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.