Variable Stars Explained

What Are They?

The Surface Of Mira

Mira, a popular variable star, is one of only three stars to have images taken showing the shape and actual surface of the star. The other two are the Sun and Betelgeuse.

More info on Mira can be found in our Variable Star of the Month archives.

Image taken with the HST by Dr. Margarita Karovksa, AAVSO Council Member, & NASA.

Variable stars are stars that change brightness. The brightness changes of these stars can range from a thousandth of a magnitude to as much as twenty magnitudes over periods of a fraction of a second to years, depending on the type of variable star. Over 30,000 variable stars are known and catalogued, and many thousands more are suspected to be variable. There are a number of reasons why variable stars change their brightness. Pulsating variables, for example, swell and shrink due to internal forces. While an eclipsing binary will dim when it is eclipsed by a faint companion; and then brightens when the occulting star moves out of the way. The different causes for light variation in variable stars provides the impetus for classifying the stars into different categories.

Variable stars are classified as either intrinsic, wherein variability is caused by physical changes such as pulsation or eruption in the star or stellar system (pulsating variables and eruptive variables, or extrinsic, wherein variability is caused by the eclipse of one star by another or by the effects of stellar rotation (eclipsing binaries and rotating variables).

Why Observe Variable Stars

Variable stars need to be systematically observed over decades in order to: determine the long-time behavior of a star, provide professional astronomers with data needed to analyze variable star behavior, to schedule observations of certain stars, to correlate data from satellite and ground-based observations, and to make computerized theoretical models of variable stars.

Research on variable stars is important because it provides information about stellar properties, such as mass, radius, luminosity, temperature, internal and external structure, composition, and evolution. This information can then be used to understand other stars. Professional astronomers have neither the available time nor the unlimited telescope access needed to gather data on the brightness changes of thousands of variable stars. Thus it is amateur astronomers utilizing visual, photographic, photoelectric, and now CCD techniques, who are making a real and highly useful contribution to science by observing variable stars and submitting their observations to the AAVSO International Database.

Types of Variable Stars

Pulsating Variables

Pulsating Variables are stars that 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. The following types of pulsating variables may be distinguished by the pulsation period, the mass and evolutionary status of the star, and the characteristics of their pulsations. Cepheids (Period: 1-70 days; Amplitude of variation: .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. The later the spectral class of a Cepheid, the longer is its period. Cepheids obey a strict period-luminosity relationship. An example of a Cepheid variable light curve is given below.

RR Lyrae stars (Period: 30-100 days; Amplitude of variation: .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.

Long Period Variables (LPVs)
(Period: 80-1000 days; Amplitude of variation: 2.5 to 5.0 mag.)
These are giant red variables that show characteristic emission lines. The spectral classes range through M, C, and S. Also known as ôMirasö after the prototype star.

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.

Cataclysmic Variables

Cataclysmic variables (also known as Eruptive variables), as the name implies, are stars that have occasional violent outbursts caused by thermonuclear processes either in their surface layers or deep within their interiors.

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, 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 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.

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.

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 Geminorum (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 ôstandstillsö. These standstills last the equivalent of several cycles, with the star ôstuckö 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 (ôsuperoutburstö) is bright, less frequent, and long, with a duration of 10 to 20 days. During superoutbursts, small periodic modulations (ôsuperhumpsö) appear.

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 Coronae 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.

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.

Rotating Stars

Rotating stars show small changes in light that may be due to dark or bright spots, or patches on their stellar surfaces (ôstarspotsö). Rotating stars are often binary systems.

Other Types of Variable Stars

The following types of stars are not recommended for observation by inexperienced observer due to either their irregularity, or the small amplitude of variation that they exhibit.

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.

Irregular variables
These stars, which include the majority of red giants, are pulsating variables. As the name implies, these stars show luminosity changes with either no periodicity or with a very slight periodicity.

The GCVS (General Catalogue of Variable Stars) classification of variable stars provides a thorough description of the different types of variable stars.