Free Articles: HD 20794 b

82 G. Eridani

82 G. Eridani
Observation data Epoch J2000 Equinox J2000
Constellation	Eridanus
Right ascension	03 19 55.65093
Declination	−43° 04′ 11.2175″
Apparent magnitude (V)	4.254
Characteristics
Spectral type	G8 V
U−B color index	+0.21
B−V color index	+0.71
Astrometry

Radial velocity (R_v)	+87.2 km/s
Proper motion (μ)	RA: 3038.34 mas/yr Dec.: 726.58 mas/yr
Parallax (π)	165.47 ± 0.19 mas
Distance	19.71 ± 0.02 ly (6.043 ± 0.007 pc)
Absolute magnitude (M_V)	5.34

Details

Mass	0.97 M_☉
Radius	0.92 R_☉
Surface gravity (log g)	4.40
Luminosity	0.62 L_☉
Temperature	5,401 K
Metallicity [Fe/H]	–0.40 dex
Rotational velocity (v sin i)	4.0 km/s
Age	6.1–12.7 Gyr

Other designations
e Eri, HD 20794, CD-43°1028, GCTP 703, GJ 139, LHS 19, LTT 1583, HR 1008, SAO 216263, FK5 119, and HIP 15510.

82 G. Eridani is a star about 20 light years away from Earth in the constellation Eridanus. It is a main sequence star with a stellar classification of G8.

Observation

In the southern-sky catalog Uranometria Argentina, 82 G. Eridani (sometimes abbreviated to "82 Eridani") is the 82nd star listed in the constellation Eridanus. The Argentina catalog, compiled by the 19th-century astronomer Benjamin Gould, is a southern celestial hemisphere analog of the more famous Flamsteed catalog, and uses a similar numbering scheme. 82 G. Eridani, like other stars near the Sun, has held on to its Gould designation, even while other more distant stars have not.

Properties

This star is slightly smaller and less massive than the Sun, making it marginally dimmer than the Sun in terms of luminosity; about 20% more luminous than Tau Ceti or Alpha Centauri B. The projected equatorial rotation rate ( $vsin i$ ) is 4.0 km/s, compared to 2 km/s for the Sun.

82 G. Eridani is a high-velocity star—it is moving quickly compared to the average—and hence is probably a member of Population II, generally older stars whose motions take them well outside the plane of the Milky Way. Like many other Population II stars, 82 G. Eridani is somewhat metal-deficient (though much less deficient than many), and is older than the Sun. It has a relatively high orbital eccentricity of 0.40 about the galaxy, ranging between 4.6 and 10.8 kiloparsecs from the core. Estimates of the age of this star ranged from 6 to 12 billion years.

This star is located in a region of low-density interstellar matter (ISM), so it is believed to have a large astropause that subtends an angle of 6″ across the sky. Relative to the Sun, this star is moving at a space velocity of 101 km/s, with the bow shock advancing at more than Mach 3 through the ISM.

Planetary system

On August 17, 2011, European astronomers announced the discovery of three planets orbiting 82 G. Eridani (HD 20794). The mass range of these planets classifies them as super-Earths; objects with only a few times the Earth's mass. These planets were discovered by precise measurement of the radial velocity of the star, with the planets revealing their presence by their gravitational displacement of the star during each orbit. None of the planets displays a significant orbital eccentricity. However, their orbital periods are all 90 days or less, indicating that they are orbiting close to the host star. The equilibrium temperature for the most distant planet, based on an assumed Bond albedo of 0.3, would be about 388 K (115 °C), which is above the boiling point of water.

Habitability

In his book Habitable Planets for Man, Stephen Dole gave 82 G. Eridani his highest estimate for habitability: 5.7%. Four other stars had this figure: Alpha Centauri B, 70 Ophiuchi A, Eta Cassiopeiae A, and Delta Pavonis. 82 G. Eridani (GJ 139) was also picked as a Tier 1 target star for NASA's proposed Space Interferometry Mission (SIM) mission to search for terrestrial-sized or larger planets.