Looking Up column: Double Cluster, an easy catch
The Double Cluster is a must for a late autumn evening sweep of the night sky, with even a small pair of binoculars. You can catch this spectacular star cluster pair with just your eyes on a dark night.
The pair is easy to find. Just look for the “Big M” - the five stars of Cassiopeia, nearly overhead in the northeastern sky at about 8 p.m. in early December. The bottom two stars of the “M” outline forms a nearly right triangle with the Double Cluster, situated to the right.
The Double Cluster is actually within the constellation Perseus, the Champion.
To the naked eyes on a dark night, the Double Cluster appears as a twin, fuzzy patch. They lay in front of the dim Milky Way Band, which passes through Cassiopeia. This is a wonderland of faint stars and star clusters within reach of binoculars and is especially rich in a telescope’s eyepiece.
The Double Cluster is also referred to separately as NGC 869 and NGC 884. The clusters are about 7,500 light-years from us. Each cluster has approximately 300 stars. The clusters are only a few hundred light-years apart; imagine the night sky from a planet orbiting any star nearby!
Binoculars begin to resolve the Double Cluster into their true nature, a whole lot of faint stars jammed near each other! Open star clusters are numerous across the heavens; each one is uniquely laid out and a delight to examine. The Pleiades is easily the most famous, a bright, compact, glittering patch of stars to behold with eyes alone.
The Double Cluster, however, is just that - two open star clusters of comparable size, richness and brightness, situated in close proximity in the sky.
The view in even a small telescope can be breathtaking. Take note of the little patterns the cluster stars make, the abundance of white and bluish-white stars and even a few dim, red stars sprinkled for good measure. You’ll find a red star smack dab between the two clusters.
Use a low-power eyepiece to give a wider field of view. That way you are more likely to see both clusters at the same time. Then exchange the eyepiece for a higher magnification and slowly scan around this area, comparing the clusters.
Then you may like to slowly venture in the sky right around the Double Cluster, using a small telescope and low power. I like to drop in on another interesting star arrangement close by, which to my imagination resembles a fish outline, but a fish sporting two whip antennae. As far as I know, this is not a true star cluster, but a chance arrangement of stars. See what you find.
I say “open star cluster” because there are also “globular star clusters,” hundreds of thousands of stars packed tight together in a ball, fraying at the edges, resembling what I get when I take a snowball at night and smash in against a dry sidewalk or asphalt road (doesn’t everyone do that?).
Perseus is an easily seen constellation, located between Cassiopeia and the Pleiades star cluster. In early evening, look up (of course; looking down the only celestial object you will see is planet Earth under your feet), in the east-northeast. Cassiopeia is on the left, high in the northeast; the Pleiades will be fairly high in the east.
At this time, the Perseus star pattern places his (Perseus’) head to the left, and stars marking his feet to the right.
The Pleiades cluster is just off the lower “foot” of Perseus.
One of the brighter and most well-known stars of Perseus is Algol, an eclipsing binary star that varies in brightness every 2 days, 20 hours and 48 minutes. Usually, the star is at maximum light, magnitude +2.3. There is a dimmer, close companion star orbiting Algol which regularly partly eclipses it, dropping its magnitude to +3.5. It stays at minimum light for only 20 minutes. It takes 4 1/2 hours for Algol to drop in brightness and another 4 1/2 to recover.
Many stars are like this, but Algol is one of the easiest to see and follow with eyes alone.
Take a look fairly low in the southwestern sky during evening twilight for Saturn and Jupiter. The two planets are getting closer and closer to each other, as seen from Earth. The apparent distance between them will shrink through Dec. 21, when the pair will look like a “double planet” - a spectacular and rare conjunction. This weekend they are about 1.8 degrees apart (about the width of three “full moons”). On Dec. 21 they will be just a tenth of a degree apart, about one-fifth the apparent width of the moon.
Jupiter is the brighter planet, to the lower right of Saturn.
Mars, dimming but still bright, is high in the south at around 8 p.m.
Venus is the bright “Morning Star” seen low in the eastern sky before dawn. Mercury is also there, but very low and not as bright.
Last quarter moon is on Tuesday, Dec. 8.
Keep looking up at the sky!
Peter Becker is managing editor at The News Eagle in Hawley, Pennsylvania. Notes are welcome at firstname.lastname@example.org. Please mention in what newspaper or website you read this column.