40 Eridani

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40 Eridani
Diagram showing star positions and boundaries of the Eridanus constellation and its surroundings
Diagram showing star positions and boundaries of the Eridanus constellation and its surroundings
A star chart of the Eridanus constellation showing the position of 40 Eridani (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Eridanus
40 Eridani A
Right ascension 04h 15m 16.31963s[1]
Declination −07° 39′ 10.3404″[1]
Apparent magnitude (V) 4.43[1]
40 Eridani B
Right ascension 04h 15m 21.786s[2]
Declination −07° 39′ 29.22″[2]
Apparent magnitude (V) 9.52[3]
40 Eridani C
Right ascension 04h 15m 21.50s[4]
Declination −07° 39′ 22.3″[4]
Apparent magnitude (V) 11.17[3]
40 Eridani A
Spectral type K0.5V[5]
U−B color index +0.45[3]
B−V color index +0.82[1]
40 Eridani B
Spectral type DA4[3]
U−B color index +0.45[3]
B−V color index +0.03[3]
40 Eridani C
Spectral type M4.5eV[6]
U−B color index +0.83[3]
B−V color index +1.67[3]
Variable type Flare star[7]
40 Eridani A
Radial velocity (Rv)−43.0[8] km/s
Proper motion (μ) RA: −2,240.12[1] mas/yr
Dec.: −3,420.27[1] mas/yr
Parallax (π)200.62 ± 0.23[1] mas
Distance16.26 ± 0.02 ly
(4.985 ± 0.006 pc)
Absolute magnitude (MV)5.93[9]
40 Eridani B
Radial velocity (Rv)−21[10] km/s
Proper motion (μ) RA: −2,228.3[2] mas/yr
Dec.: −3,377.1[2] mas/yr
40 Eridani C
Radial velocity (Rv)−46[3] km/s
Proper motion (μ) RA: −2,237[4] mas/yr
Dec.: −3,411[4] mas/yr
Primary40 Eridani A
Companion40 Eridani BC
Period (P)~8,000[11] yr
Semi-major axis (a)~400[12] AU
Primary40 Eridani B
Companion40 Eridani C
Period (P)252.1 yr
Semi-major axis (a)~35 AU
Eccentricity (e)0.410
Inclination (i)108.9°
Longitude of the node (Ω)150.9°
Periastron epoch (T)1849.6
Argument of periastron (ω)
40 Eridani A
Mass0.84 [8] M
Radius0.81[13] R
Luminosity0.46[note 1] L
Temperature5,300[8] K
Metallicity [Fe/H]−0.19[14] dex
40 Eridani B
Mass0.50[15][16] M
Radius0.014[16] R
Luminosity0.013[17] L
Temperature16,500[15] K
40 Eridani C
Mass0.20[13] M
Radius0.31[13] R
Luminosity0.008[note 1] L
Temperature3,100[18] K
Age5.6[19] Gyr
Other designations
ο² Eri, 40 Eri, ADS 3093, CCDM J04153-0739, GCTP 945, Gliese 166
A: Keid,[20] HD 26965, HIP 19849, HR 1325, LHS 23, LTT 1907, SAO 131063, BD-07° 780
B: BD-07° 781, G 160-060, HD 26976, LHS 24, LTT 1908, SAO 131065
C: DY Eri, BD-07°781 C, LHS 25, LTT 1909
Database references
SIMBADThe system

40 Eridani (abbreviated 40 Eri), also designated Omicron² Eridani (ο² Eridani, abbreviated Omicron² Eri, ο² Eri), is a triple star system in the constellation of Eridanus. Based on parallax measurements taken during the Hipparcos mission, it is less than 17 light-years from the Sun.

The primary star of the system, designated 40 Eridani A, also named Keid,[20] is easily visible to the naked eye. It is orbited by a binary pair whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, by William Herschel.[21]:p73 It was again observed by Friedrich Struve in 1825 and by Otto Struve in 1851.[11][22]

In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was a white dwarf, the first discovered.[23] Although it is neither the closest white dwarf, nor the brightest in the night sky, it is by far the easiest to observe; it is nearly three magnitudes brighter than Van Maanen's Star (the nearest solitary white dwarf), and unlike the companions of Procyon and Sirius it is not swamped in the glare of a much brighter primary.[17]


40 Eridani is the system's Flamsteed designation and ο² Eridani (Latinised to Omicron² Eridani) its Bayer designation. The designations of the sub-components - 40 Eridani A, B and C - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[24] 40 Eridani C also bears the variable star designation DY Eridani.

The system bore the traditional name Keid derived from the Arabic word qayd meaning "(egg) shells". In 2016, the IAU organized a Working Group on Star Names (WGSN)[25] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[26] It approved the name Keid for the component 40 Eridani A on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.[20]


40 Eridani A is a main-sequence dwarf of spectral type K1. 40 Eridani B and C, are a 9th magnitude white dwarf (spectral type DA4) and an 11th magnitude red dwarf flare star (spectral type M4.5e), respectively. Presumably, while B was a main-sequence star, it was the most massive member of the system, but ejected most of its mass before it became a white dwarf. B and C orbit each other approximately 400 AU from the primary star, A.[12] Their orbit has a semimajor axis of 35 AU (which is the approximate average distance between B and C) and is rather elliptical (eccentricity 0.410).[11]

As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star in Hercules, near the border with Serpens Caput.[note 2]

Potential for life[edit]

Comparison of the habitable zones of the Sun and 40 Eridani A (here labeled Vulcan, after the fictional planet from Star Trek)

The habitable zone of 40 Eridani A, where a planet could exist with liquid water, is near 0.68 (calculated from habitable zone) AU from A. At this distance a planet would complete a revolution in 223 Earth days (according to the third Kepler's law) and 40 Eridani A would appear nearly 20%[note 3] wider than the Sun does on Earth. An observer on a planet in the 40 Eridani A system would see the B/C pair as unusually bright (magnitudes -8 and -6) white and reddish-orange stars in the night sky.

It is unlikely that habitable planets exist around 40 Eridani B because they would have been sterilized by its evolution into a white dwarf. As for 40 Eridani C, it is prone to flares, which cause large momentary increases in the emission of X-rays as well as visible light. This would be lethal to Earth-type life on planets near the flare star.[12]

Planetary system[edit]

In 2018, a planet was discovered orbiting 40 Eridani A with a minimum mass of 8.47±0.47 Earth masses.[27]

The 40 Eridani planetary system
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b 8.47±0.47 M 0.22446±0.00004 42.38 ± 0.01 days 0.04+0.05

See also[edit]


  1. ^ a b From L=4πR2σTeff4, where L is the luminosity, R is the radius, Teff is the effective surface temperature and σ is the Stefan–Boltzmann constant.
  2. ^ From 40 Eridani the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=16h 15m 16.32s, Dec=07° 39′ 10.34″, which is located near the border of Hercules (constellation) and Serpens Caput, the closest bright star being Alpha Serpentis. The absolute magnitude of the Sun is 4.85, so, at a distance of 5.04 parsecs, the Sun would have an apparent magnitude .
  3. ^ From , where h is the apparent height, d is the distance of the object, and a is the actual size of the object.


  1. ^ a b c d e f g van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752Freely accessible. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ a b c d Second U.S. Naval Observatory CCD Astrograph Catalog (UCAC-2); CDS ID I/289.
  3. ^ a b c d e f g h i Gliese Catalogue of Nearby Stars, preliminary 3rd ed., 1991. CDS ID V/70A.
  4. ^ a b c d Salim, Samir; Gould, Andrew (2003). "Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog". The Astrophysical Journal. 582 (2): 1011. arXiv:astro-ph/0206318Freely accessible. Bibcode:2003ApJ...582.1011S. doi:10.1086/344822. ; CDS ID J/ApJ/582/1011.
  5. ^ Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Bubar, E. J.; McGahee, C. E.; O'Donoghue, A. A.; Knox, E. R. (2006). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample". The Astronomical Journal. 132: 161–170. arXiv:astro-ph/0603770Freely accessible. Bibcode:2006AJ....132..161G. doi:10.1086/504637. 
  6. ^ General Catalogue of Trigonometric Parallaxes, 4th ed., 1995. CDS ID I/238A.
  7. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally Published In: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S. 
  8. ^ a b c HD 26965, database entry, Geneva-Copenhagen Survey of Solar neighbourhood, J. Holmberg et al., 2007, CDS ID V/117A, accessed on line November 19, 2008; described in Nordström, B; Mayor, M; Andersen, J; Holmberg, J; Pont, F; Jørgensen, B. R; Olsen, E. H; Udry, S; Mowlavi, N (2004). "The Geneva-Copenhagen survey of the Solar neighbourhood". Astronomy & Astrophysics. 418 (3): 989–1019. arXiv:astro-ph/0405198Freely accessible. Bibcode:2004A&A...418..989N. doi:10.1051/0004-6361:20035959. 
  9. ^ Holmberg, J.; et al. (July 2009), "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics", Astronomy and Astrophysics, 501 (3): 941–947, arXiv:0811.3982Freely accessible, Bibcode:2009A&A...501..941H, doi:10.1051/0004-6361/200811191. 
  10. ^ Evans, D. S (1967). "The Revision of the General Catalogue of Radial Velocities". Determination of Radial Velocities and Their Applications. 30: 57. Bibcode:1967IAUS...30...57E. 
  11. ^ a b c d Heintz, W. D. (1974). "Astrometric study of four visual binaries". Astronomical Journal. 79: 819. Bibcode:1974AJ.....79..819H. doi:10.1086/111614. 
  12. ^ a b c "40 (Omicron2) Eridani 3] at solstation.com". Retrieved 2018-02-06. 
  13. ^ a b c Catalogue of nearest stars until 10pc, V. A. Zakhozhaj. Revised 1996. CDS ID V/101.
  14. ^ Cayrel de Strobel, G.; Hauck, B.; Francois, P.; Thevenin, F.; Friel, E.; Mermilliod, M.; Borde, S. (1992). "A catalogue of Fe/H determinations". Astronomy & Astrophysics (1991 ed.). 95: 273–336. Bibcode:1992A&AS...95..273C. 
  15. ^ a b Finley, David S.; Koester, Detlev; Basri, Gibor (1997). "The Temperature Scale and Mass Distribution of Hot DA White Dwarfs". The Astrophysical Journal. 488: 375–396. Bibcode:1997ApJ...488..375F. doi:10.1086/304668. 
  16. ^ a b Provencal, J. L.; Shipman, H. L.; Høg, Erik; Thejll, P. (1998). "Testing the White Dwarf Mass-Radius Relation with HIPPARCOS". The Astrophysical Journal. 494 (2): 759. Bibcode:1998ApJ...494..759P. CiteSeerX accessible. doi:10.1086/305238. 
  17. ^ a b Keid Archived 2007-05-14 at the Wayback Machine., Jim Kaler, STARS web page, accessed 15/5/2007, 10/12/2011.
  18. ^ Johnson, H. M.; Wright, C. D. (1983). "Predicted infrared brightness of stars within 25 parsecs of the sun". Astrophysical Journal Supplement Series. 53: 643. Bibcode:1983ApJS...53..643J. doi:10.1086/190905. 
  19. ^ Mamajek, Eric E.; Hillenbrand, Lynne A. (November 2008). "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics". The Astrophysical Journal. 687 (2): 1264–1293. arXiv:0807.1686Freely accessible. Bibcode:2008ApJ...687.1264M. doi:10.1086/591785. 
  20. ^ a b c "Naming Stars". IAU.org. Retrieved 16 December 2017. 
  21. ^ Herschel, William (1785). "Catalogue of Double Stars. By William Herschel, Esq. F. R. S". Philosophical Transactions of the Royal Society of London. 75: 40–126. JSTOR 106749. 
  22. ^ Van Den Bos, W. H. (1926). "The orbit and the masses of 40 Eridani BC". Bulletin of the Astronomical Institutes of the Netherlands. 3: 128. Bibcode:1926BAN.....3..128V. 
  23. ^ White Dwarfs, E. Schatzman, Amsterdam: North-Holland, 1958. , p. 1
  24. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707Freely accessible [astro-ph.SR]. 
  25. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016. 
  26. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14. 
  27. ^ Ma, Bo; Ge, Jian; Muterspaugh, Matthew; Singer, Michael A; Henry, Gregory W; González Hernández, Jonay I; Sithajan, Sirinrat; Jeram, Sarik; Williamson, Michael; Stassun, Keivan; Kimock, Benjamin; Varosi, Frank; Schofield, Sidney; Liu, Jian; Powell, Scott; Cassette, Anthony; Jakeman, Hali; Avner, Louis; Grieves, Nolan; Barnes, Rory; Gilda, Sankalp; Grantham, Jim; Stafford, Greg; Savage, David; Bland, Steve; Ealey, Brent (2018). "The first super-Earth Detection from the High Cadence and High Radial Velocity Precision Dharma Planet Survey". Monthly Notices of the Royal Astronomical Society. arXiv:1807.07098Freely accessible. Bibcode:2018MNRAS.tmp.1837M. doi:10.1093/mnras/sty1933. 

External links[edit]

Coordinates: Sky map 04h 15m 16.32s, −07° 39′ 10.34″