Tuesday, August 07, 2007




In most years the Perseid meteor shower is the most spectacular of the many meteor showers observable. There are usually between 50 and 80 meteors per hour on the night of its peak which this year falls on Sunday August 12 and Monday August 13. Here's a little introduction to the constellation Perseus from which these meteors seem to originate.


The exact location of any constellation varies, of course, with the season and the time of night. Perseus, however, is a circumpolar constellation, and one can locate it easily by reference to other northern sky landmarks. At this time of year it can be found in the northeast;the later at night the higher it will be seen. Draw a line from the two "pointer stars" of the Big Dipper, Dubhe and Merak, across the Polaris the North Star. Continue the line onward past the inverted pentagon of Cepheus to the "W" of Cassiopeia. Look down towards the horizon, and you'll find the "K" of Perseus. The above diagram shows the nearby constellations and the radiant (the apparent origin of the Perseid meteors). If you observe late enough at night(best just before dawn) you will also see the planet Mars beneath Perseus towards the horizon.

If this is not helpful there are a number of star maps online that can help you in locating any of the constellations. See:

and for a selection of many sky maps, some free and some not, go to Planetarium Software.
The diagram to the left illustrates the major stars of Perseus. Molly has always seen Perseus as the "K" shape described by most commentators. Others have described it as a "fleur de lis", but I have consistently been unable to see such a thing in this constellation. Perseus contains four named stars. The brightest, alpha-Per, is usually called Mirfak, from the Arabic for elbow, and sometimes Algenib, from the Arabic for "the side" (though other stars also bear this name). It is a white supergiant of 1.8 apparent magnitude and is about 620 light years (lys) away. Mirfak is 62 times the size of our sun and 5,000 times as bright.
The most famous star in Perseus is beta-Per, Algol the demon star. The name comes from the Arabic "Alghul" which means "the ghoul" or "the demon". It is often referred to as "the head of the demon" because it is supposed to represent the head of Medusa (see 'The Legend of Perseus' later in this blog). This star is the classic eclipsing binary variable star. In these systems each member of the pair periodically eclipses the other. Algol consists of a blue main sequence star and a yellow-white subgiant. This system usually shines with a magnitude of 2.1, but every 2.87 days it dims to magnitude 3.4 for 10 hours as the subgiant passes in front of the other star. this system is 105 light years away.
Xi-Per is called Menkib from the Arabic for "shoulder". It is a blue star of magnitude 4 with a distance of 150 light years. Omicron-Per is called Atik from the Arabic for "collarbone". It is a blue giant of magnitude 3.8 and is 1,000 light years away.
Epsilon-Per (to the horizon from Mirfak and to the west of Algol) is the main radiant of the Perseid meteor showers. Note that the first diagram above is incorrect in this matter, though the casual observer could hardly tell the difference. There are also, however, other radiants within the constellation, and so not all the meteors seen will be traceable back to the neighbourhood of epsilon-Per. Epsilon-Per is a binary system about 130 lys from Earth. the brighter component is a blue main sequence star of magnitude 2.9, and the dimmer is only of the 8th magnitude. Stars dimmer than magnitude 6 are invisible to the naked eye.
Stay tuned tomorrow.

No comments: