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
- 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
- RR Lyrae stars: They all have
approximately the same luminosity.
- Cepheids: Period-luminosity
relationship, longer period Cepheids are also brighter (why could
- 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).
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 olemiss.edu>,
modified 29 sep 2012