mardi 23 septembre 2008

Perseus 4

Perseus isn't a very clear asterism; some forms of the constellation have a straight line from alpha to theta, perhaps indicating Perseus' sword or sickle that he used to kill the Medusa.
However, as far back as Ptolemy's time, Perseus was seen as holding the head of Medusa, with Algol (beta Persei) being the "evil eye" of the Gorgon.
Perseus' stars are fairly bright. A good many of them go under other than Bayer names; several are notable binaries. There are also a few fine deep sky objects.

Double stars:
Epsilon Persei is rather difficult because of the dim companion: 2.9, 8.1; PA 10º, separation 8.8".
Zeta Persei is a multiple system, also with faint companions:
AB: 2.9, 9.5; 208º and separation 12.9". C is a dim 11.3, PA 286º and separation 32.8"; D is 9.5, 195º, 94.2".
Eta Persei is a fixed triple system; AB are yellow and blue.
AB: 3.8, 8.5; PA 300º, separation 28.3". C: 9.8; 268º, separation 66.6".
Struve 331 is a splendid fixed binary: 5.4, 6.8; PA 85º, separation 12.1".
It's found midway on a line between gamma Persei and tau Persei and just a bit south.
h1123 is a fine wide binary in the middle of M34: 8.5, 8.5; PA 248º, separation 20.0".
Variable stars:
Perseus had many types of variable stars, many of which are too small to notice. Below are some of the more obvious examples.
Beta Persei (Algol) is a notable EA type eclipsing binary, changing from 2.12 to 3.39 every 2.8673 days (2d, 20h 48m 56s) as the companion eclipses the primary. The eclipse lasts roughly ten hours.
The name itself, Al Ghul, means "Mischief Maker"; it is sometimes called Ras Algol, Ra's al Ghul, "The Demon's Head".
This is a very bright white star, and the first eclipsing binary every discovered, in 1669 (thus giving the name "Algol variable" to this type of star). However the theory itself of an eclipsing binary being responsible for the variations in visual magnitude had to wait until 1783; this theory was only proved correct in 1889.
The whole system is a bit more complicated than one star occulting another. A third component, Algol C, orbits both A and B about every 1.86 years, and even more companions have been proposed, but not proven.
R Persei is a long-period variable, 8.1 to 14.8 every 209.89 days. The next maximum is scheduled for mid May 1997 (then that December). In 2000 the maximum should occur the last week of October.
S Persei is an SRc type variable, 7.9 to 12 every 822 days (2.25 years). The next maximum should occur in the last half of October, 1998.

Deep Sky Objects:
M34 (NGC 1039) is a fine open cluster containing about eighty stars. The cluster is considered about 100 million years old.
The cluster is about five degrees WNW of Algol (beta Persei), or more precisely twenty-seven arc minutes west of Algol and two degrees north.
NGC 869 and NGC 884 form the well known "Double Cluster", two open star clusters side by side, easily seen by naked eye or binoculars.
The clusters are both considered babies, 869 only being about 6.5 million years old, and 884 about 11-12 million years old.
The easiest way to find them is to form a triangle, using gamma Andromedae and alpha Persei. Then the northern point becomes the twin clusters.
NGC 1499, The California Nebula, is a gaseous nebula one degree north of zeta Persei, and stretching itself in an east-west direction. Unfortunately it is extremely faint and difficult to view. In fact binoculars might afford the best chance.

Aucun commentaire: