m3 – globular cluster
Lupe Rioné, Amateur astronomer (Spain)
Constellation of Canes Venatici
33, 900 light-years
Globular clusters: Globular clusters are concentrations of thousands to millions of stars joined gravitationally. They all orbit in the halo of a galaxy, although some of them can be located near the center of the Milky Way.
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History and discovery
If we are already surprised with the Globular Cluster M2, M3, also called NGC 5272, it 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. While M1 and M2 had been previously observed by other astronomers, Messier was the first to visualize M3 on May 3, 1764. Some believe that the observation of this cluster led the French astronomer to build up a catalog of celestial bodies to differentiate them from comets, which were widely observed in his time. However, as with M2, Messier described M3 as a nebula. Finally, using his enormous reflecting telescope, William Herschel determined in 1784 that M3 was a dense cluster of thousands of stars, and since then it was reclassified as a globular cluster.
Image captured from the Querol (Tarragona) on June 27, 2020, with a reflector telescope 200/1000 and a Canon EOS 55OD camera.
Spectacular image of the globular cluster M3 . Credit: Robert J. Vanderbei
Location and observation
With an apparent magnitude of 6.3, the globular cluster M3 can be located from the northern hemisphere in the Canes Venatici Constellation, about halfway between Cor Caroli (the brightest star in this constellation) and Arturo (the brightest star in the Boyero Constellation). Like M2, M3 can be seen with the naked eye in good visibility conditions. Using small binoculars, the cluster can be seen as a small nebulous point. With a small telescope, one can appreciate its compact core which is surrounded by a kind of granular brightness. Those with a 12-inch or larger telescope can see the stars that make up the cluster’s core.
Wide-field image of the M3 Globular Cluster. Credit: Observatory of La Pobla de Tornesa (Castellon)
Beautiful image of M3. Credits: Gustavo Naharro
With an age of 8 billion years, M3 is one of the richest, largest, and most spectacular clusters, only surpassed by the Great Cluster of Hercules (M13). At a distance of about 34,000 light years, M3 is located in the galactic halo, farther away from us than the center of the Milky Way. Nevertheless, it shines 300,000 times brighter than the Sun. Its diameter is 190 light years, but its Roche limit is much larger, covering a sphere with a diameter of 760 light years. In other words, all the stars inside that sphere are gravitationally bound to the cluster. Incredible, isn’t it?
The Roche limit is the smallest distance that a body orbiting around a more massive one can be and remain whole by virtue of its gravitational cohesion.
The orbit of M3 in the galactic halo is very eccentric and extreme, and it can move up to 66,000 light years from the center, and 49,000 light years above and below the galactic plane. At its closest point to the center of our galaxy (22,000 light-years), its Roche limit is reduced to below 200 light-years. Under these conditions, it is very likely that the stars farthest from the center of the cluster will escape and orbit our galaxy separately. It is even possible that they will rush towards the galactic center.
Panoramic image where we can find the Globular Cluster M3. The powerful light you see in the lower left area is from the Moon, about to set. Credits: Querol (Tarragona)
M3 approaches the earth at a speed of about 500,000 km/h, which takes into account the travel speed of the solar system and other associated factors.
An outstanding feature of M3 is that it houses some 500,000 stars, most of which are very old. The brightest stars have a magnitude of 12.7, while the giants, located in the so-called horizontal branch, are of magnitude 15.7. In addition, this cluster is extremely rich in variable stars. To date, 212 of them have been discovered, and the periods of 186 have been determined. For this reason, M3 is the globular cluster with the most variable stars in the Milky Way. Similarly, at least 170 RR Lyrae variable stars have been discovered and used as “standard candles” to determine the distance to the cluster.
Half of the cluster’s mass is only in a 22 light-years area. Interestingly, M3 contains a relatively high number of blue straggler stars. We call them that because they are blue stars in their main phase, and they are much younger than most of the stars in the cluster. Experts estimate that these stars have gone through a dramatic change process, in which the cooler outer layers have been ripped away in close encounters with other stars, something that happens, for example, when they pass through the dense central region of the cluster. Such stars were discovered by Alan Sandage (1953) on photographic plates taken by the Hale Telescope (Mount Palomar). As you will see, the life of these beautiful globular clusters is exciting.
Somewhere, something incredible is waiting to be known