Find Perseus, the classic Greek hero in the night sky

Saturday, Feb 9, 2019 4:03 AM

Greetings, Stargazers.

There are lots of bright stars and constellations in a clear winter sky, but somehow, I haven’t written a column about the constellation Perseus. Maybe that is because the pattern of stars in Perseus isn’t much of a pattern. I only see a rather amorphous Y made up of second and third magnitude stars, so none are particularly bright. These follow the clearly distinctive W shape of Cassiopeia in the sky.

If you go outside shortly after dark, Perseus will be very close to your zenith, or just about straight overhead. If you make it outside a bit later, a more consistent way to find Perseus is on the line midway between Cassiopeia and the Pleiades, the open cluster in Taurus. They all will be sinking toward the northwestern horizon as the evening progresses.

In mythology, Perseus was the classic Greek hero, slaying great monsters and rescuing damsels in distress. He was the son of the Greek god Zeus and a mortal mother, Danae. And if you need to keep up with mythical genealogies, he was both the half-brother and great grandfather of Heracles (or Hercules, if you don’t mind mixing Greek and Roman stories).

Perseus was able to slay Medusa without being turned to stone by glimpsing her reflection in his brightly polished shield. Many of the statues of Perseus picture him holding up the severed head of Medusa.

In the constellation, Medusa’s head is being held by Perseus, and her eye is still blinking at us as the star Algol. Algol, also known as the demon star, is an eclipsing binary, and one of the few variable stars whose variation can be easily observed with the naked eye.

Every 2.86 days, there is an approximately 10-hour eclipse that causes the visible Algol to appear to dim from magnitude 2.1 down to magnitude 3.4 – a factor of 3.6 times dimmer. This happens when a larger but much cooler and therefore dimmer star passes in front of the brighter primary star. The Wikipedia page has a calendar and a description of how to calculate when to look for the next occultation.

As an aside, most variable stars show much smaller variations, and it is therefore much more challenging to see the changes. The human eye has an amazing ability to view both bright and dim things at the same time. However, a side effect is that eyes are not that great at noticing subtle differences in brightness. Changes as much as 20 percent are hard to detect. A comparison for those of you who are photographers is that 20 percent is less than a 1/3 f-stop difference.

Between Perseus and Cassiopeia is one of the more interesting binocular objects in the sky. NGC 869 and NGC 884 are two open clusters collectively known as the Double cluster. They are faintly visible to the naked eye on a clear, dark night when your eyes are well-adjusted to the dark. In mythology, they are the jeweled handle of Perseus’ sword. Why these never made Charles Messier’s list of fuzzy-looking things is a mystery, since they are much brighter than many of the clusters that are on the list.

This month

Although I am loath to use the now-popular term supermoon, there is another one this month. The full moon on the Feb. 19 more closely aligns with lunar perigee than the full moon last month, so this one will be even bigger. But I will challenge anyone to tell the size difference between the two just by looking.

Mars is still prominent in the evening sky and tonight (Saturday, Feb. 9) will be about 10 degrees above the crescent moon.

There is a planetary conjunction this month between Venus and Saturn. On the early morning of Feb. 18, the two will be separated by about 1 degree in the constellation Sagittarius.

It is hard for me to believe how quickly the year is racing by if I am already mentioning the summer constellation Sagittarius in a February column.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at hakes_c@fortlewis.edu.