R Doradus


R Doradus is a red giant variable star in the far-southern constellation Dorado. Its distance from Earth is. Having a uniform disk diameter of arcsec, it is thought to be the extrasolar star with the largest apparent size as viewed from Earth.

Variability

The visible magnitude of R Doradus varies between 4.8 and 6.6, which means it is usually visible to the naked eye, but in the infrared it is one of the brightest stars in the sky. With a near-infrared J band magnitude of −2.6, only Betelgeuse at −2.9 is brighter. In the infrared K band, it is sometimes the brightest star in the sky, although usually Betelgeuse is brighter.
It is classified as a semiregular variable star of type SRb, indicating giants with slow poorly-defined variations, often alternating between periodic and irregular brightness changes. Some studies show it alternating between periods of about 175 and 332 days, and a period of 117.3 days has also been identified. It has been likened to a Mira variable when its variations are relatively regular, although its amplitude of only 1.5 magnitudes is smaller than Mira variables.

Angular diameter

The angular diameter of R Doradus is easily measured using interferometry. Its uniform disc diameter, the diameter when interpreted as a disc of uniform brightness, when viewed at is. When viewed at and interpreted as a limb-darkened disc, the diameter is.
The angular diameter of R Doradus is larger than any other measured star. The angular diameter of the next-largest star, Betelgeuse, is around.

Properties

The Hipparcos parallax of R Doradus is, corresponding to a distance of.
The bolometric luminosity of R Doradus, derived from its bolometric flux at a distance of, is. The measured angular diameter, again assuming a distance of gives a radius of. The effective temperature corresponding to this luminosity and radius is.
Comparison of its properties with theoretical evolutionary tracks gives an age of between 6 and 14 billion years, with a current mass of between 0.7 and. Its initial mass would have been between 1 and. It is on the asymptotic giant branch having exhausted its core helium.
The radius of means that the diameter of R Doradus is is 415 million km. If placed at the centre of the Solar System, the perihelion of Mars would be within the star.
R Doradus has a projected equatorial rotation velocity of. It is calculated to take to rotate once on its axis.
R Doradus exists on the constellation boundaries of Dorado and Reticulum, also close between the bright stars Canopus and Achernar.