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 gravity, gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally calle ...

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 first constellati ...

Delphinus Delphinus is a small constellation in the Northern Celestial Hemisphere, close to the celestial equator. Its name is the Latin version for the Greek word for dolphin (). It is one of the 48 constellations listed by the 2nd-century astronomer Pt ...

. Its apparent magnitude is 13.07. With a trigonometric parallax of 113.4 ± 0.2 mas, it is about 28.76

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

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 or binary star system 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 as separate stars us ...

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 bu ...

. 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 centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

, 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 magnetic reconnection, the magne ...

, 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 0791.2 Delphini, HU J20294834+0941202

See also

References