The Double Cluster is the common name for the naked-eye open cluster

Open cluster

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...

s NGC 884

NGC 884

NGC 884 is an open cluster located 7600 light years away in the constellation of Perseus. The cluster is most likely around 12.5 million years old. It is the easternmost of the Double Cluster with NGC 869. Located in the Perseus OB1 association both clusters are located physically close to one...

 (χ Persei) and NGC 869

NGC 869

NGC 869 is an open cluster located 7600 light years away in the constellation of Perseus. The cluster is most likely around 13 million years old. It is the westernmost of the Double Cluster with NGC 884. Located in the Perseus OB1 association both clusters are located physically close to one...

 (h Persei), which are close together in the constellation Perseus

Perseus (constellation)

Perseus is a constellation in the northern sky, named after the Greek hero Perseus. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations defined by the International Astronomical Union...

. NGC 884 and NGC 869 are at distances of 7600 and 6800 light-years away, respectively, so they are also close to one another in space. The clusters' ages, based on their individual stars, are relatively young. NGC 869 is 5.6 million years old and NGC 884 is 3.2 million years old, according to the 2000 Sky Catalogue. In comparison, the Pleiades

Pleiades (star cluster)

In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...

 have an estimated age ranging from 75 million years to 150 million years. There are more than 300 blue-white super-giant stars in each of the clusters. The clusters are also blueshifted, with NGC 869 approaching Earth at a speed of 22 km/s and NGC 884 approaching at a similar speed of 21 km/s. Their hottest main sequence stars are of spectral type B0.

Mythology

Perseus is the mythological symbol of adventure. Perseus was born the son of Zeus (The “Father of Gods and men” ) and the mortal Danae. He was a demigod but not immortal. Perseus was challenged by King Polydectes of Seriphos to slay one of the Gorgons (Medusa), whose gaze turned an on-looking victim into stone. Athena, Hermes, and other gods gave Perseus a helmet, a shield, and a curved sword with studded jewels on its handle to aid him in the challenge. Along with beheading Medusa, Perseus performed other heroic deeds as well, such as saving Andromeda who was a princess chained to a rock as a sacrifice to a sea monster. Due to his great accomplishments, the gods placed Perseus among the stars, with the head of Medusa in one hand and the jeweled sword in the other. The Double Cluster represents the jeweled handle of Perseus’s sword.

History

Hipparchus, a Greek astronomer, cataloged the object (a patch of light in Perseus) as early as 130 B.C. However, the true nature of the Double Cluster wasn’t discovered until the invention of the telescope many centuries later. In the early 19th century William Herschel, the German-born English astronomer, was the first to recognize the object as two separate clusters. The Double Cluster is not included in Messier's catalog, but is included in the Caldwell catalogue of popular deep-sky objects not in Messier.

Location

The Double Cluster is circumpolar (continuously above the horizon) from most northern temperate latitudes. It is in close proximity to the constellation Cassiopeia the Queen. The Double Cluster approximately marks the illuminated point of the Perseid meteor shower, which peaks annually around August 12 or 13. Although easy to locate in the northern sky, observing the Double Cluster in its two parts requires optical aid. They are described as being an "awe-inspiring" and "breathtaking" sight, and are often cited as a target in astronomy observer's guides.

External links

• SEDS page on the Double Cluster
• AstroPhoto.net page the Double Cluster
• Dark Atmospheres Photography - Perseus Double Cluster
• The Double Cluster - Astronomy Picture of the Day
  Astronomy Picture of the Day
  Astronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
  
  2009 December 4
• DSLR Image of Double Cluster with Comet 103P/Hartley 2