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.
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.
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Variable Star Naming Conventions Variable star names are determined by a committee appointed by the International Astronomical Union (I.A.U.). The assignments are made in the order in which the variable stars were discovered in a constellation. If one of the stars that has a Greek letter name is found to be variable, the star will still be referred to by that name. Otherwise, the first variable in a constellation would be given the letter R, the next S, and so on to the letter Z. The next star is named RR, then RS, and so on to RZ; SS to SZ, and so on to ZZ. Then, the naming starts over at the beginning of the alphabet: AA, AB, and continuing on to QZ. This system (the letter J is omitted) can accommodate 334 names. There are so many variables in some constellations in the Milky Way, however, that additional nomenclature is necessary. After QZ, variables are named V335, V336, and so on. The letters representing stars are then combined with the genitive Latin form of the constellation name as given in Table 3.1. For all but the most formal usage, and for reports you submit to the AAVSO, the three letter abbreviations should be used. This system of nomenclature was initiated in the mid-1800s by Friedrich Argeländer. He started with an uppercase R for two reasons: the lowercase letters and the first part of the alphabet had already been allocated for other objects, leaving capitals towards the end of the alphabet mostly unused. Argeländer also believed that stellar variability was a rare phenomenon and that no more than 9 variables would be discovered in any constellation (which is certainly not the case!). |
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.
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.
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.
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.
Supernovae - These massive stars show sudden, dramatic, and final magnitude increases of 20 magnitudes or more, as a result of a catastrophic stellar explosion.
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.
 | 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.
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.
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.
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.
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 member 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). |
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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.
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.
| Courage! Each step forward brings us nearer the goal, and if we can not reach it, we can at least work so that posterity shall not reproach us for being idle or say that we have not at least made an effort to smooth the way for them.
- Friedrich Argeländer (1844) the “father of variable star astronomy” |