[ How the Choice was Made ][ Vulcan in Earth's Sky ][ Vulcan's Star System ][ The Planet Vulcan and its Sky ]
 

The Choice of a Star for Vulcan:
The star around which Vulcan orbits was never identified in the original series or in any of the feature films and so has never
been officially established.
In the literature generated by the series, both 40 Eridani and Epsilon Eridani have been suggested for Vulcan's star.
In the end 40 Eridani was chosen mainly because of its age.  It is about 4 billion years old (the same as our sun) , whereas
Epsilon Eridani is barely 1 billion years old.  It was thought that intelligent life and an advanced civilization would not have
had time to evolve on a younger star.

How to find Vulcan in the Earth Sky:

The trinary system 40 Eridani (Omicron 2 Eridani in some star atlases) is located in the constellation Eridanus
    at RA 4h 13m D -07d 44m; the system shines in our sky at magnitude 4.4 (easily visible, but not conspicuously so).
    Small telescopes will show 40 Eridani A next to a single white star (actually B and C, but small telescopes won't show
    them as separate). A larger telescope--say, an 8-incher--will be able to show B and C as separate, so that the system
    will look like this:
 

                                                            * (A)
                                                * (B)
                                                 * (C)

      40 Eridani B--the white dwarf--has the distinction of being the easiest-to-see white dwarf in Earth's night sky.

Vulcan's Star System:
40 Eridani--also known as Omicron 2 Eridani--is a fascinating trinary system located at a distance of only 4.9 parsecs (about
16 light-years) from Earth. The system consists of three stars: 40 Eridani A, B, and C.
The two companion stars--a 9th-magnitude white dwarf and an 11th-magnitude red dwarf--orbit each other about 400
astronomical units from the primary. They would gleam brilliantly in the Vulcan sky with apparent magnitudes -8
and -6, respectively.
 
 

	40 Eridani A	40 Eridani B	40 Eridani C	Our Sun
Description	orange main sequence dwarf	white dwarf	red dwarf	yellow
Surface Temperature (deg K)	5100	14,000	3300	6000
Spectral Class	K1e	DA4	M4.5e	G
Luminosity Class	V	WD   (VII)	V
Apparent Visual Magnitude	4.43	9.52	11.17
Absolute Visual Magnitude	5.91	11.01	12.66	4.75
Visual Luminosity (x Sol)	0.376	0.00345	0.000754	1
Mass (x Sol)	0.75	0.44	0.15	1
Diameter (x Sol)	1.063	0.019	0.28	1
Comfort Zone (in A.U.s)	0.613	0.0587	0.0275	1
Orbital Period in Comfort Zone (days)	202.515	7.83406	4.29184	365.25
Angular Size of Star in sky in CZ (degrees)	0.926825	0.172513	5.435548
starspot cycle	roughly 11 years

40 Eridani A, the primary star (and Vulcan's sun) is an orange main-sequence dwarf of spectral type K1 V, which makes it a
cousin to our own Sun. (It's interesting that the star is an orange one; in the telvesion show the sky on Vulcan is colored
orange.)

The other 2 stars in this trinary system, 40 Eridani B and C, are both situated 400 AU (about 2 light-days or 40 billion miles)
from 40 Eridani A-- an enormous distance. 40 Eridani B and C orbit *each other* every 248 earth-years, at an average
distance from each other of 44 AU (about 4.1 billion miles, greater than the distance from the Sun to Pluto). In turn, 40 Eridani
B and C--*together*--orbit 40 Eridani A every 8000 or so earth-years.

The enormous density of 40 Eridani B is typical of a white dwarf: 44% of the Sun's mass crammed into a sphere only twice the
diameter of the earth. The stellar material of 40 Eridani B is 90,000 times denser than water; a cubic inch of it would weigh
2 tons, and the surface gravity of the star turns out to be 37,000 times that of Earth.

40 Eridani C is a very faint red main-sequence dwarf of spectral type M4e V.

The Planet Vulcan and its Sky:

In order for the planet Vulcan to be centered in the comfort zone of 40 Eridani A (Vulcan's sun) it would have to be at a
distance from its star of 0.613 AU (about 57,000,000 miles - equivalent to a solar orbit just inside that of Venus in our
system).  The comfort zone is that distance from the star where water can remain in liquid form.  Liquid water is necessary
for the existence any kind of life that we know (and presumably any kind of alien life too).  Any closer to the Vulcan sun
and the water would be evaporated into steam; any further out it would be frozen into a solid.
At this distance, 40 Eridani A would appear from Vulcan as a disk 0.86 arc-degrees in diameter, about 62% larger than
the Sun appears to us.

40 Eridani B and C are much too far away from 40 Eridani A to appear as disks to an observer on Vulcan; they would
appear as points of light to the naked eye, albeit very *bright* points of light.

The apparent distance between B and C would change as they orbit each other, but to an observer on Vulcan, B and C
would always appear near each other--in fact, even when B and C are as far apart as they ever get, they would appear
together in the Vulcan sky only as far apart as the width of your fist held out at arm's length.

Both B and C are bright enough to be easily visible in Vulcan's daytime sky, though (at their distance) they wouldn't really
be contributing much to Vulcan in the way of light and heat.

40 Eridani B would appear as a blazing silver-white star shining at magnitude -7.4 in the Vulcan sky (some 16 times brighter
than the brightest that Venus ever appears in our sky).

40 Eridani C would appear as a brilliant blood-red star of magnitude -6.3 (about 6 times brighter than the bbrightest Venus
we ever see).

At night, they would shine with a combined magnitude of -7.8, some 22 times brighter than our brightest Venus--though still
only about 1/47th of the brightness of our full Moon. Enough to cast a dim shadow, maybe, but dim enough to let night
remain night.