SIGMA (σ) ORIONIS
MULTIPLE STAR

Right Ascension:
5h 38m 45s

Best Seen:
1/1 - 3/15

Declination:
-2º 36' 00" Combined
Magnitude:
3.73

Computer File:
sigmaori

Constellation:
Orion

Apparent
Separation: 41.0"

Actual

Compared to

Distance

1,150 l.y.

--

Separation

~ 1.5 trillion miles

~ 17,000 Earth/Sun

Orbital Period

varies - see below

--

Actual Brightness

--

varies - see below

Magnitude

varies - see below

--

Mass

--

--

Surface Gravity

--

--

Surface Composition (by mass)

74% hydrogen 24% helium 2% everything else

same (Sun)

Spectral Type

O9 V / B2 V / B2 V

G2 V (Sun)

Density (gram/cubic cm)

WHAT TO LOOK FOR THROUGH THE TELESCOPE:

  1. Recommended eyepiece: 24mm or 40 mm.

  2. When people look through the telescope they should see at least three stars near each other.

SIGMA ORIONIS INFORMATION:

  1. This system actually consists of five stars, three – maybe four – of which can be seen with the telescope.

  2. The “major” stars are Stars AB and Star E. They are about 1.5 trillion miles away from each other.

  3. The brighter star is actually two stars, “A” and “B.”

a. Apparent separation between stars A and B: 0.25".

b. Actual distance between the two: about 90 AU’s – they only take about 170 years to orbit each other.

c. Star A 1. Actual brightness of Star A: about 2400 times that of the Sun. 2. Magnitude of Star A: 4.1. 3. Spectral type of Star A: O9.5.

d. Star B 1. Actual brightness for Star B: about 778 times that of the Sun. 2. Magnitude of Star B: 5.2. 3. Spectral type of Star B: B0.5

  1. The star closest to Star AB is Star C. a. Apparent separation between AB and C: about 11.0". b. Actual distance between AB and C: about 3900 AU’s. c. Magnitude of Star C: ninth. d. Spectral type of Star C: A.

  2. Further away from AB, but brighter than C, are two other stars, Stars D and E.

a. Star D 1. Apparent separation between AB and D: about 12.0". 2. Actual distance between AB and D: about 4,600 AU’s. 3. Actual brightness of Star D: about 171 times that of the Sun. 4. Magnitude of Star D: seventh. 5. Spectral type of Star D: B2

b. Star E 1. Apparent separation between AB and E: about 41.0". 2. Actual distance between AB and E: about 15,000 AU’s 3. Actual brightness of Star E: about 212 times that of the Sun. 4. Magnitude of Star E: seventh. 5. Spectral type of Star E: B2.

For Greg: spacing in previous info section was intentional: kept star-specific info single spaced to save room, and hopefully to provide visual clues for what info goes with what.

More for Greg: here’s what was written previously...

  1. This system consists of five visible components (three of which can be seen with our telescope).

  2. Close pair “A” and “B” are 0.25" apart with a period of 170 years and combined mass of 35 solar masses. The spectral type of “A” is O9; “B” is B3.

  3. “C” is 10th magnitude; 11.2" away from “A” and “B”; spectral type A2.

  4. “D” is 7.5 magnitude; 12.9" away from “A” and “B”; spectral type B2

5a. “E” is 6th magnitude; 42" away from “A” and “B”; spectral type O9; and a luminosity 900 times the sun’s.

5b. This star has an 8th magnitude companion 30" away.

5c. Separation between “A” and “E” is at least 20,000 astronomical units or about 1/3 of a light year.

Item
Updated Notes

Coordinates 10-14-2002
tweaked a bit

Combined Magnitude
10-18-2002
OK with Scott’s The Flamsteed Collection

Apparent Separation 11-08-2002
based on coordinates given in Flamsteed

Distance
10-18-2002
previous: 1400 ly – BUT updated with Scott’s Flamsteed

Separation
11-08-2002
previous: actual: 1600 trillion miles – BUT “compared to sun” gives approx right answer

Orbital Period

Actual Brightness
11-08-2002
previous: 5,000 (Sun)

Magnitude
11-08-2002
too many to put in table

Mass

Surface Gravity

Surface Composition 01-06-2003
OK for all stars

Spectral Type
11-08-2002
based on Star AB and Star E

Density

Other Information
11-08-2002
info from Flamsteed, Scott’s Starlist 2000, and http://www.astro.uiuc.edu/~kaler/sow/sigmaori.html