The
visible magnitude of R Doradus varies between 4.8 and 6.3,[2] 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 bandmagnitude of −2.65, only
Betelgeuse and
Antares at −2.9 and −2.73 (respectively) are brighter.[11] In the infrared
K band, it is sometimes the brightest star in the sky, although usually
Betelgeuse is brighter.[12]
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.[5] Some studies show it alternating between periods of about 175 and 332 days, and a period of 117.3 days has also been identified.[13] 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.[14] The star was discovered to be variable in 1874 by
Benjamin Gould, and received the
variable-star designation R Doradus.[2]
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 1.25
μm is 57±5
mas.[15] When viewed at 2.3 μm and interpreted as a
limb-darkened disc, the diameter is 51.18±2.24 mas.[13]
The angular diameter of R Doradus is larger than any other measured star other than the Sun. The angular diameter of the next-largest star,
Betelgeuse, is
around 45 mas.[15]
Properties
The
Hipparcos parallax of R Doradus is 18.31±0.99
mas, corresponding to a distance of 55±3
pc.[1] The
bolometric luminosity of R Doradus, derived from its bolometric
flux at a distance of 55 pc, is 4,350±520L☉. The measured
angular diameter, again assuming a distance of 55 pc gives a radius of 298±21R☉. The
angular diameter and bolometric flux of R Doradus derive a cool surface
effective temperature of 2,710±170
K.[7]
Comparison of its properties with theoretical
evolutionary tracks gives an age of between 6 and 14 billion years. R Doradus has lost part of its mass during its evoltion, and currently has a mass of either 0.7 or 1.0 M☉. Its initial mass would be either 1 or 1.25 M☉. Because of the enlarged surface and low mass, R Doradus has a surface gravity of only 0.026% that of Earth.[a] It is on the
asymptotic giant branch having exhausted helium at its core.[7]
The radius of 298 R☉ means that the diameter of R Doradus is 415 million km (2.77
AU). 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 1±0.1
km/s. It is calculated to take 57.5
years to rotate once on its axis.[8]
R Doradus figures almost at the western limit of
Dorado, next to
Reticulum (map to be held, as standard, upwards, with south horizon in front of viewer). Like Alpha Reticuli, which outshines it, it is a little south of a line between the bright stars
Canopus and
Achernar, as is the narrow kite-shaped asterism of Reticulum.
Notes
^from a log(g) of -0.6. Should be divided by 980.655 to convert to Earth's gravity.
^
abSkrutskie, Michael F.; Cutri, Roc M.; Stiening, Rae; Weinberg, Martin D.; Schneider, Stephen E.; Carpenter, John M.; Beichman, Charles A.; Capps, Richard W.; Chester, Thomas; Elias, Jonathan H.; Huchra, John P.; Liebert, James W.; Lonsdale, Carol J.; Monet, David G.; Price, Stephan; Seitzer, Patrick; Jarrett, Thomas H.; Kirkpatrick, J. Davy; Gizis, John E.; Howard, Elizabeth V.; Evans, Tracey E.; Fowler, John W.; Fullmer, Linda; Hurt, Robert L.; Light, Robert M.; Kopan, Eugene L.; Marsh, Kenneth A.; McCallon, Howard L.; Tam, Robert; Van Dyk, Schuyler D.; Wheelock, Sherry L. (1 February 2006).
"The Two Micron All Sky Survey (2MASS)". The Astronomical Journal. 131 (2): 1163–1183.
Bibcode:
2006AJ....131.1163S.
doi:10.1086/498708.
ISSN0004-6256.
S2CID18913331.
^Wilson, Ralph Elmer (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Washington: Carnegie Institution of Washington.
Bibcode:
1953GCRV..C......0W.
^
abcVlemmings, W. H. T.; Khouri, T.; De Beck, E.; Olofsson, H.; García-Segura, G.; Villaver, E.; Baudry, A.; Humphreys, E. M. L.; Maercker, M.; Ramstedt, S. (2018). "Rotation of the asymptotic giant branch star R Doradus". Astronomy and Astrophysics. 613: L4.
arXiv:1804.04709.
Bibcode:
2018A&A...613L...4V.
doi:
10.1051/0004-6361/201832929.
S2CID52088412.