40 Eridani (also known as Omicron2 Eridani, or Keid, from the Arabic word qayd, (egg) shells) is a triple star system less than 16.5 light years away from Earth. It is in the constellation Eridanus. The primary star of the system, 40 Eridani A, is easily visible to the naked eye. The pair 40 Eridani B/C was discovered on January 31, 1783, by William Herschel., p. 73 It was again observed by Friedrich Georg Wilhelm Struve in 1825 and by Otto Wilhelm von Struve in 1851. 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.
40 Eridani A is a main-sequence dwarf of spectral type K1. Its two companion stars, 40 Eridani B and 40 Eridani C, are a 9th magnitude white dwarf (spectral type DA4) and an 11th magnitude red dwarf flare star (spectral type M4.5e) which has the variable star designation DY Eridani. 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 astronomical units from the primary star, A. Their orbit has a semimajor axis of 35 AU (which is also the approximate average distance between B and C) and is rather elliptical (eccentricity 0.410).
Although 40 Eridani B 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.
Potential for life
The primary component has a metallicity of [Fe/H]=−0.19, i.e. about 65 per cent of the solar metallicity, thus providing a probably sufficient heavy element abundance for the formation of terrestrial planets. However, no planet orbiting a member of 40 Eridani is known so far.[original research?] 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 2] 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.
- 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.
- From , where h is the apparent height, d is the distance of the object, and a is the actual size of the object.
- Hipparcos Catalogue; CDS ID I/239. Astrometric data updated from J1991.25 to J2000.0.
- Second U.S. Naval Observatory CCD Astrograph Catalog (UCAC2); CDS ID I/289.
- Gliese Catalogue of Nearby Stars, preliminary 3rd ed., 1991. CDS ID V/70A.
- 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.
- General Catalogue of Trigonometric Parallaxes, 4th ed., 1995. CDS ID I/238A.
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- Astrometric study of four visual binaries, W. D. Heintz, Astronomical Journal 79, #7 (July 1974), pp. 819–825.
- 40 (Omicron2) Eridani 3 at solstation.com, accessed May 15, 2007.
- 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.
- 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.
- Catalogue of nearest stars until 10pc, V. A. Zakhozhaj. Revised 1996. CDS ID V/101.
- Keid, Jim Kaler, STARS web page, accessed May 15, 2007.
- 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.
- Cayrel de Strobel, G.; Hauck, B.; Francois, P.; Thevenin, F.; Friel, E.; Mermilliod, M.; Borde, S. (1992). "A catalogue of Fe/H determinations - 1991 edition". Astronomy & Astrophysics 95: 273–336. Bibcode:1992A&AS...95..273C. —metallicity for component A
- 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:0807.1686. Bibcode:2008ApJ...687.1264M. doi:10.1086/591785.
- Catalogue of Double Stars, William Herschel, Philosophical Transactions of the Royal Society of London 75 (1785), pp. 40–126
- 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.
- White Dwarfs, E. Schatzman, Amsterdam: North-Holland, 1958. , p. 1
- Keid at Jim Kaler's STARS.
- 40 (Omicron2) Eridani 3 at solstation.com.
- Omicron(2) Eridani entry at the Internet Stellar Database.