Book contents
- Frontmatter
- Dedication
- Contents
- Foreword
- Preface
- User Guide
- Charles Messier
- The Observations
- The Catalog
- Statistics of the Messier objects
- Visual observation of the Messier objects
- Photography of the Messier objects
- The 110 Messier objects
- M 1
- M 2
- M 3
- M 4
- M 5
- M 6
- M 7
- M 8
- M 9
- M 10
- M 11
- M 12
- M 13
- M 14
- M 15
- M 16
- M 17
- M 18
- M 19
- M 20
- M 21
- M 22
- M 23
- M 24
- M 25
- M 26
- M 27
- M 28
- M 29
- M 30
- M 31
- M 32
- M 33
- M 34
- M 35
- M 36
- M 37
- M 38
- 39
- M 40
- M 41
- M 42
- M 43
- M 44
- M 45
- M 46
- M 47
- M 48
- M 49
- M 50
- M 51
- M 52
- M 53
- M 54
- M 55
- M 56
- M 57
- M 58
- M 59
- M 60
- M 61
- M 62
- M 63
- M 64
- M 65
- M 66
- M 67
- M 68
- M 69
- M 70
- M 71
- M 72
- M 73
- M 74
- M 75
- M 76
- M 77
- M 78
- M 79
- M 80
- M 81
- M 82
- M 83
- M 84
- M 85
- M 86
- M 87
- M 88
- M 89
- M 90
- M 91
- M 92
- M 93
- M 94
- M 95
- M 96
- M 97
- M 98
- M 99
- M 100
- M 101
- M 102
- M 103
- M 104
- M 105
- M 106
- M 107
- M 108
- M 109
- M 110
- Glossary of technical terms
- Index of figures
- Index of sources
M 75
from The 110 Messier objects
Published online by Cambridge University Press: 05 August 2015
- Frontmatter
- Dedication
- Contents
- Foreword
- Preface
- User Guide
- Charles Messier
- The Observations
- The Catalog
- Statistics of the Messier objects
- Visual observation of the Messier objects
- Photography of the Messier objects
- The 110 Messier objects
- M 1
- M 2
- M 3
- M 4
- M 5
- M 6
- M 7
- M 8
- M 9
- M 10
- M 11
- M 12
- M 13
- M 14
- M 15
- M 16
- M 17
- M 18
- M 19
- M 20
- M 21
- M 22
- M 23
- M 24
- M 25
- M 26
- M 27
- M 28
- M 29
- M 30
- M 31
- M 32
- M 33
- M 34
- M 35
- M 36
- M 37
- M 38
- 39
- M 40
- M 41
- M 42
- M 43
- M 44
- M 45
- M 46
- M 47
- M 48
- M 49
- M 50
- M 51
- M 52
- M 53
- M 54
- M 55
- M 56
- M 57
- M 58
- M 59
- M 60
- M 61
- M 62
- M 63
- M 64
- M 65
- M 66
- M 67
- M 68
- M 69
- M 70
- M 71
- M 72
- M 73
- M 74
- M 75
- M 76
- M 77
- M 78
- M 79
- M 80
- M 81
- M 82
- M 83
- M 84
- M 85
- M 86
- M 87
- M 88
- M 89
- M 90
- M 91
- M 92
- M 93
- M 94
- M 95
- M 96
- M 97
- M 98
- M 99
- M 100
- M 101
- M 102
- M 103
- M 104
- M 105
- M 106
- M 107
- M 108
- M 109
- M 110
- Glossary of technical terms
- Index of figures
- Index of sources
Summary
Degree of difficulty 4 (of 5)
Minimum aperture 50mm
Designation NGC 6864
Type Globular cluster
Class I
Distance 77,840 ly (R2005) 62,000 ly (1996)
Size 160 ly
Constellation Sagittarius
R.A. 20h 6.1min
Decl. –21° 55′
Magnitude 8.6
Surface brightness –
Apparent diameter 7′
Discoverer Méchain, 1780
History This faint globular cluster was discovered by Pierre Méchain on the 27th of August 1780, and he described it as a nebula. Messier confirmed Méchain's find with two observations, made on the 5th and 18th of October the same year, but his impression was one of very faint stars. This seems unlikely, though; only William Herschel was truly able to resolve this star cluster with his much larger apertures, four years later. He characterized M 75 as a miniature version of M 3. Smyth appeared disappointed and described M 75 as a “lucid white mass among some glimpse stars.” He criticized Messier as “courageous” to have included this cluster in his catalog.
Heinrich d'Arrest had 11 inches of aperture at his disposal, twice as much as Smyth. Hence, he wrote about M 75 that it was “very bright, much brighter in the center. Accompanied by many small stars, one with mag 12.” He also determined a visual diameter of 75″ to 80″.
Curtis again gave a description of the photographic appearance of M 75: “A bright, compact globular cluster 2' in diameter, greatly condensed at center.”
Astrophysics At a distance of 78,000 light-years, M 75 is the second-farthest globular cluster in the Messier catalog. Its location is 40,000 light-years beyond the galactic center, below the galactic plane. Its size is a reasonable 160 light-years; M 75 has 500,000 solar masses and a luminosity of 160,000 Suns. This globular cluster is remarkable for its very compact appearance and concentration towards the center, which gave it the highest concentration classification of class I according to Shapley. Typical for a cluster of the outer galactic halo, M 75 is very metal-poor, which means that all elements heavier than helium have a very much lower abundance than found in the Sun.
- Type
- Chapter
- Information
- Atlas of the Messier ObjectsHighlights of the Deep Sky, pp. 262 - 263Publisher: Cambridge University PressPrint publication year: 2008