddenly fade by
1–9 magnitudes (2.5 to 4000 times dimmer) before recovering to their
initial brightness over months to years. Most are classified as yellow
supergiants by luminosity, although they are actually post-AGB stars,
but there are both red and blue giant R CrB stars.
R Coronae Borealis (R CrB) is the prototype star.
DY Persei variables are a subclass of R CrB variables that have a periodic variability in addition to their eruptions.
Wolf–Rayet variables
Wolf–Rayet stars are massive hot stars that sometimes show
variability, probably due to several different causes including binary
interactions and rotating gas clumps around the star. They exhibit broad
emission line spectra with
helium,
nitrogen,
carbon and
oxygen lines. Variations in some stars appear to be stochastic while others show multiple periods.
Gamma Cassiopeiae variables
Gamma Cassiopeiae
(γ Cas) variables are non-supergiant fast-rotating B class emission
line-type stars that fluctuate irregularly by up to 1.5 magnitudes
(fourfold change in luminosity) due to the ejection of matter at their
equatorial regions caused by the rapid rotational velocity.
Flare stars
In main-sequence stars major eruptive variability is exceptional. It is common only among the
flare stars, also known as the
UV Ceti
variables, very faint main-sequence stars which undergo regular flares.
They increase in brightness by up to two magnitudes (six times
brighter) in just a few seconds, and then fade back to normal brightness
in half an hour or less. Several nearby red dwarfs are flare stars,
including
Proxima Centauri and
Wolf 359.
RS Canum Venaticorum variables
These are close binary systems with highly active chromospheres,
including huge sunspots and flares, believed to be enhanced by the close
companion. Variability scales ranges from days, close to the orbital
period and sometimes also with eclipses, to years as sunspot activity
varies.
Cataclysmic or explosive variable stars
Images showing the expansion of the light echo of a red variable star, the V838 Monocerotis
Supernovae
Supernovae
are the most dramatic type of cataclysmic variable, being some of the
most energetic events in the universe. A supernova can briefly emit as
much energy as an entire
galaxy,
brightening by more than 20 magnitudes (over one hundred million times
brighter). The supernova explosion is caused by a white dwarf or a star
core reaching a certain mass/density limit, the
Chandrasekhar limit,
causing the object to collapse in a fraction of a second. This collapse
"bounces" and causes the star to explode and emit this enormous energy
quantity. The outer layers of these stars are blown away at speeds of
many thousands of kilometers an
hour. The expelled matter may form nebulae called
supernova remnants. A well-known example of such a nebula is the
Crab Nebula, left over from a supernova that was observed in
China and
North America in 1054. The core of the star or the white dwarf may either become a
neutron star (generally a
pulsar) or disintegrate completely in the explosion.
Supernovae can result from the death of an extremely massive
star, many times heavier than the Sun. At the end of the life of this
massive star, a non-fusible iron core is formed from fusion ashes. This
iron core is pushed towards the Chandrasekhar limit till it surpasses it
and therefore collapses.
A supernova may also result from mass transfer onto a
white dwarf
from a star companion in a double star system. The Chandrasekhar limit
is surpassed from the infalling matter. The absolute luminosity of this
latter type is related to properties of its light curve, so that these
supernovae can be used to establish the distance to other galaxies. One
of the most studied supernovae is
SN 1987A in the
Large Magellanic Cloud.
Novae
Novae
are also the result of dramatic explosions, but unlike supernovae do not
result in the destruction of the progenitor star. Also unlike
supernovae, novae ignite from the sudden onset of thermonuclear fusion,
which under certain high pressure conditions (
degenerate matter) accelerates explosively. They form in close
binary systems,
one component being a white dwarf accreting matter from the other
ordinary star component, and may recur over periods of decades to
centuries or millennia. Novae are categorised as
fast,
slow or
very slow, depending on the behaviour of their light curve. Several
naked eye novae have been recorded,
Nova Cygni 1975 being the brightest in the recent history, reaching 2nd magnitude.
Dwarf novae
Dwarf novae are double stars involving a
white dwarf in which matter transfer between the component gives rise to regular outbursts. There are three types of dwarf nova:
- U Geminorum stars,
which have outbursts lasting roughly 5–20 days followed by quiet
periods of typically a few hundred days. During an outburst they
brighten typically by 2–6 magnitudes. These stars are also known as SS Cygni variables after the variable in Cygnus which produces among the brightest and most frequent displays of this variable type.
- Z Camelopardalis stars, in which occasional plateaux of brightness called standstills are seen, part way between maximum and minimum brightness.
- SU Ursae Majoris stars, which undergo both frequent small outbursts, and rarer but larger superoutbursts. These binary systems usually have orbital periods of under 2.5 hours.
DQ Herculis variables
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.
Z Andromedae variables
These symbiotic binary systems are composed of a red giant and a hot
blue star enveloped in a cloud of gas and dust. They undergo nova-like
outbursts with amplitudes of up to 4 magnitudes. The prototype for this
class is
Z Andromedae.
AM CVn variables
AM CVn variables are symbiotic binaries where a white dwarf is
accreting helium-rich material from either another white dwarf, a helium
star, or an evolved main-sequence star. They undergo complex
variations, or at times no variations, with ultrashort periods.
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
Ellipsoidal variables
These
are very close binaries, the components of which are non-spherical due
to their mutual gravitation. 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.
Stellar spots
The surface of the star 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 we observe brightness variations of a few tenths of magnitudes.
FK Comae Berenices variables
BY Draconis stars are of spectral class K or M and vary by less than 0.5 magnitudes (70% change in luminosity).
Magnetic fields
Alpha-2 Canum Venaticorum variables
Alpha-2 Canum Venaticorum (α
2 CVn) variables are
main-sequence
stars of spectral class B8–A7 that show fluctuations of 0.01 to 0.1
magnitudes (1% to 10%) due to changes in their magnetic fields.
SX Arietis variables
Stars in this class exhibit brightness fluctuations of some 0.1
magnitude caused by changes in their magnetic fields due to high
rotation speeds.
Optically variable pulsars
Few
pulsars have been detected in
visible light. These
neutron stars
change in brightness as they rotate. Because of the rapid rotation,
brightness variations are extremely fast, from milliseconds to a few
seconds. The first and the best known example is the
Crab Pulsar.
Eclipsing binaries
How eclipsing binaries vary in brightness
Extrinsic variables have variations in their brightness, as seen by
terrestrial observers, due to some external source. One of the most
common reasons for this is the presence of a binary companion star, so
that the two together form a
binary star. When seen from certain angles, one star may
eclipse the other, causing a reduction in brightness. One of the most famous eclipsing binaries is
Algol, or Beta Persei (β Per).
Algol variables
Algol variables undergo eclipses with one or two minima separated by
periods of nearly constant light. The prototype of this class is
Algol in the
constellation Perseus.
Double Periodic variables
Double periodic variables exhibit cyclical mass exchange which causes
the orbital period to vary predictably over a very long period. The
best known example is
V393 Scorpii.
Beta Lyrae variables
Beta Lyrae (β Lyr) variables are extremely close binaries, named after the star
Sheliak.
The light curves of this class of eclipsing variables are constantly
changing, making it almost impossible to determine the exact onset and
end of each eclipse.
W Serpentis variables
W
Serpentis is the prototype of a class of semi-detached binaries
including a giant or supergiant transferring material to a massive more
compact star. They are characterised, and distinguished from the similar
β Lyr systems, by strong UV emission from accretions hotspots on a disc
of material.
W Ursae Majoris variables
The stars in this group show periods of less than a day. The stars
are so closely situated to each other that their surfaces are almost in
contact with each other.
Planetary transits
Stars with
planets
may also show brightness variations if their planets pass between Earth
and the star. These variations are much smaller than those seen with
stellar companions and are only detectable with extremely accurate
observations. Examples include
HD 209458 and
GSC 02652-01324, and all of the planets and planet candidates detected by the
Kepler Mission.
