R Doradus (HD 29712 or P Doradus) is a

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

in the far-southern

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Dorado Dorado () is a constellation in the southern sky. It was named in the late 16th century and is now one of the 88 modern constellations. Its name refers to the dolphinfish (''Coryphaena hippurus''), which is known as ''dorado'' in Spanish, altho ...

. Its distance from Earth is . Having a uniform disk diameter of , it is thought to be the extrasolar star with the largest apparent size as viewed from Earth.

Variability

The

visible magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

of R Doradus varies between 4.8 and 6.6, which means it is usually visible to the naked eye, but in the

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

it is one of the brightest stars in the sky. With a near-infrared J band

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

of −2.6, only

Betelgeuse Betelgeuse is a red supergiant of spectral type M1-2 and one of the largest stars visible to the naked eye. It is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion ...

at −2.9 is brighter. In the infrared K band, it is sometimes the brightest star in the sky, although usually

is brighter. It is classified as a

semiregular variable star In astronomy, a semiregular variable star, a type of variable star, is a giant or supergiant of intermediate and late (cooler) spectral type showing considerable periodicity in its light changes, accompanied or sometimes interrupted by various irre ...

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 Mira variables (named for the prototype star Mira) are a class of pulsating stars characterized by very red colours, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitude at visual wavelen ...

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 Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...

. 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 other than the Sun. The angular diameter of the next-largest star,

, is around .

Properties

The

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...

parallax of R Doradus is , corresponding to a distance of . The

bolometric luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a s ...

of R Doradus, derived from its

bolometric A bolometer is a device for measuring radiant heat by means of a material having a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley. Principle of operation A bolometer ...

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...

at a distance of , is . The measured angular diameter, again assuming a distance of gives a radius of . The

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

corresponding to this luminosity and radius is . Comparison of its properties with theoretical

evolutionary Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes, which are passed on from parent to ...

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 The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

having exhausted its core helium. The radius of means that the diameter of R Doradus is 415 million km (). If placed at the centre of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

, the

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...

would be within the star. R Doradus has a projected equatorial

rotation velocity In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an object ...

of . It is calculated to take to rotate once on its axis.

References

External links

Swinburne Astronomy Online; information about R Doradus

{{DEFAULTSORT:R Doradus Dorado (constellation) M-type giants Mira variables Doradus, P Doradus, R 029712 1492 CD-62 00175 021479 J04364544-6204379 Emission-line stars