Heads up ! “Keid” , home of the Vulcans .

The river of Eridanus flows out beside the base of Orion. A bit of dark sky and omicron one and two are visible. Otherwise it’s a star hop from Orion. It’s a surprisingly lovely binary that from x50 splits open into a triple. All this with 4″ aperture, giving lots of colour.

SAO 131063/65 RA 04h15.3m. -07 39′.

Separation, A-BC 83″.  BC 8.9″. This bright star has a famous double companion , a white dwarf with a red dwarf. Orange yellow and indigo blue colours here.


From Sky Safari pro,

“Omicron2 Eridani, 40 Eridani – Keid
Appearing as an unassuming star of magnitude 4.43, 40 Eridani is actually a triple star system 16.4 light years away in the constellation Eridanus. 40 Eridani is also known as ο2 Eri, or Keid, from the Arabic word “qayd”, meaning (egg) “shells”. Its closeness makes the primary star of the system, 40 Eridani A, one of the few class K dwarfs visible to the naked eye.


40 Eridani A is an orange-yellow main sequence star of spectral type K1 V, stably fusing hydrogen into helium at its core (like the Sun). Keid A has a cool surface temperature of 5100 K, a low luminosity of 0.4 suns, and a mass around three-fourths solar. Such stars abound in space, but they are so faint that few can be seen without a telescope. Indeed, 40 Eridani is famed not for Keid A itself but for its much fainter companions. The pair 40 Eridani B/C was discovered in 1783 by William Herschel. Easily seen with a small instrument, 40 Eridani B lies 83″ away. It is a white dwarf of magnitude 9.50, by far the most visible of the breed. The distance between the white dwarf (40 Eri B) and the K star (40 Eri A) is at least 400 AU, and their orbital period is at least 7200 years.

More remarkably, the white dwarf has a companion too: a dim, class M4.5 Ve, magnitude 11.2, hydrogen-fusing red dwarf, called 40 Eridani C. The two orbit with a 252-year period, and are now about as far apart as they get (around 9″). Their orbit has an eccentricity of 0.410, and their distance varies from 21 AU to 49 AU, averaging 35 AU apart. Computed from this orbit, the white dwarf (40 Eridani B) has a mass of just 0.50 suns. The little star shines with 1.3% the luminosity of our Sun, a high temperature of 16,700 K, and a terribly small radius just 1.48 times the Earth’s. The red dwarf, 40 Eridani C, is much less massive than the white dwarf, at only 0.16 suns. Curiously, the actual luminosities of the two stars are comparable; the cool (about 3500 K) red dwarf’s is 2.2% of the Sun’s, most of it in the infrared. Like many of its cousins, 40 Eri C is a flare star, and has the variable star designation DY Eridani. Its magnetic field occasionally short-circuits, causing the star to brighten suddenly all across the spectrum.


White dwarf stars are the final product in the evolution of sunlike stars. Ordinary stars fuse hydrogen to helium in their cores. When the hydrogen is gone, they become giants and fuse the helium to carbon and oxygen. They then eject their outer envelopes, leaving the low-mass, ultra-dense, carbon-oxygen core as a white dwarf. Typical white dwarfs are only about the size of the Earth, and have extraordinary densities, typically averaging a ton per cubic centimeter. Although 40 Eri B is the least massive of the three “classic” white dwarfs (along with Sirius B and Procyon B), it is the largest of the trio. This is consistent with its lower mass, which produces a weaker gravitational pull and hence less squeezing. Its resulting average density is a quarter ton per cubic centimeter. Since higher mass stars live shorter lives than lower mass stars, the 40 Eri B must originally have been the most massive of the three, containing perhaps one solar mass of material. As a red giant, 40 Eridani B would have dominated the system, but ejected most of its mass before becoming a white dwarf. The K star (40 Eri A) will be next to go, while dim red 40 Eri C will last for a seeming eternity.

Fictional Planets

In Star Trek, 40 Eridani is the home star system of the Vulcans. How does science compare to science fiction? The “habitable zone”, where a planet could exist with liquid water, is around 0.61 AU from 40 Eridani A. At this distance, a planet would orbit in 203 Earth days, and 40 Eridani A would appear about 30% wider than the Sun does in Earth’s sky. An observer on a planet orbiting 40 Eridani A would see the B/C pair as unusually bright, white and red stars of magnitudes -8 and -6. They would be visible in daylight, though not bright enough to diminish the darkness at night. 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 40 Eri B or C. Any planets orbiting 40 Eri B would likely have been destroyed or sterilized by its evolution into a red giant and then a white dwarf. The red dwarf, 40 Eri C, is prone to flares that would be lethal to life on a planet in its habitable zone.”


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