101 must-see cosmic objects: M4

Scorpius the Scorpio holds many of the most beautiful deep sky objects in the sky, including the spectacular globular cluster M4. This group of stars stands out for several reasons. First, at 7,200 light-years away, it’s the closest globular cluster to our solar system. It’s also a bit small for globulars, measuring about 75 light-years in diameter. For comparison, the Hercules Cluster and the Grandmaster of globulars, Omega Centauri, are both about 150 light-years in diameter.

Among the approximately 100,000 stars that inhabit M4 is a hidden treasure: the PSR B1620−26 pulsar. A member of a binary system, PSR B1620−26 is associated with a white dwarf. Researchers have confirmed that an exoplanet is orbiting these two stellar remnants. Because planets are thought to form shortly after their parent stars, and because M4 is estimated to be around 12 to 13 billion years old, it is one of the oldest exoplanets ever discovered.

M4 has earned a footnote in astronomical history for being the first globular cluster ever resolved. It was also the only globular of the 29 included in Messier’s catalog that he was able to resolve into stars.

Best of all, it couldn’t be easier to find. M4 is nestled just over a degree west of bright Antares (Alpha [α] Scorpii), the heart of Scorpio. Using low power, move Antares just outside the eastern edge of your field and you will see the spherical shape of the cluster take shape. Even an old 2.4 inch refractor can resolve some stars at 50x.

With an opening of 4 inches or more, a curious characteristic appears. M4 displays a bright bar cutting through the center of its disk, apparently splitting the cluster into two halves. Try a moderate magnification of around 75x to 100x to get the best view of this split, which is the result of a chain of 11th magnitude cluster stars lining up right after each other.


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