R Doradus (HD 29712 or P Doradus) is a red giant

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

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 first constellati ...

Dorado, close to the border with

Reticulum Reticulum is a small, faint constellation in the southern sky. Its name is Latin for a small net, or reticle—a net of crosshairs at the focus of a telescope eyepiece that is used to measure star positions. The constellation is best viewed b ...

. 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 of R Doradus varies between 4.8 and 6.3, which means it is usually visible to the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...

, but in the

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

it is one of the brightest stars in the sky. With a near-infrared J band magnitude of −2.65, only

Betelgeuse Betelgeuse is a red supergiant star in the constellation of Orion (constellation), Orion. It is usually the List of brightest stars, tenth-brightest star in the night sky and, after Rigel, the second brightest in its constellation. It i ...

and

Antares Antares is the brightest star in the constellation of Scorpius. It has the Bayer designation α Scorpii, which is Latinisation of names, Latinised to Alpha Scorpii. Often referred to as "the heart of the scorpion", Antares is flanked by ...

at −2.9 and −2.73 (respectively) are 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 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 wave ...

when its variations are relatively regular, although its amplitude of only 1.5 magnitudes is smaller than Mira variables. The star was discovered to be variable in 1874 by Benjamin Gould, and received the variable-star designation R Doradus.

Angular diameter

The angular diameter of R Doradus is easily measured using

interferometry Interferometry is a technique which uses the ''interference (wave propagation), interference'' of Superposition principle, superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important inves ...

. 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 and distances of ...

parallax of R Doradus is , corresponding to a distance of . The bolometric luminosity of R Doradus, derived from its bolometric

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 in physics. For transport phe ...

at a distance of , is . The measured

angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the ''visual an ...

, again assuming a distance of gives a radius of . The

and bolometric flux of R Doradus derive a cool surface

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 ...

of . Comparison of its properties with theoretical

evolutionary Evolution is the change in the heritable characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certa ...

tracks gives an age of between 6 and 14 billion years. R Doradus has lost part of its mass during its evolution, and currently has a mass of either . Its initial mass would be either . Because of the enlarged surface and low mass, R Doradus has a surface gravity of only 0.026% that of Earth. 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 helium at its core. 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 "Sola ...

, the

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

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. While such value is very slow if compared to stars like the Sun, it is more than 100 times faster for a typical AGB star, suggesting R Doradus may have a close companion modifying its rotation. Using ALMA facilities, researchers at Chalmers University, in July to August 2023, were able to record the movement of hot gas bubbles on the surface of the star. Such bubbles, witness of the convective activity linked to deep nuclear fusion, would have a life of about a month, and a size more than 75 times that of the Sun.

Notes

References

External links

Swinburne Astronomy Online; information about R Doradus

{{DEFAULTSORT:R Doradus Dorado M-type giants Mira variables Doradus, P Doradus, R 029712

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