M55 is a globular cluster located in eastern Sagittarius close to the Capricornus and Microscopium constellation boundaries. At magnitude +6.7, it's beyond naked eye visibility but bright enough to be seen with binoculars. However, since there aren't any particular bright stars close by, it can be a difficult target to locate. With a declination of -30 degrees, this cluster is one of the more southerly objects in the Messier catalogue and therefore challenging for observers at northern temperate latitudes. It's best seen during the months of June, July and August.
M55 was discovered by Nicholas Louis de Lacaille on June 16, 1752 from South Africa. Charles Messier then catalogued it on July 24, 1778. From Paris, Messier had difficulty finding M55 and in the end it took him 14 years to spot it!
One method to locate M55 is to begin with the "teapot" asterism of Sagittarius. Start by locating stars Kaus Media (δ Sgr - mag. +2.7) and Ascella (ζ Sgr - mag. +2.6). Imagine a line connecting Kaus Media to Ascella and then curve this line eastwards and slightly southwards for about 17 degrees to arrive at M55.
When viewed through 7x50 or 10x50 binoculars, M55 appears as a diffuse non-stellar ball of light without a bright core. Under dark sky conditions it looks somewhat like a round hazy comet. The globular has a low surface brightness, which means that only a small amount of light pollution can render it invisible. Through a 100mm (4-inch) scope it appears loose and hints at resolution. A 200mm (8-inch) instrument resolves many stars and through even larger scopes the cluster bursts into life, revealing thousands of pinpoints of light spread across the complete surface. In total, it spans 19 arc minutes of apparent sky but appears considerably smaller through the eyepiece.
M55 is located 17,600 light-years distant and has a spatial diameter of about 94 light-years. Only about 55 variable stars have been discovered in a cluster that's estimated to contain 100,000 stars. It's approximately 12,700 billion years old.
M55 Data Table
|Object Type||Globular cluster|
|RA (J2000)||19h 39m 59s|
|DEC (J2000)||-30d 57m 44s|
|Apparent Size (arc mins)||19 x 19|
|Number of Stars||100,000|