m9 – globular cluster
Lupe Rioné, Amateur astronomer (Spain)
Constellation of Ophiuchus
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
Many of the objects that Charles Messier added to his catalog were previously unveiled by other astronomers. However, M9 is one of Messier’s original discoveries. He spots it on May 28, 1764, describing it at first as a nebula in the constellation of Ophiuchus, due to his inability to observe stars in it.
Almost 20 years later, Mr. William Herschel was able to observe it at 250 times magnification, more than twice what Messier had at his disposal. In this way, Herschel was able to distinguish some of the stars that make up this celestial body, and noted that the vast majority of them seemed to accumulate in the center. M9 was later observed by British Admiral William Henry Smyth, an amateur astronomer who in 1844 published his famous Cycle of Celestial Objects.
Hubble Space Telescope image of the globular cluster M9. In this incredible image, you can even make out individual stars that make up the cluster core, as well as a large number of stars in the cluster. Credit: NASA/ESA/Hubble.
Artistic representation of the location of the globular cluster M9 almost in the center of the Milky Way. The orange dot is the location of the solar system.
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. In fact, M9 is so close to the galactic center that its gravitational influence slightly deforms it. For this reason, if we look closely, we will observe M9 slightly flattened, rather than spherical.
With respect to our solar system, it has an estimated distance of 25,800 light-years, and it is moving away from us at a radial velocity (with respect to the Sun) of approximately 800,000 km/h. Its angular diameter of 12 arcminutes corresponds to an extension of about 90 light-years in diameter. M9 is classified as a class VIII globular cluster because its stars are compressed toward the center of the structure.
Like all globular clusters, M9 contains the oldest stars in our galaxy, which differ widely in composition compared to the Sun. It should be noted that elements such as iron (which forms the core of our planet), carbon, and oxygen are very scarce in globular clusters. Beyond hydrogen and helium, the heaviest elements are formed in the cores of stars, and in the explosions of novae and supernovae.
Thanks to the power of the Hubble Space Telescope, it has been possible to observe that M9 contains about 250,000 stars. Most of them are red giants, but it also has many yellow and blue stars. We know that the color of a star is linked to its temperature. The redder the star, the cooler it is, and the bluer the star, the hotter it is. We also know that M9 hosts at least 13 variable stars (Cepheids), 10 of which were found by Baade. The brightest star has an apparent magnitude of 13.5 (observable with a telescope of about 150 mm) and its red giants on the horizontal branch register an apparent magnitude of 16.2. Like all globular clusters M9, it is a true example of the grandeur and perfection of these objects that were born at the same time as our galaxy.
Wide field image of M9. Credit: Jesús Sanabria.
The globular cluster M9 is more difficult to observe than other clusters, so the amateur must be patient. It is not easy to locate it because of its position in the constellation Ophiuchus (a very large constellation). To find your way around, it is necessary to locate the star Sabik. Once found, just look at the area about 3 degrees southeast of Sabik, where you should see a small triangular formation of stars. M9 lies slightly above the one farther south.
Even with very dark skies, it is impossible to observe it with the naked eye, although with 10×50 binoculars it can already be visible as a faint, tiny cloud. With a 100 mm telescope, we can already see the central part with a diameter of 3 arcminutes and a slightly oval shape, whose image weakens towards the edges. The 200 and 250 mm telescopes already show the globular cluster with about 7 or 8 minutes of arc, where the more compact central region can be seen. With an amateur telescope of 300 mm and up, the core can be completely resolved.
Location of M9. Credit: ESO, IAU and Sky &Telescope
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