HU Delphini, also known as Gliese 791.2, is a

star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speak ...

in the

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 ...

Delphinus Delphinus (Pronounced or ) is a small constellation in the Northern Celestial Hemisphere, close to the celestial equator. Its name is the Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-Europea ...

. Its apparent magnitude is 13.07. With a

trigonometric parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to perspective (graphica ...

of 113.4 ± 0.2 mas, it is about 28.76

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s (8.82

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s) away from the Solar System. HU Delphini is a

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

with a well-defined period of 538.6 days. The orbit has been derived from astrometry as well as through spectral observations, although that has been difficult because of the high

projected rotational velocity Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulge ...

. It is also fairly eccentric, at 0.558. Both stars in the system are red dwarfs. The primary component of the system is only 23.7% as massive as the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, so it is fully convective. As a result, there are frequent

starspot Starspots are stellar phenomena, so-named by analogy with sunspots. Spots as small as sunspots have not been detected on other stars, as they would cause undetectably small fluctuations in brightness. The commonly observed starspots are in gene ...

s on its surface, especially near poles. While the normal surface temperature of the primary is 3000 K, the starspots themselves are cooler: only 2700 K. It is also a

flare star A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in th ...

, the first flare been detected in August 2000.Natsvlishvili R. Sh., Karapetian A. A. A possible flare of a red dwarf HU Del
/ref> The secondary star has a mass of 11.4% that of the Sun.

References

{{DEFAULTSORT:HU Delphini M-type main-sequence stars Binary stars Flare stars Delphinus (constellation) 0791.2 Delphini, HU J20294834+0941202

See also

References