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·Astronomy: Observatory - WHERE IN THE WORLD

  Variable Star: Astronomy: Variable Star - CLASSES OF VARIABLES
Posted on Saturday, December 11 @ 01:28:14 CST by astronomy
  Variable Star


There are three broad classes ot variable stars: pulsating, eruptive, and eclipsing.

  • Pulsating variables brighten and fade as their outer layers rhythmically contract and expand. They are further classified according to the pattern of their periods.
    A well-known pulsating variable. Delta Cephei. is the prototype for the Cepheid variables. These have regular periods ranging from one to several days. The longer a
    Cepheid's period, the greater its absolute magnitude. This relationship is so reliable that astronomers use Cepheids as "standard candles" to calculate the distances to nearby galaxies.
    Long-period stars, such as Mira, are red giants that pulsate like Cepheids do, but their cycles are less regular and their periods are longer, ranging from 80 days to 5 years.
    Other pulsating variables forming their own subtypes include RR Lyrae and RV Tauri. Semi-regular and irregular variables are also kinds of pulsating variables.
  • Cataclysmic, or eruptive, variables exhibit sudden and large brightness outbursts. Many are novae, binary systems that erupt once in a cycle that can last thousands of years. Dwarf novae, such as U Geminorum, erupt more frequently, with cycles lasting weeks. Supernovae are often classed as cataclysmic, but their outbursts happen just once and destroy the star.
    Sometimes included in the cataclysmic category are the peculiar R Coronae Borealis stars. At irregular intervals, these rapidly fade by as much as 6 to 8 magnitudes, possibly because of an outbreak of carbon soot in their atmosphere.
  • Eclipsing variables, such as Beta Lyrae, are binary systems in which one star eclipses the other during each orbital period. From Earth, we see this as a periodic decrease in light output, followed by a return to normal brightness.



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