RR Lyrae variables are variable stars often used as standard candles.
RR Lyrae are pulsating horizontal branch stars, with a mass of around half the Sun's. RR Lyrae stars shed mass prior to becoming RR Lyrae and consequently, RR Lyrae were once stars with similar or slightly less mass than the sun, around 0.8 solar masses. They pulse in a manner similar to Cepheid variables, but some important differences exist.
RR Lyrae are old, relatively low mass stars. Therefore, they are much more common than Cepheids, but less luminous. The average absolute magnitude of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun. Their period is shorter, typically less than one day, sometimes ranging down to seven hours.
There are three main types of RR Lyrae stars:
- RRab
- RRc, which are shorter periods.
- RRd, which are double-mode pulsators.
The relationship between an RR Lyrae's variability period and absolute magnitude makes them good standard candles for relatively near objects, especially within the Milky Way. They are extensively used in globular cluster studies, but difficult to observe in external galaxies, because of their intrinsic faintness. This type of variable is named after the prototype, the variable RR in the Lyra constellation. They were formerly called "cluster variables" because of their common occurrence in globular clusters.
See also
External links
- APOD four-frame animation of RR Lyrae variables in globular cluster M3
- Animation of RR Lyrae-Variables in globular cluster M15
- [1]animation RR Lyrae in the central part of the globular cluster M15
- RR Lyrae stars
- [2]2 night animation RR Lyrae at the globular cluster M3, 10min intervals.
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