M54 is a globular cluster located in Sagittarius that’s a staggering 87,400 light years from Earth. It was discovered by Charles Messier on July 24, 1778 and was for many years thought to be part of the Milky Way but is now believed to belong to the nearby Sagittarius Dwarf Elliptical Galaxy. It owns the distinction of being the first extragalactic globular cluster ever discovered, even though it wasn't recognized as such for over 200 years. Despite its vast distance, M54 is visible in binoculars albeit faintly (mag. +7.9). The fact that it can be seen in binoculars at all from such a distance is incredible and is due to its large intrinsic size and high absolute brightness. With a diameter of over 300 light-years diameter, this globular is enormous and one of the largest known.
Finding M54 is easy as it lies within the teapot asterism of Sagittarius. The starting point is to focus on the base of the teapot and image a line connecting Ascella (ζ Sgr - mag. +2.6) with Kaus Australis (ε Sgr - mag. +1.8). Positioned about 1.75 degrees along this line and slightly north is M54. With a declination of -30 degrees, the globular is best seen from the Southern Hemisphere during the months of June, July and August. From northern temperate latitudes, it’s a much more difficult target as it never rises very high above the southern horizon.
Binocular observers may at first overlook this fascinating globular cluster. Although the core is bright, the globular looks small and may appear starlight on initial observation. However, further inspection shows a small degree of nebulosity - like an out of focus star - hinting at the true nature of this massive object. A small 80mm (3.1-inch) telescope under good seeing reveals a bright point like core surrounding by nebulosity that tails off gradually from the centre. Larger scopes of the order of 150mm (6-inches) or 200mm (8-inches) may reveal twists and knots especially under good seeing conditions. However, due to its considerable distance from Earth even the largest amateur telescopes fail to resolve M54.
M54 has a luminosity of 850,000 Sun's and an absolute magnitude of -10. At such a large distance from Earth, the brilliance of M54 is not obvious and this is also reflected in the apparent magnitude of its brightest stars; a mere +15.5. In reality, it does contains over 1 million stars of which at least 82 variable stars, mostly of the RR Lyrae type and there are also two semi-regular red variable stars with periods of 77 and 101 days respectively.
M79 in Lepus is the only other extragalactic globular cluster in the Messier catalogue and is part of the tiny Canis Major Dwarf galaxy.
M54 Data Table
|Object Type||Globular cluster|
|RA (J2000)||18h 55m 03s|
|DEC (J2000)||-30d 28m 42s|
|Apparent Size (arc mins)||12 x 12|
|Number of Stars||>1 Million|
|Notable Feature||Likely belongs to the Sagittarius Dwarf Elliptical Galaxy|