40 Eridani
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| Observation data Epoch J2000 |
|
|---|---|
| Constellation | Eridanus |
| Right ascension | 04h 15m 16.3s |
| Declination | -07° 39' 10" |
| Apparent magnitude (V) | 4.43/9.52/11.17 |
| Characteristics | |
| Spectral type | K1 V/DA VII/M4.5e V |
| B-V color index | 0.82/0.04/1.64 |
| U-B color index | 0.44/-0.68/0.88 |
| Variable type | None/None/Flare star |
| Astrometry | |
| Radial velocity (Rv) | -43.0 km/s |
| Proper motion (μ) | RA: -2239.33 mas/yr Dec.: -3419.86 mas/yr |
| Parallax (π) | 199.00 ± 0.77 mas |
| Distance | 16.39 ± 0.06 ly (5.03 ± 0.02 pc) |
| Absolute magnitude (MV) | 5.92/11.01/12.66 |
| Details | |
| Mass | 0.89/0.50/0.16 M☉ |
| Radius | 0.85/0.02 R☉ |
| Luminosity | 0.36/0.0033 L☉ |
| Temperature | 5,100/14,000/3,300 K |
| Metallicity | ? |
| Rotation | ? |
| Age | ? years |
| Other designations | |
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 constellation Eridanus and was discovered to be a double star in 1783 by William Herschel. In 1851, Otto Wilhelm von Struve discovered that one of the stars in the double was itself a double.
In 1910 it was discovered that component B was actually a white dwarf star, which meant it had already evolved through its main sequence stage. In fact this was the first white dwarf star to be discovered. While it was a main sequence star, this was the most massive member of this system but it ejected most of its mass toward the end of its life.
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[edit] General information
The primary star of 40 Eridani is an orange main sequence dwarf of spectral type K1, observed to be about 4 billion years old. The two companion stars, 40 Eridani B, a 9th magnitude white dwarf (spectral type A VII) and 40 Eridani C an 11th magnitude red dwarf (spectral type M4e V), orbit each other 400 astronomical units from the primary star. Component C is a flare star with variable star designation DY Eridani.
[edit] Potential for life
The habitable zone (where a planet could exist with liquid water) of 40 Eri A is near 0.61 AU. At this distance a planet would complete a revolution in 203 earth days, and the star would appear about 62% wider than Sol does on Earth. An observer on a planet in the 40 Eri A system would see the B/C pair as unusually bright (magnitudes -8 and -6) white and red stars in the night sky. This is not bright enough to diminish the darkness at night, though they would be visible during daytime. (By comparison, Earth's full moon is magnitude -12.6, and Venus at its brightest is -4.) 40 Eri C is prone to flares, which cause large momentary increases in the X-ray band as well as visible light. It is extremely unlikely that habitable planets exist around the B or C stars: planets circling 40 Eri B would likely have been destroyed or sterilized by its evolution into a white dwarf, while planets in C's habitable zone would be tidally locked with one side always facing the star and the other in permanent night.
