Ptolemy Cluster
Encyclopedia
Messier 7 or M7, also designated NGC 6475 and sometimes known as the Ptolemy Cluster, is an open cluster
of star
s in the constellation
of Scorpius
.
The cluster is easily detectable with the naked eye
, close to the "stinger" of Scorpius
. It has been known since antiquity
; it was first recorded by the 1st-century astronomer Ptolemy
, who described it as a nebula
in 130 AD. Giovanni Batista Hodierna observed it before 1654 and counted 30 star
s in it. Charles Messier
catalogued the cluster in 1764 and subsequently included it in his list of comet
-like objects as 'M7'.
Telescopic
observations of the cluster reveal about 80 stars within a field of view of 1.3° across. At the cluster's estimated distance of 800–1000 light years this corresponds to an actual diameter of 18–25 light years. The age of the cluster is around 220 million years while the brightest star is of magnitude
5.6.
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...
of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Scorpius
Scorpius
Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...
.
The cluster is easily detectable with the naked eye
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
, close to the "stinger" of Scorpius
Scorpius
Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...
. It has been known since antiquity
Ancient history
Ancient history is the study of the written past from the beginning of recorded human history to the Early Middle Ages. The span of recorded history is roughly 5,000 years, with Cuneiform script, the oldest discovered form of coherent writing, from the protoliterate period around the 30th century BC...
; it was first recorded by the 1st-century astronomer Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...
, who described it as a nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
in 130 AD. Giovanni Batista Hodierna observed it before 1654 and counted 30 star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s in it. Charles Messier
Charles Messier
Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of deep sky objects such as nebulae and star clusters that came to be known as the 110 "Messier objects"...
catalogued the cluster in 1764 and subsequently included it in his list of comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
-like objects as 'M7'.
Telescopic
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
observations of the cluster reveal about 80 stars within a field of view of 1.3° across. At the cluster's estimated distance of 800–1000 light years this corresponds to an actual diameter of 18–25 light years. The age of the cluster is around 220 million years while the brightest star is of magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
5.6.