Perseus and Andromeda

    These two constellations were included in Ptolemy’s Almagest, the famous classical list of 48 ‘original’ constellations drawn up in Alexandria about 150 AD. (Ptolemy was an Alexandrian who lived from c.90 to 168 AD).

     In mythology Andromeda was the beautiful daughter of King Cepheus and Queen Cassiopeia, who unwisely boasted about Andromeda’s beauty and thereby offended the Sea God, Neptune. So Neptune sent a sea monster, Krakon, to ravage their kingdom. The Oracle was consulted and it was said that the only way to preserve the kingdom was to sacrifice Andromeda by chaining her to a rock so that she could be eaten by the monster. This was done, and the monster was about to devour her when our hero, Perseus, saves the day. Perseus was returning home after killing the Gorgon, Medusa. He was carrying home Medusa’s head as a trophy when he happens to see the chained Andromeda about to be devoured by Krakon. In the nick of time Perseus displays the head, the monster is turned to stone, Andromeda is saved.

   As this is a classical story the hero gets the spoils, since he marries Andromeda, and they remain in the sky, happy ever after.                       

   King Cephus and Queen Cassiopeia are, of course, constellations. The sea monster is said to be the constellation Cetus, while the Gorgon’s head is marked by the winking Demon Star, Algol. ( Beta Perseus ).

   Andromeda extends from R.A. 2 hours 35 minutes back to 22 hours 55 minutes, and Dec. 21 degrees to 58 degrees. Perseus extends from R.A. 4 hours 45 minutes to 1 hour 25 minutes, and Dec. 31 degrees to 59 degrees. And, as appropriate for a marriage of this sort, the two constellations adjoin each other along a curvy common boundary, approximately R.H. 2 hours 30 minutes and 50 degrees Dec.

   To me, Perseus resembles a giraffe, and is quite conspicuous, even though the brightest star (alpha) is only mag 1.8 and the famous Winking  Demon star, Algol, is mag 2.1 to 3.3. Algol is the type example of an eclipsing binary. It eclipses its partner every 2.5 days. Every 2.5 days approximately, Algol fades from 2.1 mag to 3.3mag  over a period of 5 hours, holds steady at 3.3 mag for 20 minutes, then over the next 5 hours goes back to 2.1 mag. This feature has been known since ancient times, although the reasoning is fairly recent. [On the text sheet there is a lettered and numbered sketch map of stars and clusters of the constellation.]. (Slide).

    ‘e’ Persei is a red semi-variable, ranging from mag 3.8 to mag 4.2..’K’ is a good comparison star, mag 4.0. Besides the double cluster (H VI 33, 34) and M 34, there are other clusters, the California Nebula, and numerous other double and variable stars. Also ‘E‘ Perseus is the radiant for the early August Perseids meteor shower. So for all, Perseus provides plenty of astronomy.

    Now let is look at the distaff side of the marriage. Andromeda, as a constellation, is pretty shapeless, its three brightest stars being… [ alpha, beta & gamma ?], all mag 2,0 to 2.1. [There follows another Greek lettered and numbered sketch map of the constellation.] (Slide).

     ‘Theta’ is a fine ‘easy’ telescopic double separation of 10 arc seconds. The primary is orange, 2 mag , while the companion is blue, 5 mag, and in turn also a double, although much harder to separate.

    ‘Beta’  is a red giant.

   A notable variable is ‘R’, near the [ Greek lettered ]  triangle, which varies from mag 6 down to mag 12.

  And, of course there is ‘M31’. I’m sure everybody has seen pictures of the Great Spiral Andromeda galaxy, complete with two satellite spirals, M 32 and NGC 205. I am also sure everybody has also seen it ‘live’, either with the naked eye or through binoculars or telescope. Well, not exactly ‘live’, as the light left it 1.5 million years ago. Our nearest full sized neighbouring galaxy, a short step for the cosmologist, a giant leap for the casual observer.


      [ Paul concludes with a final, third slide, and invites questions or comments.].