Variable Stars

Don't all stars change over time? Over millions of years yes, but some vary over periods of months, days, ..., sometimes even hours!
Why would stars vary? Some are going through an unstable period in their lives (more than just the usual astroseismology oscillations, or "starquakes"), others just look like they vary, but what we're seeing is something different. For example, the first "variable star" ever discovered was Algol, which in reality is a (strange) eclipsing binary star.

  Non-Periodic Variable Stars

  • What are they? Stars whose luminosity changes in time, but not in a regularly repeating pattern.
  • Different types: Individual young stars like T-Tauri stars or stars with irregular obscuring dust disks, older ones moving off the main sequence (like d Scorpii) and type II supernovae, and some binary systems, like novas and type I supernovae. Supernovae only blow up once.

Periodic Variable Stars

  • RR Lyrae stars: Short-period pulsation, up to one day; they all have about the same luminosity.
  • Cepheid variables: Longer pulsation, from one day to a couple of months; Examples are d Cephei, the original one, and Polaris.
  • Other kinds: There are long-period variables: They, which vary over months or years (the main example is Mira, in Cetus, which varies by a factor 1000 in brightness!), and short-period sub-dwarf B stars that pulsate like jello every few minutes to two hours.
  • Reason: Instability (evolution on the HR diagram), and light-absorbing chemicals in their atmospheres.

  How Are They Useful?

  • RR Lyrae stars: They all have approximately the same luminosity.
  • Cepheids: Period-luminosity relationship, longer period Cepheids are also brighter (why could that be?).
  • Use as standard candles: Variable stars (mainly the brighter Cepheids, see M100), can be used to find distances out to about 30 Mpc (with HST).

What about other variables? Some are also unstable, post-main sequence stars whose variation is chaotic rather than regular; in some cases (like V838 Mon) brightness changes may occur when the star engulfs a planet. A very different, important group is that of binary stars which appear to be varying because they are being eclipsed, or because of their motion.

page by luca bombelli <bombelli at>, modified 29 sep 2012