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Variable Stars


Abstract

Variable Stars

Keywords: stars,

Introduction

Some stars have a variable brightness, and are called variable stars. Some of the important parameters of variability include the timescale over which variation occurs, the regularity of the variations, the degree (magnitude) of variation, and the wavelengths effected. Many of these features can be captured in a light curve (find some examples?) A fundamental distinction to make is between extrinsic variables where the light output from the star remains constant but appears to vary from our vantage point for some other reason, and intrinsic variables where the actual output of light from the star varies.

 
 

Related Topics


Further Reading

Related Pages

  • Stellar Magnitude
  • Herzsprung-Russell Diagram
  • Novae
  • Luminous Blue Variables
  • Stellar Winds ???

Other Web Sites

  • GCVS
 
 

Extrinsic Variables

extrinsic variables where the light output from the star remains constant but appears to vary from our vantage point for some other reason, and

Eclipsing Variables

These stars vary in brightness because they are periodically eclipsed. The prototype eclipsing variable is Algol in Perseus (b Persei; R.A. 03 08.2, dec. +40 57; mag. 2.2 to 3.4; period 2.87 days; spectrum B + G), comprising a blue dwarf and a fainter yellow subgiant.

Another eclipsing binary e.g. Zeta Aurigae

EA Light Curves

 

Ellipsoidal Variables

Ellipsoidal variables are binary stars whose components are ellipsoidal in shape owing to their closeness and strong mutual attraction, with the result that the cross-sections of their surfaces presented to us vary. The light variation is slight but exactly periodic. Eclipses do not occur because our line of sight does not lie in the orbital plane.

Intrinsic Variables

intrinsic variables where the actual output of light from the star varies.

Whereas extrinsic variables can be of almost any star type, intrinsics occupy distinct areas on a Hertzsprung-Russell diagram. The main types are pulsating variables and eruptive variables.

Pulsating Variables

Of the stars which vary in intrinsic brightness, most (about 90%) do so because of changes in their size, and are known collectively as pulsating variables. There are several types; some of the common types are described below.

Cepheid Variables

Cepheids are F-K supergiants that show asymmetric but synchronised variation in light and radial velocity, attributed to radial pulsation of the whole star. The period is typically between 1 and 80 days.

Consider an analogy to a heavy piston supported by compressed gas. If the piston is pushed down from its equilibrium position and suddenly released, it begins to oscillate. At first it moves upwards, driven by the excessively compressed air beneath. However, the inertia of the heavy piston carries it beyond the equilibrium point, overshooting the rest position. Now the gas is insufficiently compressed to support the weight of the piston, so it begins to fall again. In an ideal closed system, it oscillates forever, but in both our analogy and in most real stars, energy is continually radiated away; the equilibrium position is overshot by a smaller amount with each iteration, until the oscillations cease altogether. However, if the system is kept energetic, e.g. by irradiation, the oscillations can continue indefinitely. This is the basis of the Eddington-Zhevakhin model for cepheid variables. "In 1953 Zhevakhin ... showed that in the case of a Delta Cephei star the effects of opacity properties of surface regions [which prevents energetic radiation from the core escaping] were sufficiently strong to overcome the damping effect in the rest of the star and to allow the star to oscillate" (Kippenhahn 1992, p. 109).

The pulsations are mainly driven in the subsurface layers where singly ionised helium is being further ionised (Smith 1995, p. 219).

Conditions producing the cepheid phenomenon seem to be restricted to a particular temperature range only a few hundred degrees wide, translating to a narrow strip on the HRD: the so-called instability strip.

The prototype cepheid variable is Delta Cephei (R.A. ???, dec. ???). However, there are several sub-types besides the classical cepheids, most notably the W Virginis stars which occupy a slightly different position on the Hertzsprung-Russell diagram. The importance of cepheid variables is that their period is related to their absolute brightness, allowing their use to calculate distance from their apparent brightness.

RR Lyrae Stars

RR Lyrae variables are old population II blue giant stars (type A, rarely type F), frequently found in globular clusters, which vary by 0.5 to 1.5 magnitudes in less than one and a half days. The prototype, RR Lyrae itself, varies from apparent magnitude 7.4 to 8.6 in 0.57 days.

Mira Stars

Old red giant and supergiant stars frequently pulsate, but less regularly than the types described above. Mira (Omicron Ceti; R.A. 02 19.3, dec. -02 59; mag. 1.7 to 10.1; period 332.0 days; spectrum M) is the prototype for the most common of all variables which have periods ranging from about three months to two years, during which they can vary by several magnitudes. The exact period and amplitude differs slightly from cycle to cycle.

Semi-Regular and Irregular Stars

Semi-Regular and irregular stars are giants and supergiants having variations in brightness which show periodicity only on short time scales (semi-regular) or else no periodic behaviour at all (irregular). Semi-regular stars are divided into sub-types based on their spectral type; irregulars can be of almost any spectral type at all.

A particular group, T-Tauri stars, are irregular variables with amplitudes ranging from 1m to 4m. These are stars being newly formed, before reaching the ZAMS and commencing to 'burn' hydrogen (read more).

Erupting Variables

Finally, some stars vary in brightness owing to eruptions associated with the ejection of matter from their surfaces. Novae and supernovae are the most spectacular examples.

Somewhat less spectacular, but dramatic enough for all that, are luminous blue variable stars like Eta Carinae. At its most luminous, Eta Carinae has equalled the apparent brightness of Canopus, although it has since faded but a factor over 1000. The star is known as a "mass loss" star; periods of brightness occur when the star ejects part of its outer surface. The ejecta from its last period of great brightness (about 1830 to 1840) now forms a small nebula, the Homunculus Nebula. Eta Carinae is the most extreme star we know from our galaxy. It is an amazing three million times more luminous than the sun (Malin 1993, pp. 153-155; read more).

Mass loss stars can sometimes be identified from the 'bubbles' they blow in the interstellar medium. Two examples are Henize 44 and Henize 70 in the LMC.

References

Kippenhahn, Rudolph 1992: 100 Billion Suns: The Birth, Life, and Death of the Stars. Princeton University Press.

Smith, Robert C. 1995: Observational Astrophysics. Cambridge. 443 pp.


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