M75 (mag. +8.7) is a very distant and compact globular cluster located in eastern Sagittarius. At 67,500 light years from Earth it's one of the more remote Messier globulars and hence appears faint and small from our perspective. It's located far beyond the galactic centre (46,700 light-years) and almost on the opposite side of the galaxy to us. Despite this, M75 is 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 after and added it to his catalogue a few weeks later. It was William Herschel who first resolved M75 into stars describing it (like M62 and M70) as a "miniature version of M3."
M75 is located right 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 Sgr - mag. +4.5, 60 Sgr - mag. +4.8, 62 Sgr - mag. +4.4 and ω Sgr - mag. +4.7). The globular is best seen during the months of June, July and August from the Southern Hemisphere and the tropics.
For binocular observers M75 appears stellar in nature. A 100mm (4-inch) telescope at high magnifications will start to pick out a certain degree of fuzziness. The cluster however appears small, visually spanning only 3 arc minutes in diameter. It's roughly comparable in size and brightness to the Messier globulars located within the teapot (M54, M69 and M70). To begin resolution, larger telescopes of the order of 250mm (10-inch) or more 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 plus and is estimated to contain 400,000 stars. Classified as class I, M75 is one of the more densely concentrated globular clusters known. However, despite been faint and small it's a remarkable globular given its incredible, extragalactic-like 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|
|Number of Stars||400,000|
|Notable Feature||Distant globular and one of the most densely concentrated clusters known|