The Double Cluster is more formally known as h and Chi Persei, or NGC 884 and NGC 869. It resides in the northern part of the constellation Perseus, quite close to the constellation Cassiopeia the Queen.
The Double Cluster has to rate as one of the most magnificient deep-sky objects NOT to have been listed in the famous Messier catalogue. Of course, Charles Messier (1730-1817) was only interested in cataloguing deep-sky objects that could be mistaken for comets. Apparently, he thought nobody would see this pair of glittery clusters as a comet in the sky.
Although considered a deep-sky jewel, the Double Cluster is visible to the unaided eye in a dark country sky. If you zoom in on these rather feeble-looking stellar blobs with binoculars or a wide view telescope, they all of a sudden turn into two glorious star clusters.
How to find it
At mid and far northern latitudes, the Double Cluster is circumpolar – above the horizon every night of the year at any hour of the night. Yet, the Double Cluster is harder to see when it’s close to the horizon, so it’s best to look when the Double Cluster is somewhat high in the sky.
For general reference, the Double Cluster is high in the sky when the Big Dipper is low, and vice versa. Because the Big Dipper is lowest in the northern sky on late autumn and early winter evenings, the Double Cluster is highest in the northern sky at these times. The Double Cluster is pretty much always visible at evening except in late spring and summer.
Even in middle August, you can see the Double Cluster from late night till dawn. The annual Perseid meteor shower peaks around August 12 or 13, and if you trace the paths of these Persied meteors backward, they appear to originate near the Double Cluster.
To locate the Double Cluster, find the W or M-shape constellation Cassiopeia the Queen. (It looks like a W when low in the sky and an M when high in the sky.) Draw an imaginary line from the star Navi (Gamma Cassiopeiae) through the star Ruchbah (Delta Cassiopeiae), and go about 3 times the Navi/Ruchbah distance to locate the Double Cluster.
Science
The Double Cluster is thought to be over 7,000 light-years distant, and to be separated from one another by a few hundred light-years. It is amazing that we can see these stars at all across this great span of space. Each cluster contains a few hundred stars, harboring young, hot supergiant suns that are many thousands of times more luminous than our sun.
Astronomers tell us that the Double Cluster lies within the Perseus arm of the Milky Way galaxy. However, our solar system resides in the inner part of the Orion arm. Therefore, when we look at the Double Cluster, we are looking through our local spiral arm and all the way to the next spiral arm outward from the galactic center.
The two star clusters making up the Double Cluster are called NGC 869 (h Persei) and NGC 884 (chi Persei).
h Persei’s position is Right Ascension: 2h 19m; Declination: 57o 9′ north
chi persei’s position is Right Ascension: 2h 22.4m; Declination: 57o 7′ north
I have been seeing the double cluster now for over a month and showing it to others as my star parties over the lake at Union Reservour in Longmont–Tonight it was Jupiter and the double cluster that impressed everyone! Also saw three meteors shoot underneath Cassie.
Marvelous night!
I have just observed this wonderful deep sky object tonight after watching the transit of Io over the disk of Jupiter.
Does anyone know anything about the very conspicuous red star located in between the two clusters, such as star type, age, and which of the two clusters it belongs to?
It was a very nice night for observing – cool autumn temperatures with the crickets still chirping.
Raven,
I’ve noticed a few red stars in the direction of the Double Cluster myself. I’m guessing the star that you are referring to is FZ Persei. Possibly, this is a red supergiant, though I believe its distance is unknown. You may be interested in the article “Red supergiants, neutrinos and the Double Cluster” by Tristram Brelstaff at http://adsabs.harvard.edu/.
Bruce
Hi Bruce,
A million thanks for your reply ! I’ve been reading the article that you recommended and feel that I really hit upon an interesting celestial find. All those red stars add a nice touch to the cluster and make it a splendid showpiece, just as nice as the Pleiades. But it was the central red star that stood out and hopefully I learn more about it. In any case, this is one cluster I’ll be returning to again and again.
Thanks again. :)
Raven
Raven,
Please share with me anything you learn about that central red star.
Bruce
I sure will, Bruce. And hopefully soon. Thanks again !
- Raven
Hi Bruce,
A friend of mine in my local astronomy club and who specializes in deep sky objects verified that this particular star is indeed FZ Persei. It is part of the lesser dense cluster of the two, or NGC 884. Its age is believed to be about 11 million years old and its distance is 7650 light years away. It is of course a variable star, ranging from 9.8 to 10.77 with a period of 184 days. The odd part of this star is that it is redder when it is dimmer. Maybe this explains why this one red star caught my attention and not the others, but I haven’t estimated the magnitude when I saw it last time, nor do I know where on the magnitude period chart it currently is at.
There are about 80 papers written on this star. If you go to SIMBAD at:
http://simbad.harvard.edu/simbad/sim-fid
and type in “v* fz per” in the identifier box, submit it, and then scroll down to “References.” A list of those papers will be displayed. I haven’t checked them all out yet, since the information I’m relaying to you was just given to me yesterday. But it looks like there is a ton of great information there, with the oldest paper published in 1892 and the most recent published in 2008.
- Raven
Raven,
It’s amazing how much I learn from EarthSky readers. Thank you for all the information.
Bruce
You’re very welcome, Bruce. I’ve learned a lot from you too, including your other articles you’ve written.
hey i know a lot about double cluster and did you know:
right ascension:2h 20m
distance:7kly(2300Pc)
apparent magnitude(V):4.3
apparent dimensions(V):60′
other designations:h+xPersei, Caldwell 14