 Hertzsprung-Russell diagram adapted from Powell. The instability strip is shown and contains the Cepheid variable region (in red) and the RR Lyrae region (in blue). RR Lyrae variables are variable stars often used as standard candles. Image File history File links HRDiagram_InsStrip. ...
Image File history File links HRDiagram_InsStrip. ...
Most stars are of nearly constant luminosity. ...
A standard candle is an astronomical object that has a known luminosity. ...
RR Lyrae are pulsating Horizontal branch stars, with a mass of around half the Sun's. It should be noted that RR Lyrae stars shed mass prior to becoming RR Lyrae and consequently, RR Lyrae were once stars with similar or slightly greater mass than the sun. They pulse in a manner similar to Cepheid variables, but some important differences exist. The horizontal branch (HB) is a stage of stellar evolution which immediately follows the red giant branch. ...
The Sun (or Sol) is the star at the center of our Solar system. ...
A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. ...
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, and sometimes down to eight or nine hours. In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us. ...
The Sun (or Sol) is the star at the center of our Solar system. ...
There are three main types of RR Lyrae stars: RRab, RRc and RRd. The RRc stars are of shorter period and the RRd stars 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 clusters studies, but difficult to observe in external galaxies. A globular cluster is a spherical bundle of stars (star cluster) that orbits a galaxy as a satellite. ...
NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ...
This type of variable is named after the prototype, the variable RR in the Lyra constellation. Lyra (Latin for Lyre) is one of the 48 constellations listed by Ptolemy, and is one of the 88 modern constellations recognized by the International Astronomical Union. ...
See also
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