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Alpha (α) Capricorni

Algedi
Optical Double Star
Right Ascension 20h 17m 51s Best Seen 9/1 - 11/15
Declination -12° 31' 06" Combined Magnitude 4.30
Apparent Separation 389" Constellation Capricorn

Actual		 Compared
to Sun
Distance 686 / 109 ly --
Actual Brightness -- 584 / 32
Magnitude 4.44 / 3.58 --
Mass -- 5 / 2.5
Surface Gravity -- --
Surface Composition (by mass) 74% hydrogen
24% helium
2% everything else		 same
Spectral Type G3 Ib / G8 III 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 two stars next to each other. One star should be slightly brighter than the other.

Alpha Capricorni Information:

  1. This is 4th magnitude optical double star in the constellation Capricorn. They look close together, but they are not actually related to each other.

    • Both are rich, yellowish-orange
    • Temperatures for both are around 8,500⁰ F, similar to our Sun
    • Both have stopped fusing hydrogen in their cores and are preparing to fuse their internal helium to carbon.

  2. This star is also called Algedi (or Al Giedi), from the Arabic meaning “the kid” (a young goat) and it refers to the whole constellation of Capricorn.

  3. Since both stars can be seen with the unaided eye, it is unclear whether “Algedi” refers to both stars, or just the brightest star.

  4. Note that despite their similar magnitude, one is much farther – and much more luminous – than the other.

  5. Though most stars with the name “alpha” are the brightest star in their constellation, with the combined magnitude of the two stars, Alpha Capricorni is actually the third brightest star in Capricorn (delta capricorni is the brightest). It could be named “alpha” because of its position in the constellation, resulting in it being the first star named.

    a. Alpha1 Cap
    Spectral Type G3, evolved yellow supergiant
    Distance 690 light years away (light traveling since the dark ages of the 1300s)
    Temperature 8700 °F
    Luminosity 930 Suns, 176 Suns total
    Diameter 40 Suns (and 5x bigger than Alpha2 Cap)
    Mass 5 solar masses

    b. Alpha2 Cap
    Spectral Type G8, yellow giant
    Distance 109 light years away (light traveling since early 1900s)
    Temperature 8050 °F
    Luminosity 43 Suns visual, 48 Suns total
    Diameter 8 Suns
    Mass 2.5 solar masses
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References
Item Updated Notes
Coordinates 2002-11-26 split difference between two
Combined Magnitude 2002-11-26 with Scott’s The Flamsteed Collection
Apparent Separation 2002-11-26 from coordinates
Distance 2002-11-26 from Flamsteed and SIMBAD
Actual Brightness 2002-11-26 from Flamsteed
Magnitude 2002-11-26 from Flamsteed
Mass 2002-11-26 http://www.astro.uiuc.edu/~kaler/sow/algedi.html
Surface Gravity --
Surface Composition 2003-01-06 OK for all stars
Spectral Type 2002-11-26 a Cap1: SIMBAD says G3 Ib -- Flamsteed says G3 Ib
a Cap2: SIMBAD says G6/G8 III -- Flamsteed says G9 III
Density --
Other Information 2002-11-26 http://www.astro.uiuc.edu/~kaler/sow/algedi.html
INCLUDE THIS ?? : Through a telescope, both components of Alpha Capricorni have faint companions. Plus, SIMBAD says both stars are a “star in double system.” It is unclear whether the components and their companions form optical doubles or multiple stars.