M75, magnitude +8.7, is a very distant and compact globular cluster located in eastern Sagittarius. At 67,500 light-years, it's one of the remotest Messier globulars and from our perspective appears faint and small. This cluster lies far beyond the galactic centre (46,700 light-years) and almost on the opposite side of the galaxy to the Solar System. Despite this, it's intrinsically bright and on nights of good seeing and transparency can be glimpsed with a pair of 7x50 or 10x50 binoculars.
M75 was discovered by Pierre Méchain on the night of August 27, 1780. Charles Messier observed it soon afterwards and added it to his catalogue a few weeks later. It was William Herschel who first resolved M75 into stars, describing it as a "miniature version of M3." He also gave the same description to M62 and M70.
M75 is located at the Sagittarius-Capricornus border. It's positioned about 23 degrees northeast of the centre of the Sagittarius teapot asterism and 5.5 degrees north and a little east of a small group of four faint naked-eye stars (59 Sagittarii - mag. +4.5, 60 Sagittarii - mag. +4.8, 62 Sagittarii - mag. +4.4 and ω Sgr - mag. +4.7).
The cluster is best seen during the months of June, July and August from southern and tropical locations.
For binocular observers, M75 appears stellar in nature. A 100mm (4-inch) telescope at high magnifications will reveal a certain degree of fuzziness. However, the cluster is small and spans visually only 3 arc minutes in diameter. It's roughly comparable in size and brightness to the other Messier globulars located within the teapot, M54, M69 and M70. To begin resolution, large reflectors of at least 250mm (10-inch) aperture are required.
In total M75, covers 6.8 arc minutes of apparent sky, which corresponds to a spatial diameter of 134 light-years. It's very old at 13 billion years and is estimated to contain 400,000 stars. Classified as class I, M75 is one of the most densely concentrated globulars known. However despite been faint and small, it's a remarkable object to observe given its incredible distance.
M75 Data Table
|Object Type||Globular cluster|
|RA (J2000)||20h 06m 05s|
|DEC (J2000)||-21d 55m 17s|
|Apparent Size (arc mins)||6.8 x 6.8|
|Age (years)||>13 Billion|
|Number of Stars||400,000|
|Notable Feature||Distant globular and one of the most densely concentrated clusters known|