# Messier 75

Messier 75
Observation data (J2000 epoch)
ClassI[1]
ConstellationSagittarius
Right ascension 20h 06m 04.85s[2]
Declination−21° 55′ 17.85″[2]
Distance68 kly (20.9 kpc)[3]
Apparent magnitude (V)+9.18[4]
Apparent dimensions (V)6.8
Physical characteristics
Absolute magnitude−8.57[2]
Metallicity${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = −1.29[6] dex
Other designationsGCl 116, M75, NGC 6864[4]

Messier 75 or M75, also known as NGC 6864, is a globular cluster of stars in the southern constellation Sagittarius. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects that same year.[7]

M75 is at a distance of about 67,500[3] light years away from Earth and is 14,700[6] light years away from, and on the opposite side of, the Galactic Center.[8] Its apparent size on the sky translates to a true radius of some 67 light years.[5] M75 is classified as class I, meaning it is one of the more densely concentrated globular clusters known. It shows a slow rotation around an axis that is inclined along a position angle of −15°±30°.[3] The absolute magnitude of M75 is about −8.5 or some 180,000 times more luminous than the Sun.[7]

The cluster has a half-light radius of 9.1 ly (2.80 pc)[6] with a core radius of about 1.6 ly (0.5 pc) and appears not to have undergone core collapse yet. The mass density at the core is 7.9×104 M·pc−3.[2] There are 38 RR Lyrae variable stars and the cluster appears to be Oosterhoff-intermediate in terms of metallicity.[8] 62 candidate blue stragglers have been identified in the cluster field, with 60% being in the core region.[2]

Messier 75 is part of the Gaia Sausage, the hypothesized remains of a dwarf galaxy that merged with the Milky Way.[9] It is a halo object with an orbital period of 0.4 Gyr around the galaxy and a large eccentricity of 0.87. The apocenter is 57 kly (17.5 kpc) – close to the current separation.[3]

## References

1. ^ Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.
2. Contreras Ramos, R.; et al. (April 2012), "The Unimodal Distribution of Blue Straggler Stars in M75 (NGC 6864)", The Astrophysical Journal, 748 (2): 9, arXiv:1201.4959, Bibcode:2012ApJ...748...91C, doi:10.1088/0004-637X/748/2/91, 91.
3. Koch, Andreas; et al. (August 2018), "Kinematics of outer halo globular clusters: M 75 and NGC 6426", Astronomy & Astrophysics, 616: 9, arXiv:1805.06894, Bibcode:2018A&A...616A..74K, doi:10.1051/0004-6361/201833110, A74.
4. ^ a b "NGC 6864". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-16.
5. ^ a b From trigonometry: radius = distance × sin( diameter_angle / 2 ) = 67 ly.
6. ^ a b c van den Bergh, Sidney (February 2012), "Sizes of Galactic Globular Clusters", The Astrophysical Journal, 746 (2): 4, Bibcode:2012ApJ...746..189V, doi:10.1088/0004-637X/746/2/189, 189.
7. ^ a b Frommert, Hartmut; Kronberg, Christine (September 2, 2007), "Messier 75", SEDS Messier pages, Students for the Exploration and Development of Space (SEDS), retrieved 2018-12-05.
8. ^ a b Corwin, T. M.; et al. (May 2003), "M75, A Globular Cluster with a Trimodal Horizontal Branch. II. BV photometry of the RR Lyrae Variables", The Astronomical Journal, 125 (5): 2543–2558, arXiv:astro-ph/0301542, Bibcode:2003AJ....125.2543C, doi:10.1086/374232.
9. ^ C., Myeong, G.; et al. (August 2018), "The Sausage Globular Clusters", The Astrophysical Journal Letters, 863 (2): 5, arXiv:1805.00453, Bibcode:2018ApJ...863L..28M, doi:10.3847/2041-8213/aad7f7, L28.