Chapter 3 — About Variable Stars

The Naming of Variable Stars

examples:  SS Cyg Z Cam alpha Ori V2134 Sgr

Click here for a table that lists all of the official constellation names and abbreviations.

There are also some special kinds of star names. For instance, sometimes stars are given temporary names until such time as the editors of the General Catalogue of Variable Stars assign the star a permanent name. An example of this would be N Cyg 1998-a nova in the constellation of Cygnus which was discovered in 1998. Another case is of a star that is suspected but not confirmed to be variable. These stars are given names such as NSV 251 or CSV 3335. The first part of this name indicates the catalogue in which the star is published, while the second part is the catalogue entry number for that star.

Variable Star Designations

examples: 2138+43
1405-12A 0214-03 1151+58

Note that in one example given, the designation is followed by the letter "A". This is because there is another variable in the proximity, with the designation 1405-12B which was discovered later.

Click here for more information on the Harvard Designation of variable stars.

Types of Variable Stars

Generally, long period and semiregular pulsating variables are recommended for beginners to observe. These stars have a wide range of variation. Also, they are sufficiently numerous that many of them are found close to bright stars, which is very helpful when it comes to locating them.

A brief description of the major types in each class is covered in this chapter. There is also mention of the star's spectral type. If you are interested in learning more about stellar spectra and stellar evolution, you can find information on these subjects in basic astronomy texts or in some of the books mentioned in Appendix 3.

PULSATING VARIABLES

Cepheid — delta Cep

Cepheids - Cepheid variables pulsate with periods from 1 to 70 days, with light variations from 0.1 to 2 magnitudes. 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 the period-luminosity relationship. Cepheid variables may be good candidates for student projects because they are bright and have short periods.

RR Lyrae stars - These are short-period (.05 to 1.2 days), pulsating, white giant stars, usually of spectral class A. They are older and less massive than Cepheids. The amplitude of variation of RR Lyrae stars is generally from 0.3 to 2 magnitudes.

RV Tauri stars - These are yellow supergiants having a characteristic light variation with alternating deep and shallow minima. Their periods, defined as the interval between two deep minima, range from 30 to 150 days. The light variation may be as much as 3 magnitudes. 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 - Long Period Variables (LPVs) are pulsating red giants or supergiants with periods ranging from 30-1000 days. They are usually of spectral type M, R, C or N. There are two subclasses; Mira and Semiregular.

Mira - These periodic red giant variables vary with periods ranging from 80 to 1000 days and visual light variations of more than 2.5 magnitudes.

Mira (omicron Ceti)

Semiregular - These are giants and supergiants showing appreciable periodicity accompanied by intervals of semiregular or irregular light variation. Their periods range from 30 to 1000 days, generally with amplitude variations of less than 2.5 magnitudes.

Semiregular — Z UMa

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.

CATACLYSMIC VARIABLES

Supernovae - These massive stars show sudden, dramatic, and final magnitude increases of 20 magnitudes or more, as a result of a catastrophic stellar explosion.

SN 1987A

Novae - These close binary systems consist of an accreting white dwarf as a primary and a low-mass main sequence star (a little cooler than the Sun) as the secondary star. Explosive nuclear burning of the surface of the white dwarf, from accumulated material from the secondary, causes the system to brighten 7 to 16 magnitudes in a matter of 1 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 A or F giant stars.

Nova — V1500 Cyg

A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star eta Carinae. This star was the site of a giant outburst about 150 years ago, when it became one of the brightest stars in the southern sky. Though the star released as much visible light as a supernova explosion, it survived the outburst.

Recurrent Novae - These objects are similar to novae, but have two or more slightly smaller-amplitude outbursts during their recorded history.

Recurrent Nova — RS Oph

Dwarf Novae - These are close binary systems made up of a red dwarf-a little cooler than our Sun, a white dwarf, and an accretion disk surrounding the white dwarf. The brightening by 2 to 6 magnitudes is due to instability in the disk which forces the disk material to drain down (accrete) onto the white dwarf. There are three main subclasses of dwarf novae; U Gem, Z Cam, and SU UMa stars.

U Geminorum - After intervals of quiescence at minimum light, they suddenly brighten. Depending on the star, the eruptions occur at intervals of 30 to 500 days and last generally 5 to 20 days.

U Gem

Z Camelopardalis - These stars are physically similar to U Gem stars. They 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.

Z Cam

SU Ursae Majoris - Also physically similar to U Gem stars, 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.

SU UMa

U Geminorum		To the left are 20-second exposures of U Gem before outburst and after the start of an outburst. Images were taken by AAVSO Director Arne Henden, USRA/USNO, using a CCD with a V filter on the U. S. Naval Observatory 1.0-m telescope in Flagstaff, AZ. Beneath the photos is the artist, Mark A. Garlick's rendition of a CV system (note the sun-like star to the right, the white dwarf, and the accretion disk surrounding the white dwarf).

Symbiotic stars - These close binary systems consist of a red giant and a hot blue star, both embedded in nebulosity. They show semi-periodic, nova-like outbursts, up to three magnitudes in amplitude.

Symbiotic — Z And

R Coronae Borealis - These rare, luminous, hydrogen-poor, carbon-rich, supergiants 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.

R CrB

ECLIPSING BINARY STARS

Eclipsing Binary — beta Per

ROTATING STARS

What is a Light Curve?

Information about the periodic behavior of stars, the orbital period of eclipsing binaries, or the degree of regularity (or irregularity) of stellar eruptions, can be directly determined from the light curve. More detailed analysis of the light curve allows astronomers to calculate such information as the masses or sizes of stars. Several years or decades of observational data can reveal the changing period of a star, which could be a signal of a change in the structure of the star.

Phase Diagrams

• Visit the light curve generator