A team of astronomers using the MPG/ESO 2.2-m telescope at ESO’s La Silla Observatory in Chile has captured a new image of the open star cluster Messier 7, also known as NGC 6475.
Messier 7 is a small cluster of about 100 stars located in the constellation of Scorpius, about 800 light-years away.
The cluster is about 200 million years old and spans some 25 light-years across.
The first to mention this stellar object was the Roman mathematician and astronomer Claudius Ptolemy, as early as 130 AD, who described it as a “nebula following the sting of Scorpius.” In his honor, Messier 7 is sometimes called Ptolemy’s Cluster.
In 1764, Charles Messier included it as the seventh entry in his Messier catalogue.
Later, in the 19th century, John Herschel described the appearance of this object as seen through a telescope as a “coarsely scattered cluster of stars.”
In the new image Messier 7 stands out against a very rich background of hundreds of thousands of fainter stars, in the direction of the centre of the Milky Way.
Open star clusters like Messier 7 are groups of stars born at almost the same time and place, from large cosmic clouds of gas and dust in their host galaxy. These groups of stars are of great interest to scientists, because the stars in them have about the same age and chemical composition. This makes them invaluable for studying stellar structure and evolution.
An interesting feature in the new image of Messier 7 is that, although densely populated with stars, the background is not uniform and is noticeably streaked with dust. This is most likely to be just a chance alignment of the cluster and the dust clouds.
Although it is tempting to speculate that these dark shreds are the remnants of the cloud from which the cluster formed, the Milky Way will have made nearly one full rotation during the life of this star cluster, with a lot of reorganization of the stars and dust as a result.
So the dust and gas from which Messier 7 formed, and the star cluster itself, will have gone their separate ways long ago.