Cepheid Variables

Abstract

The defining characteristics of cepheid variable stars are presented and interpreted in an evolutionary context. Some occurences are listed with their known physical properties.

Keywords: Cepheid, variable, delta Cephei, W Virginis

Introduction

Although interesting in their own right, from a stellar modelling perspective, much of the importance of cepheid variables derives from the relation between their period and their absolute brightness. This relationship means that, once calibrated, they can be used as 'standard candles' to find the distances to distant stellar associations – including other galaxies.

The period-luminosity relationship was first discovered by the American Henrietta Swan Leavitt in the course of studying cepheids in the Small Magellanic Cloud. Leavitt, knowing that all of the d cepheids in the SMC lie at roughly the same distance (in the same way that all of the towns in England are roughly the same distance from Wellington) was able to deduce the period-luminosity relation from measuring their periods against their apparent brightness, then calibrating the relation from cepheids for which the distance was known by other means (presumably parallax).

Using this relationship, and making appropriate adjustments for intervening dust, it is possible to deduce the distance to any cepheid variable from its apparent brightness. In this way, for example, Edwin Hubble following up on work performed initially by John C. Duncan (see Pagels 1985, p. 95) first determined the distance to the galaxy in Andromeda, thus finally laying to rest the belief that the "spiral nebulae" were part of our own galaxy.

Classical Cepheids

These are population I yellow or red supergiant stars (type A to F at maximum, G to K at minimum) with periods ranging from 2 to 40 days and varying in brightness by up to one magnitude. For example, d Cephei itself varies from mag. 3.6 to 4.3 over 5.4 days. (At dec. 58 25, it is a good object for observation in the northern hemisphere.)

W-Virginis Stars

Whereas the classical d type cepheids are fairly closely concentrated in the galactic plane, other cepheids have been found at high galactic latitudes with considerable velocity components perpendicular to the galactic plane, suggesting they belong to population II. These are the W-Virginis stars. Their light curves and periods - mostly about 18 days - are similar to classical cepheids, but there are some differences as shown in figures 1 and 2.

For a given period, the W-Virginis stars are about 1.5 to 2.0 magnitudes less than classical d cepheids. On the average, W-Virginis stars are of an earlier spectral type, and emission lines sometimes occur in their spectra.

Characteristics

refer Kaler 1989 pp. 156 et seq.

Spectra

xxx

Mass and Luminosity

xxx

Variability/Mass Loss

xxx

Interpretation

xxx

Occurrence/Examples

xxx

Study of ZZ Carinae

ZZ (or l) Carinae (R.A. 09 43.9, dec. -62 16.6; epoch 1950) is a d Cephei type variable. It varies from about 4^m.33 to 5^m.51 over the period of 35.5412 days. At maximum it belongs to spectral type F8; at minimum to type K01b.

The star was monitored over several weeks with 7x50mm binoculars. Magnitude was estimated visually by comparison with neighbouring stars.

A finder chart, raw data, and a graph follows.

The finder chart shows iota (i) Carinae, upsilon (u) Carinae, and N Velorum as the main reference points. Several of the stars nearby l Carinae (such as R Carinae) cannot be used as comparison stars since they are themselves variable. Next to N Vel. is IC 2488, an open cluster of around 70 stars at 7 ^m.4. The open cluster IC 2602 around q Carinae (not shown) is another convenient reference point for those of us who cannot see a damned thing once we remove our spectacles, and have to star hop even to find the right constellation!

Fig. 5

Cloudy nights plagued the observation period. Also, the early observations were compromised by inexperience with evaluating magnitudes and other "traps for young players" (e.g. magnitude comparisons with other variable stars). There is not much sign of the correct 35 day period, although the fact that the star is variable around magnitude 4 over a few tens of days is clear.

References

Burnham, Robert (1989a): Burnham’s Celestial Handbook. Volume 1 – Andromeda to Cetus. Dover.

— (1989b): Burnham’s Celestial Handbook. Volume 2 – Chamaeleon to Orion. Dover.

— (1992): Burnham’s Celestial Handbook. Volume 3 – Pavo to Vulpecula. Dover.

Carroll, Bradley W.; Ostlie, Dale A. (1996): Introduction to Modern Astrophysics. Addison Wesley.

Kaler, James B. (1997): Stars and Their Spectra. Cambridge. (Corrected paperback ed.) 300 pp.

Tirion, Wil (1991): Cambridge Star Atlas 2000.0. Cambridge.