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Transcript
General Astronomy
Variable Stars
Variable Stars
The observations of differences in the brightness of variable stars, start from the
antiquity.
• In 134 B.C, Iparchus observed the flash of a nova.
• Regular observations of variations in brightness of stars were not been reported
until 1572 A.D.
– This year, Fabricius observed a star in Cetus that was not shown in any atlas. Some
months later, the star disappeared and it was rediscovered in 1603. This is the star
Mira, the first variable star known.
•
•
In 1669, Montanari discovered the variation of Algol. Arabians had probably
observed the variations in brightness of ß Perseus, a hypothesis that can explain
the Arabian name of Algol (al ghual - demon).
Discoveries became more and more frequent subsequently
– in 1844, 18 variables were known
– by 1920, 2054 variables were known
•
There are now over 28,000 stars known to be variable, and 14,000 more that are
suspected to be changing in brightness in our galaxy, the Milky Way.
Chi Cygni
Ranges from 12th magnitude to nearly 3.5th magnitude over
a period of about 406 days (Long Period Variable)
At minimum
At maximum
Types of Variable Stars
• There are two types of variable stars
– intrinsic
• variation is due to physical changes in the star
or stellar system
– extrinsic
• variability is due to the eclipse of one star by
another or the effects of stellar rotation.
Much of this information comes from the AAVSO web site at
www.aavso.org
Types of Variable Stars
Intrinsic Variability
• Pulsating
– Show periodic expansion and contraction of their
surface layers.
– Pulsations may be radial or non-radial.
• A radially pulsating star remains spherical in shape, while
a star experiencing non-radial pulsations may deviate
from a sphere periodically.
• Eruptive
– Stars that have occasional violent outbursts
caused by thermonuclear processes either in their
surface layers or deep within their interiors
– Also known as Cataclysmic Variables
Pulsating Variables
• Cepheids
– (Period: 1-70 days; Amplitude of variation: 0.1 to 2.0
mag.)
– These massive stars have high luminosity and are of
F spectral class at maximum, and G to K at minimum.
– Cepheids obey a strict period-luminosity relationship.
– Two types: Classical and W Virginis
Pulsating Variables
• RR Lyrae stars
– Period: .2 to 1.0 days; Amplitude of variation: 0.3 to 2 mag.
– These are short-period, pulsating, white giant stars, usually of
spectral class A.
– They are older and less massive than Cepheids.
• RV Tauri stars
– Period: 30-100 days; Amplitude of variation: up to 3.0 mag
– These are yellow supergiants having a characteristic light
variation with alternating deep and shallow minima. Their
periods are defined as the interval between two deep minima.
Some of these stars show long-term cyclic variations from
hundreds to thousands of days. Generally, the spectral class
ranges from G to K.
Pulsating Variables
• Mira (Omicron Ceti)
– Long Period Variables (LPVs)
– Period: 80-1000 days; Amplitude of variation: 2.5 to 7.0 mag.
– These are giant red variables that show characteristic emission lines.
The spectral classes range through M, C, and S.
• SemiRegular
– Period: 30-1000 days; Amplitude of variation: 1.0 to 2.0 mag.
– These are giants and supergiants showing appreciable periodicity
accompanied by intervals of irregular light variation.
Eruptive Variables
• Supernovae
– Period: none; Amplitude of variation: 20+
– These massive stars show sudden, dramatic, and final
magnitude increases as a result of a catastrophic stellar
explosion.
• Novae
– Period: 1-300+days; Amplitude of variation: 7-16 mag.
– These close binary systems consist of a main sequence, Sunlike 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 the initial brightness over several years or decades.
Near maximum brightness, the spectrum is generally similar to
that of an A or F giant star.
Eruptive Variables
• Recurrent Novae
– Period: 1-200+days; Amplitude of variation: 7-16 mag.
– These objects are similar to novae, but have two or more slightly
smaller-amplitude outbursts during their recorded history.
• Symbiotic Stars
– Period: semi-periodic; Amplitude of variation: up to 3 mag.
– 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
• R Corona Borealis
– Period: irregular; Amplitude of variation: up to 9 mag.
– 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.
Eruptive Variables
• 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.
– There are three sub-classes of dwarf novae:
• U Gemenorium
– (Period: 30-500 days: Amplitude range variation: 2-6 mag.)
– After intervals of quiescence at minimum light, they suddenly
brighten. The duration of outburst is generally from 5 to 20 days.
• Z Camelopardalis
– These systems show cyclic variations, interrupted by intervals of
constant brightness called “standstills”. These standstills last the
equivalent of several cycles, with the star “stuck” at the brightness
approximately one-third of the way from maximum to minimum
• SU Ursae Majoris
– These systems have two distinct kinds of outbursts: one is faint,
frequent, and short, with a duration of 1 to 2 days; the other
(“superoutburst”) is bright, less frequent, and long, with a duration
of 10 to 20 days. During superoutbursts, small periodic modulations
(“superhumps”) appear
Eruptive Variables
• Gamma Cassiopea
– These are hot variables with Be spectra. They are probably
very young and still slightly eruptive.
– They have characteristically fast rotation, which broadens the
spectra lines. At the equator, the rotation velocity is only
slightly less than escape velocity; if there is a slight eruption, a
cloud of hydrogen escapes.
– The type star, γ Cas is the brightest of the class, but it includes
other well-known stars, such as Pleione (also known as BU
Tau) in the Pleiades cluster. Frequent ejections of material
have created a shell around Pleione.
– The brightness variations in these stars are small, being less
than 0.5 magnitude, but from time to time absorption of light
by the shell may cause a deeper decline.
Eclipsing Binary Stars
These are binary systems of stars with an orbital
plane lying near the line-of-sight of the observer.
The components periodically eclipse one another,
causing a decrease in the apparent brightness of
the system as seen by the observer.
The period of the eclipse, which coincides with the
orbital period of the system, can range from
minutes to years.
Examples: Algol, Beta Persei
Eclipsing Binary Stars
Beta Lyrae (Sheliak) is an eclipsing contact binary star system.
Its two component stars are close enough that material from
the photosphere of each is pulled towards the other, drawing
the stars into an ellipsoid shape. Beta Lyrae is the prototype
for this class of eclipsing binaries, whose components are so
close together that they deform by their mutual gravitation.
Rotating Stars
Rotating stars are often binary systems,
which undergo small amplitude changes
in light that may be due to dark or bright
spots or patches on their stellar surface,
or may be due to thermal or chemical
inhomogeniety of stellar atmospheres
caused by magnetic fields.
Flare Stars
•
•
•
•
•
These stars are dim, red dwarfs that exhibit unusually violent flare
activity.
Flares occur sporadically, with successive flares spaced anywhere from
an hour to a few days apart. It only takes a few minutes for a flare to
reach peak brightness, and in fact more than one flare can occur at time.
It may turn out that most red dwarfs are flare stars, and that red
dwarfs without violent flare activity are the exception rather than the
rule.
The first known flare stars (V1396 Cygni and AT Microscopii) were
discovered in 1924. However, the best-known flare star (UV Ceti) was
discovered in 1948, and today flare stars are sometimes known as UV
Ceti variables.
The Sun's nearest stellar neighbor Proxima Centauri is a flare star, as is
another near neighbor Wolf 359. Barnard's Star, the second nearest
star system, is also suspected of being a flare star. Because they are so
intrinsically faint, all known flare stars are within about 60 light years
from Earth.