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Stars - Variable Stars

When stars initiate core helium fusion, the star can become unstable and pulsate. In fact, there are several regions on an H-R diagram that are instability zones. If a stars evolution passes through one of these zones, pulsations can occur:

(Image Credit - Swinburne Astronomy Online)

Variable stars are named after a prototype star that exhibits a particular pulsation period.

What makes the star pulsate are the expanding and contracting of a stars outer shell. As the star increases in size, so does luminosity - the opposite for contraction. Increased heat from inside the star causes the expansion - the expansion then causes cooling. The cooled layers contract only to be heated again.

One of the more famous variables stars is the family of Cepheid variable stars. Discovered by Henrietta Leavitt, these stars have a pronounced Period-Luminosity relationship. Longer period Cepheid were brighter than shorter period Cepheids. Because they are luminous, they can be seen over great distanced. Using the distance modulus, it is possible to use the Cepheid variable as a ruler for interstellar measurements.

(Image credit: Brooks/Cole Thomson Learning)

Cepheid rulers have been used for nearby galaxies, but Type 1a supernova are better for intergalactic distances.

RR Lyrae variable also make good rulers, and are commonly found in globular clusters. However, they pulsate faster and have about a 1 magnitude variation in brightness.

Classes of Variable Stars:

Variable Type: Population Type: Period of Variability:
Long-Period Variables Population I and Population II 100 - 700 days
Classical Cepheids Population I 1 - 50 days
Type II Cepheids (W Virginis Stars) Population II 2 - 45 days
RR Lyrae Stars Population II 1.5 - 24 hours
delta Scuti Stars Population I 1 - 3 hours
beta Cephei Stars Population I 3 - 7 hours
ZZ Ceti Stars Population I 100 - 1000 seconds

Another class of Variable star is the cataclysmic variable star. These are destructive events. There are no know patterns to these types of variable stars:

  • Classical Nova - recurrent explosions of a star-white dwarf binary pair
  • recurrent nova - a white dwarf binary that continues pulling material from a host star
  • Supernova - the life's end of a large star

Only the recurring nova can be termed a "variable star" although the variability is unpredictable.

More on this is available in Stellar Evolution in both the low mass and high mass sections.

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