m7 – Open cluster

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Lupe Rioné, Amateur astronomer (Spain)

Type

Open cluster

Location

Scorpio Constellation

Distance

980 light-years

Size

25 light-years

Open clusters: Also called galactic clusters, because they are found within the galaxy, open clusters are groups of only tens or hundreds of young stars formed from the same nebula. They have a random, usually asymmetric, structure, and their stars are gravitationally bound to each other.

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History and discovery

So far, all the Messier Catalog objects take us back to the 17th and 18th centuries to tell the story of their discovery. However, M7 is the exception. This object is also known as the Ptolemaic Cluster, because it was the Alexandrian astronomer Claudius Ptolemy who first described it in his work the Almagest in 130 AD. In his work, he describes it as a “nebulous cluster behind the sting of Scorpius. For this reason, M7 is the first object in the Messier Catalog to be discovered in antiquity.

In 1654, the astronomer Giovanni Batista Hodierna managed to list about 30 stars in M7, and later Edmund Halley also included it in his Catalog. Charles Messier discovered it on his own in 1764: “I have determined on the same night (May 23-24) the position of another cluster of stars which is more considerable and larger (compared to M6). This cluster is also visible to the naked eye as a nebula: but on examining it with a refractor, the nebulosity disappears and one perceives, in its place, a cluster of small stars, among them one that stands out more brightly above the others (…)”

M7 open cluster. Credit: ESO (image obtained by the Wide-field Imager)

M7 cluster. Credit: Ezequiel Bellocchino, Telescope: Orión ópticos ODK IO F6.8, Camera: FLI ML 8300. Baader filters

Characteristics

Also known as NGC 6475, M7 is an open cluster, that is, a group of gravitationally bound stars formed by the same molecular cloud. It is a very sparse cluster, located in a very rich region of the Milky Way. With a magnitude of 3.3, M7 is much brighter than M6 and is observable with the naked eye. The cluster is approximately 980 light-years away, and although it is officially assigned a diameter of 25 light-years, some experts suggest that its gravitational influence may extend as far as 50 light-years!

It is estimated that the cluster may harbor as many as 150 stars of different magnitudes. Although many of them formed about 220 million years ago, they have evolved in various ways. The ones that can be seen without much difficulty are blue giants, the youngest stars. However, evidence suggests there is abundant less massive brown dwarfs (F and K types) observable in the X-ray spectrum. As is characteristic of open clusters, M7 is still a fervent cradle of stars. The brightest star in this cluster is a yellow giant of spectral type G8 of magnitude 5.6. Interestingly, some estimates denote that the cluster approaches us at a speed of 14 km/sec.

There are no variable stars in M7, but four magnetic stars of spectral type Ap and Bp (the “p” stands for peculiar) have been detected. The magnetic stars are not Magnetars (a type of neutron star), but bodies possessing a strange mixture of metals, including strontium or europium. Besides, these stars have much slower rotation periods than usual.

M7 cluster and the center of the milky way in the background. Credir: Oliver Stein

OBSERVATION

M7 is one of the brightest and most beautiful open clusters we can observe in our galaxy. Being close to the Milky Way center, it has a beautiful star-studded background when photographed in places with very dark skies. Its considerable brightness and size allow us to locate it with the naked eye to the north of the Scorpion’s tail, near Sagittarius, and clearly differentiates it from M6.

Its central figure describes a lying H. With astronomical binoculars, the different brightnesses of the stars in the cumulus can be distinguished, while with a telescope, the view will be overwhelming. Many experts recommend not to force the magnification to appreciate this cluster at its best. With very large telescopes, the cluster’s observation is complicated by the numerous background stars in the center of our galaxy. The best time to observe M7 is between April and October in the southern hemisphere, while in the northern hemisphere, the best time is during the summer months.

M7 cluster and the center of the milky way in the background. Credir: Oliver Stein

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M10 – Globular Cluster | Cúmulo Globular

M10 is a very bright and star-rich cluster. Located in the constellation Ofiuco, it is estimated to be approximately 16,000 light-years from the center of our galaxy. An analysis of its radial velocity allows us to deduce that it is moving away from Earth at more than 271,440 km/h.

M9 – Globular Cluster | Cúmulo Globular

Like all globular clusters, M9 is a spectacle in itself, and one of the oldest and most intriguing objects in our galaxy. M9 stands out as one of the closest globular clusters to the center of our galaxy, only 5,500 light-years away.

M8 – Lagoon Nebula | Nebulosa de la Laguna

Like all emission nebulae, M8 or NGC 6523 is a gigantic HII region, that is, a spectacular cradle of stars. With a magnitude of 5.8, this incredible nebula boasts many large, hot stars, whose ultraviolet radiation sculpts the gases and cosmic dust, giving it its characteristic shape.

M7 – Open Cluster | Cúmulo Abierto

Also known as NGC 6475, M7 is an open cluster, that is, a group of gravitationally bound stars formed by the same molecular cloud. It is a very sparse cluster, located in a very rich region of the Milky Way.

M6 – Open Cluster | Cúmulo Abierto

This beautiful cluster comprises more than 200 stars and has a diameter of about 12-20 light-years. It is estimated that its stars were formed about 100 million years ago, which means that while dinosaurs roamed the planet, M6 was just beginning to shine in the sky.

M5 – Globular Cluster | Cúmulo Globular

The globular cluster M5 is probably one of the most interesting because it is one of the oldest, largest, and brightest clusters. It is located in the constellation of The Serpent (Serpens) at a distance of approximately 24,500 light-years.

M4 – Globular Cluster | Cúmulo Globular

Also called NGC 6121, M4 is the closest globular cluster to the Earth. M4 contains about 100,000 stars, and like most globular clusters, it is a very old structure, about 12-13 billion years old.

M3 – Globular Cluster | Cúmulo Globular

M3, also called NGC 5272, is one of the largest and most spectacular clusters that we can observe in our galaxy. In fact, M3 was the first original discovery by Charles Messier.

M2 – Globular Cluster | Cúmulo Globular

M2 is a globular star cluster and the second object in the Messier catalog. Also called NGC 7089, it was discovered in 1746 by French astronomer Jean-Dominique Maraldi.

M1 – Crab Nebula | Nebulosa del Cangrejo

M1 is the remnant of a supernova explosion that was observed by Chinese astronomers during the Song Dynasty in the 11th century.
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