M55 is a globular cluster located in eastern Sagittarius towards its border with Capricornus and Microscopium. At magnitude +6.7, it's beyond naked eye visibility but bright enough to be seen with binoculars. However, it's not an easy globular to locate since there aren't any particular bright stars nearby. With a declination of -30 degrees, M55 is one of the more southerly objects in Messier's catalogue and therefore especially difficult for observers based at northern temperate latitudes. It's best seen from southern or equatorial latitudes during the months of June, July and August.
M55 was discovered by Nicholas Louis de Lacaille on June 16, 1752 while observing from South Africa. Charles Messier then catalogued it on July 24, 1778. From Paris, Messier had difficulty finding M55, it took him 14 years to spot it!
Finding M55 can be challenging. One method 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). Then imagine a line from Kaus Media moving eastwards towards and passing through Ascella. Curve this line for another 17 degrees to arrive at M55.
When viewed through 7x50 or 10x50 binoculars, M55 appears as a diffuse ball of light, non-stellar without a bright core. Under dark sky conditions, it appears like a round hazy comet. However, M55 has a low surface brightness meaning that only a small amount of light pollution renders it practically invisible to binocular observers. Through a 100mm (4-inch) scope, it appears loose and hints at resolution. M55 in total covers 19 arc minutes of apparent sky but appears much smaller through the eyepiece. A 200mm (8-inch) instrument will resolve many stars with the cluster bursting into life; a magnificent swarm of thousands of pinpoints of light spread across the complete surface.
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 the cluster that's estimated to contain 100,000 stars. It's about 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|