Perseus

by Shaphan Shank | Jan 2, 2024 | 0 comments

Double Cluster, NGC 869 & NGC 884
Double Cluster, NGC 869 & NGC 884. Photo © Boris Stromar|Dreamstime.com.

To the ancient Greeks, the stars of Perseus represented a hero. To me, this constellation bears little resemblance to a hero or to any other recognizable object. However, although its shape is not outstanding, Perseus contains an outstanding array of stars and deep sky objects. Perseus lies along the band of the Milky Way just east of the constellation Andromeda and just west of Capella, the brightest star that is currently high overhead at mid-northern latitudes.

Mirphak, the brightest star in Perseus, is surrounded by a large group of stars called the Alpha Persei Association. This scattered group is actually an open star cluster, though it has no Messier (M) or NGC number like most smaller clusters. The Alpha Persei Association is too large to be a good telescopic target, but it is an attractive target for binoculars.

Most of the time, Algol is the second-brightest star in Perseus. However, this star is an eclipsing binary. Although it looks like a single star, Algol consists of two stars that orbit each other every few days in an orbital plane that is aligned with Earth. When the component stars line up with Earth, Algol’s apparent brightness drops from magnitude 2.1 to magnitude 3.4 because we are only seeing light from one of the stars instead of both stars. The individual component stars of Algol are much too close together to be visually separable, but the overall brightness changes are easy to observe by regularly comparing the brightness of Algol to that of surrounding stars. Algol’s eclipses occur every 2.87 days, and each eclipse lasts about 10 hours. Algol appears dimmest near the middle of each eclipse.

A bright open star cluster, M34, lies about 5° northwest of Algol. M34 is attractive when viewed at low magnification with a telescope, but it is also a good binocular target. Binoculars will show the cluster as a fuzzy patch of light with several of the brighter stars resolved.

Perseus contains lots of double stars. One of the best of these is Eta Persei, the northernmost star in the figure of Perseus. Eta Per consists of a bright orange primary star that is moderately separated (28 arcseconds) from a fainter bluish-white secondary star. A telescope at low magnification will comfortably split this double.

The Double Cluster (NGC 869 & 884) is unquestionably the most impressive deep sky object (or pair of objects) in Perseus. True to its name, the Double Cluster is a pair of very similar open star clusters that lie several hundred light years apart and about 7,500 light years from Earth. This pair of clusters lies just under 5° northwest of Eta Persei, right between the constellations of Perseus and Cassiopeia. The Double Cluster is clearly visible to the unaided eye as a fuzzy little patch of dim light. Binoculars will begin to resolve a few of the stars in the clusters. A telescope will reveal two glittering masses of stars that sit just close enough together to fit in the same field of view at low magnification.

Several nebulas lie within the bounds of Perseus, but only one of these is easily visible with a telescope. This nebula, the Little Dumbbell (M76), is a planetary nebula that lies about 1° north of the star Phi Persei, in the northwestern part of Perseus. M76 is fairly large compared to many other planetary nebulas, but it is still a pretty small object. A mid-sized telescope at high magnification should show this nebula as an hourglass- or dumbbell-shaped cloud that is brighter at the ends than in the middle. Larger telescopes may show fainter arcs of gas curving away from the bright ends of the nebula. An OIII filter will boost the contrast of this nebula, but the filter is not required to see the nebula well.

star map of Perseus

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