m10 – globular cluster

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Also known as NGC 6254, M10 was discovered by Charles Messier on May 29, 1764, one night after observing the globular cluster M9. At that time he defined it as a “starless nebula” near the belt of the constellation Ophiuchus and was struck by its extremely round shape. There are not many notes by Messier about M10, only its position in the firmament and a brief description.

With a larger and more accurate telescope, the German astronomer William Herschel observed M10 in much greater detail and found it to be a grouping of individual stars. He described it as “a beautiful cluster of stars compressed together.” Later, Harlow Shapley was the first to calculate an approximate distance to the globular cluster, estimating about 33,000 light-years. We now know that this estimate was off, and the data indicate that the star cluster is 14,300 light-years away.

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. This velocity originates from the combination of its orbital velocity around the Milky Way’s nucleus and the velocity of the Sun and the Earth.

We know that M10 takes about 140 million years to complete its orbit around the galaxy, and about 53 million years to cross the plane of the Milky Way. For comparison, the Sun takes about 250 million years to complete an orbit around the galaxy and about 70 million years to cross the galactic plane.

The data indicate that M10 contains approximately 100,000 stars, and about 15% of these are binary stars located in the core. Binary stars tend to be located in the center of the cluster because of their mass. Consequently, the core of M10 contains a huge concentration of blue straggler stars, most of them formed 2-5 billion years ago. They are called stragglers because they appear to be much younger than the cluster to which they belong. M10 is a cluster very poor in variable stars. Only four stars of this type have been catalogued; a curious fact, since in other globular clusters they are found in much greater numbers.

M10 has a diameter of about 83 light-years, and its central region, the most striking of this celestial body, measures 35 light-years. Its apparent magnitude is 6.4, which makes it difficult to see with the naked eye unless we have very dark skies. Its spectral type is F3, which means that in photographs it appears golden due to the large number of red giant stars (yellowish or orange) that it contains.

On the other hand, it belongs to class VII in the Shapley-Sawyer Concentration Classification. This is a classification system on a scale of one to twelve that uses Roman numerals for globular clusters according to their concentration. The most concentrated clusters are classified as Class I, decreasing to Class XII as the concentration decreases. In addition, it is a cluster with a fairly low metallicity (metallicity is the amount of elements that can be found in a star beyond hydrogen and helium). This data is relevant because it allows us to calculate the age of the globular cluster, approximately 11.4 billion years.

It is not strange that among the first objects catalogued and discovered by Charles Messier there were so many globular clusters, since many of them could resemble the scalp of a comet. M10, for example, is only 3 degrees 20′ southeast of M12, another globular cluster, and Messier may have identified them as 2 simultaneous comets. In fact, the astronomer drew up his famous catalog to differentiate these objects (which remained stationary and did not move against the background of stars) from comets.

With astronomical binoculars, M10 appears close to M12, farther south and much brighter. To locate it we will start from the 1st magnitude star Antares (Scorpius alpha) and head north, about 25 degrees, until we find M10.

With a telescope the process is the same, but the cluster can be seen in much more detail, and its dense core can be distinguished. With a 200+ telescope, M10 will dazzle us with its thousands of stars, from the outer halo to the dense core. In addition to this spectacle, there is the rich stellar background that populates the vicinity of this beautiful globular cluster.

Location of M10. Credit: ESO, IAU and Sky &Telescope