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 16.320s[1]
Declination −07° 39′ 10.34″[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]
Spectral type K1V[1] / DA4[3] / M4.5eV[5]
U−B color index 0.45[3] / -0.68[3] / 0.83[3]
B−V color index 0.82[1] / 0.03[3] / 1.67[3]
Radial velocity (Rv) -43.0[6] / ? / -46[3] km/s
Proper motion (μ) RA: 
-2239.72[1] / -2228.3[2] /
-2237[4] mas/yr
-3420.35[1] / -3377.1[2] /
-3411[4] mas/yr
Parallax (π) 198.26±0.84[1] mas
Distance 16.45±0.07 ly
(5.04±0.02 pc)
Absolute magnitude (MV) 5.92 / 11.01 / 12.66
Companion 40 Eridani BC
Period (P) ~8,000[7] yr
Semi-major axis (a) ~400[8] AU
Companion 40 Eridani C
Period (P) 252.1[7] yr
Semi-major axis (a) ~35[7] AU
Eccentricity (e) 0.410[7]
Inclination (i) 108.9[7]°
Longitude of the node (Ω) 150.9[7]°
Periastron epoch (T) 1849.6[7]
Argument of periastron (ω)
Mass 0.84 [6] / 0.50[9][10] / 0.20[11] M
Radius 0.81[11] / 0.014[10] / 0.31[11] R
Luminosity 0.46[note 1] / 0.013[12] / 0.008[note 1] L
Temperature 5,300[6] / 16,500[9] / 3,100[13] K
Metallicity [Fe/H]=−0.19[14]
Age 5.6[15] Gyr
Other designations
Database references
SIMBAD The system

40 Eridani (abbreviated 40 Eri), also designated Omicron² Eridani (ο² Eridani, abbreviated Omicron² Eri, ο² Eri) is a triple star system less than 16.5 light years away from Earth, in the constellation of Eridanus.

The primary star of the system, designated 40 Eridani A, also named Keid,[16] is easily visible to the naked eye. It is orbited by a binary system whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, by William Herschel.[17], p. 73 It was again observed by Friedrich Struve in 1825 and by Otto Struve in 1851.[7][18]

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.[19] 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.[12]


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).[20] 40 Eridani C also bears the variable star designation DY Eridani.

40 Eridani A's traditional name Keid derives from the Arabic word qayd meaning "(egg) shells". In 2016, the IAU organized a Working Group on Star Names (WGSN)[21] to catalogue and standardize proper names for stars. The WGSN approved the name Keid for this star on 12 September 2016 and it is now so entered in the IAU Catalog of Star Names.[16]


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.[8] 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).[7]

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 zone of 40 Eridani A (here labeled Vulcan, after a planet from Star Trek that is depicted to be in this star’s system) with the habitable zone of the Solar System.

The primary component has a metallicity of [Fe/H]=−0.19, i.e. about 65 percent of the solar metallicity, thus providing a probably sufficient heavy element abundance for the formation of terrestrial planets.[citation needed] However, no planet orbiting a member of 40 Eridani is known so far.

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. This is not bright enough to diminish the darkness at night, though they would be visible in daylight (assuming an Earth-normal atmosphere). (By comparison, Earth's full moon is magnitude −12.6, and Venus at its brightest is −4.7.)

It is extremely unlikely that habitable planets exist around the B star because planets circling 40 Eridani B would probably have been destroyed or 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.[8]

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 Epsilon Indi 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 h Hipparcos Catalogue; CDS ID I/239. Astrometric data updated from J1991.25 to J2000.0.
  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 Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog, Samir Salim and Andrew Gould, Astrophysical Journal 582, #2 (January 2003), pp. 1011–1031; CDS ID J/ApJ/582/1011.
  5. ^ General Catalogue of Trigonometric Parallaxes, 4th ed., 1995. CDS ID I/238A.
  6. ^ 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 The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ~14 000 F and G dwarfs, B. Nordström, M. Mayor, J. Andersen, J. Holmberg, F. Pont, B. R. Jørgensen, E. H. Olsen, S. Udry, and N. Mowlavi, Astronomy and Astrophysics 418 (May 2004), pp. 989–1019.
  7. ^ a b c d e f g h i j Astrometric study of four visual binaries, W. D. Heintz, Astronomical Journal 79, #7 (July 1974), pp. 819–825.
  8. ^ a b c 40 (Omicron2) Eridani 3 at solstation.com, accessed May 15, 2007.
  9. ^ a b The Temperature Scale and Mass Distribution of Hot DA White Dwarfs, David S. Finley, Detlev Koester, and Gibor Basri, Astrophysical Journal 488 (October 10, 1997), pp. 375–396.
  10. ^ a b Testing the White Dwarf Mass-Radius Relation with HIPPARCOS, J. L. Provencal, H. L. Shipman, Erik Hog, and P. Thejll, Astrophysical Journal 494 (February 20, 1998), pp. 759–767.
  11. ^ a b c Catalogue of nearest stars until 10pc, V. A. Zakhozhaj. Revised 1996. CDS ID V/101.
  12. ^ a b Keid, Jim Kaler, STARS web page, accessed 15/5/2007, 10/12/2011.
  13. ^ Predicted infrared brightness of stars within 25 parsecs of the sun, H. M. Johnson and C. D. Wright, Astrophysical Journal Supplement 53 (November 1983), pp. 643–711.
  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. —metallicity for component A
  15. ^ 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:0807.1686free to read. Bibcode:2008ApJ...687.1264M. doi:10.1086/591785. 
  16. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  17. ^ Catalogue of Double Stars, William Herschel, Philosophical Transactions of the Royal Society of London 75 (1785), pp. 40–126
  18. ^ The orbit and the masses of 40 Eridani BC, W. H. van den Bos, Bulletin of the Astronomical Institutes of the Netherlands 3, #98 (July 8, 1926), pp. 128–132.
  19. ^ White Dwarfs, E. Schatzman, Amsterdam: North-Holland, 1958. , p. 1
  20. ^ 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:1012.0707free to read [astro-ph.SR]. 
  21. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016. 

External links[edit]

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