40 (Omicron2) Eridani 3

JPL, CalTech, NASA


Larger illustration


Astronomers have identified 40
(Omicron2) Eridani A as a prime
target for the Terrestrial Planet
Finder (TPF), now planned for
launch between 2014 and 2020.

That Star A was not simply a single star was discovered in 1783 by Sir William Friedrich Wilhelm Herschel (1738-1822, portrait), who was born Friedrich Wilhelm Herschel and who subsequently discovered the planet Uranus in 1781 -- which led to his appointment in 1782 as private astronomer to the King of England. Star A's companion itself was discovered to be a binary pair BC in 1851 by Otto Wilhelm von Struve (1819-1905), who succeeded his father (Friedrich Georg Wilhelm) as director of Russia's Pulkova Observatory, made the first accurate determination of the constant of precession, and discovered some 500 binary stars. Finally, Star B became the first "white dwarf" to be discovered in 1910 when astronomers -- Henry Norris Russell (1877-1957), Edward Charles Pickering (1846-1919), and Williamina ["Mina"] Paton Stevens Fleming (1857-1911) -- realized that this dim star was of spectral type A rather than M (Ken Croswell, 1995, page 55).

© Torben Krogh & Mogens Winther,



(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)




Star A is an orange-red dwarf
star, like Epsilon Eridani
at left center of meteor.

40 Eridani A

This main sequence, orange-red dwarf (K1 Ve) may have about 89 percent of Sol's mass (RECONS), about 85 percent of its diameter (Johnson and Wright, 1983, page 655), and 36 percent of its luminosity. The star appears to be 46 percent to 102 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 282). The system shows a radial velocity of about 25 miles per second (mps) -- about 40 km per second or kps -- in recession, but the true space velocity is about 62 mps (or 100 kps). Star A and the binary pair BC have a wide separation of about 418 AUs and an orbital period of some 8,000 years (Wulff Dieter Heintz, 1974). Some alternative useful star catalogue numbers for the star are: Omi2 Eri A, 40 Eri A, HR 1325, Gl 166 A, Hip 19849, HD 26965, BD-07 780, SAO 131063, LHS 23, and LTT 1907.

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA

40 Eridani B is a young white dwarf (a remnant stellar core which enriched its closer companion, Eridani C,
with elements heavier than hydrogen when it cast off its outer gas layers) like planetary nebula NGC 2440.

40 Eridani B

A white dwarf (DA4 /VII). It has a mass estimated from about 50.1 percent of Sol's (Provencal et al, 1998), only two percent of its diameter, and 33/10,000th of its brightness. The B and C pair of stars have an "average" separation of about 35 AUs (of a semi-major axis) in a highly elliptical orbit (e= 0.410) that lasts about 252 years and swings between 21 and 49 AUs (Heintz, 1974). (See an animation of the orbits of Stars A, B, and C and their potentially habitable zones, with a table of basic orbital and physical characteristics.) Some alternative useful star catalogue numbers for the star are: Omi2 Eri B, 40 Eri B, Gl 166 B, HD 26976, BD-07 781, G 160-60, LHS 24, ADS 3093, W 33, and Struve 518.

NASA -- larger image

40 Eridani C is a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.

Life Around a Flare Star

Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (40 Eridani C) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf does normally.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.03 AU for Star C), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.