October 30, 2007


Perseus is one of many children of Zeus in Greek mythology, and he is part of a royal family currently in the Northern and Eastern parts of the sky which includes Andromeda, Cassiopeia and Cepheus. Pegasus and the sea monster, Cetus, also play a role in Perseus' tale, and they are also found nearby in the sky. As I mentioned previously, during October, Perseus can be found in the Northeastern sky by following the long skinny constellation Andromeda down toward the Northeast horizon from the Great Square of Pegasus. Several bright stars in Perseus form an arc known as the Segment of Perseus, and to the right of the arc is Beta Persei, or Algol.

The constellation Perseus is also known as the Champion, and holds the Gorgon's head in one hand, represented by Algol. Algol is an eclipsing binary star, and its "winking" during occasional eclipses, may have made Greek storytellers imagine the baleful eye of the Medusa. According to the mythology, Zeus appeared to Danae, who was imprisoned by her father, and Zeus rained down on her as a shower of gold. There is some speculation that this story is related to the Perseid meteor shower, which occurs every August when the Earth is moving in the general direction of this constellation (and so the meteors appear to originate from this part of the sky).

In the excellent book Star Names by R. H. Allen, he mentions that the story of Perseus and Cetus may have been the foundation for the story of Saint George and the Dragon. In that tale, Saint George rescued a princess from a dragon and captured it. Returning to the village of the princess, he promised them that if they would convert to Christianity, he would slay the dragon for them. They did, and he did. Like all legends, though, this one is probably a retelling of an even older story.

Alpha Persei (Mirphak, Mirfak or sometimes Algenib) is a yellow giant star, the brightest in the binocular-worthy Alpha Persei cluster, a collection of over 50 bright stars within about a 3 degree diameter area. These stars all share the same proper motion, similar to the much larger Sco-Cen Association, and so this is likely a cluster of stars formed long ago from the same giant molecular cloud about 500 light years away. This cluster is right in the plane of the Milky Way (the feet of Perseus are standing on this starry road), so the field is very target-rich for a small telescope on a dark night.

Beta Persei (Algol) is also known as the Demon Star, perhaps the most famous of all eclipsing binaries, about 100 light years away. The name Algol literally comes from a shortened version of the Arabic for the Demon's Head. Ancient astrologers considered this a very unlucky star, which Burnham speculates means that they noticed its variability even in early times. Algol's magnitude dips from about 2.1 to 3.4 for several hours every 3 days or so, a result of a near total eclipse of the primary star by a dimmer companion. Recent studies have revealed the primary to be about a 100 solar mass B star, and the companion a subgiant just a bit larger and more massive than our Sun. The story on this system is very complex and interesting, still with plenty of mysteries to unravel.

Among the deep-sky objects in this part of the sky is M34, a bright open star cluster a few degrees West of Algol near the border with Andromeda, best seen with binoculars or low-power eyepieces (due to the lack of lots of background faint stars). It is about 1400 light years away, so the stars you are seeing here are pretty bright. The whole cluster is fairly young, maybe 100 million years old, so a little older than the Pleiades.

M76 is also called the Little Dumbbell Nebula and is the faintest of all the Messier objects, about 1800 light years away. This is a double-lobed planetary nebula, which has lots of arcs and filaments being thrown off the central star, shown by deeper images. Also present in this part of the sky is the vast California Nebula, about five times larger in length than the diameter of the full moon, it shows up very nicely in deep photographs near the loose Zeta Persei cluster, about 8 degrees North of the Pleiades. I should also mention the reflection nebula NGC 1333, which APOD does a great job with here.

The most commonly observed cluster in Perseus is definitely the Double Cluster, also known as "h and xi Persei". It is about halfway between Perseus and Cassiopeia in the sky, right where the sword hand of Perseus is typically depicted (his other hand holds the Gorgon's head), about 8000 light years distant, barely visible to the naked eye as a fuzzy patch under the best conditions. Burnham considers it strange that Messier never included it in his catalog, and I agree.

The brightest members are all hot supergiants (we can guess sizes of stars by looking at spectral lines, a story for another time), with luminosities nearly 60,000 times that of our Sun, comparable to Rigel. Based on our knowledge of galactic structure, it seems as though the Double Cluster is one of the few objects we can easily observe that exists in the next arm over in our galaxy, moving outward from the direction of the galactic center (over toward Sagittarius, nearly 180 degrees away on the sky).

The clusters are likely not directly linked to one another. One is about 1000 light years more distant than the other and perhaps twice as old, so it is just an alignment coincidence here. The younger of the pair of clusters is probably one of the ten or so youngest clusters of stars in our galaxy.

The final object I'll talk about in this region is extragalactic: Perseus A. It is the brightest of a small galaxy cluster located about 2 degrees East of Algol, about 300 million light years distant. This elliptical galaxy is somewhat distorted in shape and a very bright source of radio emission. It is speculated that we are witnessing a galactic collision, based on the internal motions of different parts of this object. This is a very well-studied object and clearly has a complex story to tell, one we are still trying to piece together.

In sum, with a small telescope and a dark sky, you'll find plenty worth observing in this region of the sky besides Comet Holmes.

Posted by Observer at October 30, 2007 04:51 PM

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

chi, not xi, Persei. xi Per is an O star, used to be observed a lot for looking at interstellar absorption lines (in the visible) and comparing that to other ISM tracers (millimeter wave molecular lines, etc.).

Posted by: Feff on November 7, 2007 06:11 PM

See, if your office were still down the hall from mine, I wouldn't have to make that kind of mistake! You know, I'll bet if you had a year to kill at some observatory out in the sticks, you could crank out one hell of an updated Burnham book.

Posted by: Observer on November 7, 2007 11:45 PM

No, I doubt that. I don't know much about galaxies, and I haven't done any observing in the southern hemisphere at all. Star clusters are sort of my cup of tea, though.

Posted by: Feff on November 8, 2007 11:04 AM