The Double Cluster (also known as Caldwell 14) consists of the
open cluster
An open cluster is a type of star cluster made 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, an ...
s
NGC 869 and
NGC 884 (often designated h Persei and χ (chi) Persei, respectively), which are close together in the constellation
Perseus
In Greek mythology, Perseus (Help:IPA/English, /ˈpɜːrsiəs, -sjuːs/; Greek language, Greek: Περσεύς, Romanization of Greek, translit. Perseús) is the legendary founder of Mycenae and of the Perseid dynasty. He was, alongside Cadmus ...
. Both visible with the naked eye, NGC 869 and NGC 884 lie at a distance of about 7,500 light years in the
Perseus Arm
The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called the Scutum–Centaurus Arm. The Perseus Arm begins from the distal end of the long Milky Way central bar. Previously thought to be 13,000 lig ...
of the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
galaxy.
Membership
NGC 869 has a mass of 4,700 solar masses and NGC 884 weighs in at 3,700 solar masses; both clusters are surrounded with a very extensive halo of stars, with a total mass for the complex of at least 20,000 solar masses.
They form the core of the
Perseus OB1 association of young hot stars.
Based on their individual stars, the clusters are relatively young, both 14 million years old.
In comparison, the
Pleiades
The Pleiades (), also known as The Seven Sisters, Messier 45 and other names by different cultures, is an asterism and an open star cluster containing middle-aged, hot B-type stars in the north-west of the constellation Taurus. At a distance ...
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
blueshift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...
ed, with NGC 869 approaching Earth at a speed of and NGC 884 approaching at a similar speed of . Their hottest main sequence stars are of
spectral type
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
B0. NGC 884 includes five prominent
red supergiant
Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Ant ...
stars, all variable and all around 8th magnitude:
RS Persei, AD Persei, FZ Persei, V403 Persei, and V439 Persei.
History
Greek astronomer
Hipparchus
Hipparchus (; el, Ἵππαρχος, ''Hipparkhos''; BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equi ...
cataloged the object (a patch of light in Perseus) as early as 130 BCE. To Bedouin Arabs the cluster marked the tail of the smaller of two fish they visualized in this area, and it was shown on illustrations in
Abd al-Rahman al-Sufi's ''
Book of Fixed Stars
The ''Book of Fixed Stars'' ( ar, كتاب صور الكواكب ', literally ''The Book of the Shapes of Stars'') is an astronomical text written by Abd al-Rahman al-Sufi (Azophi) around 964. Following the translation movement in the 9th centu ...
''.
However, the true nature of the Double Cluster was not discovered until the invention of the telescope, many centuries later. In the early 19th century
William Herschel
Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline ...
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
The Caldwell catalogue is an astronomical catalogue of 109 star clusters, nebulae, and galaxies for observation by amateur astronomers. The list was compiled by Patrick Moore as a complement to the Messier catalogue.
While the Messier catal ...
of popular deep-sky objects.
The clusters were
designated h Persei and χ Persei by
Johann Bayer in his ''
Uranometria
''Uranometria'' is a star atlas produced by Johann Bayer. It was published in Augsburg in 1603 by Christoph Mangle (''Christophorus Mangus'') under the full title ''Uranometria: omnium asterismorum continens schemata, nova methodo delineata, a ...
'' (1603). It is sometimes claimed that Bayer did not resolve the pair into two patches of nebulosity, and that χ refers to the Double Cluster and h to a nearby star. Bayer's ''Uranometria'' chart for Perseus does not show them as nebulous objects, but his chart for Cassiopeia does, and they are described as ''Nebulosa Duplex'' in Schiller's ''Coelum Stellatum Christianum'', which was assembled with Bayer's help.
Location
The Double Cluster is
circumpolar
Circumpolar may refer to:
* Antarctic region
** Antarctic Circle
** the Antarctic Circumpolar Current
** Subantarctic
** List of Antarctic and subantarctic islands
** Antarctic Convergence
** Antarctic Circumpolar Wave
** Antarctic Ocean
* Arctic ...
(continuously above the horizon) from most northern temperate latitudes. It is in proximity to the constellation
Cassiopeia. This northern location renders this object invisible from locations south of about 30º south latitude, such as New Zealand, most of Australia and South Africa. The Double Cluster is approximately the
radiant 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.
Mythology
Perseus was a famous hero of Greek mythology, a son of the Greek god
Zeus
Zeus or , , ; grc, Δῐός, ''Diós'', label=Genitive case, genitive Aeolic Greek, Boeotian Aeolic and Doric Greek#Laconian, Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label=Genitive case, genitive el, Δίας, ''D ...
. Along with beheading Medusa, Perseus performed other heroic deeds such as saving princess Andromeda who was chained to a rock as a sacrifice to a sea monster, Cetus. The gods commemorated Perseus by placing him 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 his sword.
References
External links
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{{Caldwell catalogue
Open clusters
014b
Asterisms (astronomy)
Astronomical objects known since antiquity