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Sigma (σ) Orionis

Multiple Star System
Right Ascension 5h 38m 45s Best Seen 1/1 - 3/15
Declination -2° 36' 00" Combined Magnitude 3.73
Apparent Separation 41.0" Constellation Orion

Actual		 Compared
to Sun
Distance 1,150 ly --
Separation ~ 1.5 trillion miles 17,000 au
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
Spectral Type 09 V / B2 V / B2 V G2 V
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"
    Apparent Separation ~ 0.25"
    between A and B
    Actual Distance ~ 90 au
    between A and B
    only takes about 170 years to orbit each other

    c. Star A
    Actual brightness about 2400 times that of the Sun
    Magnitude 4.1
    Spectral Type O9.5

    d. Star B
    Actual brightness about 778 times that of the Sun
    Magnitude 5.2
    Spectral Type B0.5

    e. Star C (star closest to Star AB)
    Apparent Separation ~ 11.0"
    between AB and C
    Actual Distance ~ 3,900 au
    between AB and C
    Magnitude ~ 9
    Spectral Type A

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

    a. Star D
    Apparent Separation ~ 12.0"
    between AB and D
    Actual Distance ~ 4,600 au
    between AB and D
    Actual Brightness ~ 171 x Sun
    Magnitude ~ 7
    Spectral Type B2

    b. Star E
    Apparent Separation ~ 41.0"
    between AB and E
    Actual Distance ~ 15,000 au
    between AB and E
    Actual Brightness ~ 212 x Sun
    Magnitude ~ 7
    Spectral Type B2
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References
Item Updated Notes
Coordinates 2002-10-14 tweaked a bit
Combined Magnitude 2002-10-18 OK with Scott’s The Flamsteed Collection
Apparent Separation 2002-11-08 based on coordinates given in Flamsteed
Distance 2002-10-18 previous: 1400 ly – BUT updated with Scott’s Flamsteed
Separation 2002-11-08 previous: actual: 1600 trillion miles – BUT "compared to sun" gives approx right answer
Orbital Period --
Actual Brightness 2002-11-08 previous: 5,000 (Sun)
Magnitude 2002-11-08 too many to put in table
Mass --
Surface Gravity --
Surface Composition 2003-01-06 OK for all stars
Spectral Type 2002-11-08 based on Star AB and Star E
Density --
Other Information 2002-11-08 info from Flamsteed, Scott’s Starlist 2000, and http://www.astro.uiuc.edu/~kaler/sow/sigmaori.html