What are Variable Stars?
Quite simply, variable stars are stars which vary in brightness over
long or short periods of time, with amplitudes of variation from a thousandth
of a magnitude to as much as 20 magnitudes. Variable stars may be found
to be periodic (with a single period or multiple periods at once) with
little or no irregularity in their light curves (plots of brightness
vs. time), semi-periodic with some irregularity and periodicity, and
irregular with no apparent periodicity in their light curves whatsoever.
Depending on the type of variable star being observed (see below), periods
can range from a fraction of a second to years. In light of this, one
can say that nearly all stars are variable stars to a certain extent
since all stars possess constantly changing physical properties and
features affecting their brightness either periodically or irregularly.
Types of Variable Stars
Variable stars are classified according to the main cause of their
variation in brightness. Most generally, variable stars are classified
as either intrinsic or extrinsic. For intrinsic variables,
variability is caused by physical changes intrinsic to the star (eg.
radial or non-radial vibration or "pulsation"; eruptions in
the star), while for extrinsic variables, variability is caused by changes
which are external to the star as a whole (eg. the effects of stellar
rotation; two stars eclipsing each other in a binary system, causing
an apparent dimming in the brightness of the system).
are classified as either pulsating or eruptive variables.
In pulsating variables, variation in brightness is
caused by periodic expansion and contraction in the star's surface layers.
These pulsations may be radial (in which case the star retains its spherical
shape) or non-radial (resulting in periodic deviations from a spherical
shape). Pulsating variables themselves can be divided up into different
types according to their pulsation period, mass, and evolutionary status:
Cepheids: These stars have periods of
1-70 days with amplitudes of variation from 0.1 to 2.0 magnitudes. Cepheids
obey a strict period-luminosity relationship with Cepheids of higher
luminosities having longer periods, both depending on the radius. Therefore,
by measuring the period of a Cepheid variable, one can obtain its luminosity,
and by measuring its apparent brightness one may deduce how far away
RR Lyrae stars: These stars have periods
of 0.2-1.2 days with amplitudes of variation from 0.3 to 2 magnitudes.
These pulsating variables are white giant stars of spectral class A.
RV Tauri stars: These stars have periods
of 30-150 days with amplitudes of variation up to 3.0 magnitudes. They
are yellow supergiants generally of spectral classes from G to K.
Long Period Variables (Miras): These stars
have periods of 80-1000 days with amplitudes of variation from 2.5 to
5.0 magnitudes. They are giant red variables with spectral classes ranging
through M, C, and S.
Semiregular stars: These stars have periods
of 30-1000 days with amplitudes of variation from 1.0 to 2.0 magnitudes.
They are giants and supergiants displaying periodicity superimposed
with intervals of irregular light variation.
Small-Amplitude Pulsating Red Giants (SAPRGs):
These stars have periods of 5-100 days with amplitudes of variation
from 0.05 to 1 magnitude. As their name (also called small-amplitude
red variables) suggests, these stars are red giants. Due to their instability,
a majority of red giants physically expand and contract (pulsate) periodically
as a result of convective processes. Convection in red giants involves
the cyclic motion of huge cells of hot gas. This results in periodic
changes in luminosity with small amplitudes of variation. The pulsations
may be radial or non-radial as mentioned above. These stars may also
In eruptive variables,
variation in brightness is caused by occasional violent eruptions as
a result of complex processes deep within the interior of the star or
in the surface layers. The most common types of eruptive variables are:
Supernovae: These stars show sudden, dramatic,
and final magnitude increases as a result of a catastrophic stellar
explosion. Thus, there is no period, and amplitudes of variation are
Novae: These close binary systems consist
of a main sequence, Sun-like star and a white dwarf. They increase in
brightness by 7 to 16 magnitudes in a matter of one to several hundred
days. After the outburst, the star fades slowly to its initial brightness
over several years or decades. Near maximum brightness, the spectrum
is generally similar to that of an A or F giant star. Periods are typically
1-300+ days, and amplitudes of variation are 7-16 magnitudes.
Recurrent Novae: These objects are similar
to novae, but have two or more slightly smaller-amplitude outbursts
during their recorded history. Periods are 1-200+ days, and amplitudes
of variation are 7-16 magnitudes.
Dwarf Novae: These are close binary systems
made up of a Sun-like star, a white dwarf, and an accretion disk surrounding
the white dwarf. The accretion disk "erupts" every few weeks.
Symbiotic Stars: These close binary systems
consist of a red giant and a hot blue star, both embedded in nebulosity.
They show nova-like outbursts, up to three magnitudes in amplitude,
and are semi-periodic.
R Coronae Borealis Stars: These are rare,
luminous, hydrogen-poor, carbon-rich, variables that spend most of their
time at maximum light, occasionally fading as much as nine magnitudes
at irregular intervals. They then slowly recover to their maximum brightness
after a few months to a year. Members of this group have F to K and
R spectral types.
Flare Stars: Also known as UV Ceti stars,
these are intrinsically faint, cool, red, main-sequence stars that undergo
intense outbursts from localized areas of the surface. The result is
an increase in brightness of two or more magnitudes in several seconds,
followed by a decrease to its normal minimum in about 10 to 20 minutes.
Extrinsic variables are
classified as either eclipsing binary or rotating
variables. In eclipsing binaries, the object in question
is a binary star system with an orbital plane lying near the line-of-sight
of the observer, and light variation is due to the two stars making
up the system eclipsing each other. The system appears to dim when one
star passes in front of the other (in relation to the observer on earth)
and subsequently brightens when the star moves out of the way so that
both stars are exposed and contributing to the overall brightness of
the system. The period of variation coincides with the orbital period
of the system and can range from a few minutes to several years.
In rotating variable
stars, variation in brightness is usually small and results in the rotation
of the star exposing dark or bright spots, or patches ("starspots")
on its surface. Rotating variable stars are often binary systems.
For general information on variable stars, www.aavso.org/vstar/
has a lot of useful topics. For more detailed information about specific
stars the variable
star of the month archive has an excellent description of many stars.
For a review of Astronomical Terms, click here.
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