Eta² Hydri, Latinized from η² Hydri and often catalogued as HD 11977, is a

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

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 ...

of Hydrus. It is visible to the naked eye with an apparent visual magnitude of +4.68, and is one of the least

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 ...

s known. The distance to Eta² Hydri is approximately 222

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 based on

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

measurements, but it is drifting closer to the Sun with a

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

of −18 km/s. The

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

of Eta² Hydri is G8IIIb, which indicates this is a

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...

giant star. It is located in the clump zone of the

HR diagram HR, Hr or hr may refer to: Arts and media Film and television * ''H.R. Pufnstuf'', a children's television series from 1969 * ''HR'', a 2013 television drama starring Alicia Silverstone * HR, a criminal organisation in the American TV series ''Pe ...

, which indicates it has already ascended the red giant branch and is now generating energy by core helium fusion surrounded by a hydrogen fusing shell. Based on its mass, it was probably a class A star (similar to

Vega Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, an ...

or Fomalhaut) when it was on the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

. It is estimated to be around 1.3 billion years old and has expanded to 10 times the Sun's diameter, though is only around 1.9 times as massive as the Sun. The star has a leisurely rotation, with a period of no more than 230–270 days. As of 2005, an extrasolar planet was confirmed to be orbiting the star.

Planetary system

In 2005, the giant planet Eta² Hydri b was found in orbit around Eta² Hydri by measuring radial velocity variations. This object has at least 6.3 times the mass of the Sun and is orbiting with a period of . It is the first giant planet to be found in orbit around an intermediate-mass giant star.

References

External links

* {{DEFAULTSORT:Eta2 Hydri G-type giants Horizontal-branch stars Planetary systems with one confirmed planet Hydrus Hydrus, eta2 Durchmusterung objects 011977 008928 0570

Planetary system

See also

References

External links