R Doradus
R Doradus is the name of a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...

 Mira variable
Mira variable
Mira variables , named after the star Mira, are a class of pulsating variable stars characterized by very red colors, pulsation periods longer than 100 days, and light amplitudes greater than one magnitude in infrared and 2.5 magnitude in visual...

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 in the far-southern constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

Dorado is a constellation in the southern sky. It was created in the late 16th century and is now one of the 88 modern constellations. Its name refers to the dolphinfish , which is known as dorado in Spanish, although it has also been depicted as a swordfish...

, although visually it appears more closely associated with the constellation 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...

. Its distance from Earth is 204 ± 9 light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s. Having an angular diameter
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

 of 0.057 ± 0.005 arcsec, and given its distance, it is currently believed to be the star with the second largest apparent size as viewed from Earth (right after the Sun). The diameter of R Doradus is 515 ± 70 million km (3.46 AU) or 370 ± 50 times the diameter of the Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. If placed at the center of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, the orbit of Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 and most of the main asteroid belt would be entirely contained within the star.

The visible magnitude of R Doradus varies between 4.8 and 6.6, which makes it usually just visible to the naked eye, but in the infrared it is one of the brightest stars in the sky and its total luminosity is 6500 ± 1400 times that of the sun.

An anomaly?

The finding that this star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

 has the largest angular measurement presented by the ESO appears to contradict the results of typical methods of calculating the total luminosity and radius of a star.
The star is approximately 200 light years away and has a maximum apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 5.73, which seems to imply an absolute magnitude
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...

 of 1.75 and hence a radius 18 times that of the sun rather than 350 times. The discrepancy arises because of the very cool surface temperature of the star, which results in most of its radiation being emitted in the infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 rather than at visible wavelengths. Applying the appropriate bolometric correction
Bolometric Correction
In astronomy, a bolometric correction is a correction that must be made to the absolute magnitude of an object in order to convert an object's visible magnitude to its bolometric magnitude. Mathematically, such a calculation can be expressed: BC = M_b - M_v\!\,...

reveals that its true total luminosity is about 6500 times that of the Sun, rather than 17 times, as estimated from the visual brightness alone.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.