Degree of difficulty 3 (of 5)

Minimum aperture 30mm

Designation NGC 6333

Type Globular cluster

Class VIII

Distance 46,090 ly (RR Lyr, 1999)

Size 150 ly

Constellation Ophiuchus

R.A. 17h 19.2min

Decl. –18° 31′

Magnitude 7.6

Surface brightness –

Apparent diameter 11′

Discoverer Messier, 1764

History Charles Messier discovered M 9 on the 28 th of May 1764, as a “nebula without star” and added: “it is round & its light faint, 3' diameter.” Exactly 20 years later, William Herschel reported that this object was actually a very rich star cluster. In the 1830s, Admiral Smyth observed M 9 in more detail and noted: “This fine object is composed of a myriad of minute stars, clustering into a blaze in the center, and wonderfully aggregated, with numerous outliers seen by glimpses.” Lord Rosse remarked: “The outline not round; on south side is an outlying portion separated from the chief portion by a dark passage.” The German observer Heinrich d'Arrest, by contrast, believed he saw an “almost double-core cluster” and noted an elongation in the north-south direction.

In 1918, Curtis remarked that M 9 maintained the relatively small diameter of 3' on photographic plates. But modern studies, based on much deeper exposures, have almost quadrupled this number.

Astrophysics M 9 has a distance of about 14,000 light-years from the galactic center, situated on its far side, just beyond the galactic bulge. The distance to us is 46,000 light-years, and its diameter of 150 light-years is quite average, as is its total mass of about 300,000 solar masses.

Because of its position behind the fields of dusty interstellar clouds in Ophiuchus, M 9 suffers from about one magnitude of interstellar absorption, in particular in its northern and western parts. Also, according to Shapley, M 9 did not appear exactly spherical, he rather regarded it as slightly elliptical (0.9).

The brightest individual star of M 9 reaches magnitude 13.5, the mean magnitude of the 25 brightest cluster members is 15.5. 16 RR Lyrae stars have been cataloged for M 9, and two other variables, one of which is a type II cepheid.

Degree of difficulty 4 (of 5)

Minimum aperture 50mm

Designation NGC 205

Type Galaxy

Class dSph Irr

Distance 2.57 Mly (see M 31) 2.82 Mly (PN, 2000)

Size 16,000 ly

Constellation Andromeda

R.A. Oh 40.4min

Decl. +41° 41′

Magnitude 8.0

Surface brightness 22.8mag/arcsec2

Apparent diameter 21,9′×11,0′

Discoverer Messier, 1773

History M 110 was discovered by Charles Messier on the 10th of August 1773, but it never occurred to him to include it in his catalog. Almost thirty years after his initial observation, he finally published the following note in the Connaissances des Temps, 1801: “On the 10th of August, 1773, I studied the beautiful nebula in the girdle of Andromeda (No. 31) under a very good sky, using my achromatic refractor with a 68× magnification, in order to produce a drawing like the one for the nebula in Orion. I saw the nebula which LeGentil had discovered on Oct. 29, 1749 (No. 32). In addition, I saw a new, fainter situated north of the large one. It seemed unbelievable to me, that the faint nebula would not have been noticed by astronomers and myself since the discovery of the large one by Simon Marius in 1612, since the small one is in the same field of the telescope when observing the large one.” Finally, it was Kenneth Glyn Jones, in 1966, who suggested making this galaxy the last addition to the Messier catalogue, as No. 110.

An independent discovery of M 110 was made by Caroline Herschel on the 27th of August 1783. Her brother William then described her object as “a very considerable, broad, pretty faint, small nebula. It shows the same faint color as the great one, and is, no doubt, in the neighbourhood of it.” Admiral Smyth characterized M 110 in 1836 as “a large faintish nebula of an oval form,” and Reverend Webb wrote: “Large faint oval nebula, best with low powers, a very large field includes it with M 32 and M 31. Seems to sparkle.”

Degree of difficulty 1 (of 5)

Minimum aperture Naked eye

Designation NGC 6475

Type Open cluster

Class I3m

Distance 980 ly (K2005) 980 ly (proper motion, 2002) 910 ly (Hipparcos, 1999)

Size 23 ly

Constellation Scorpius

R.A. 17h 53.9min

Decl. –34° 49′

Magnitude 3.3

Surface brightness –

Apparent diameter 80′

Discoverer Ptolemy, 130 BC

History M 7 was already known to Ptolemy as a nebulous patch following the sting of the scorpion when he compiled his star catalog, the Almagest, in 130 BC. 1000 years later, the Persian scholar Al Sufiassigned it a brightness of magnitude 4.5.

Because of its very southerly position, M 7 remained unnoticed by many European observers. Hodierna counted 30 stars in 1654, and he was probably the first to recognize M 7's true nature as a star cluster. Lacaille observed it in 1752 from South Africa and wrote: “Group of 15 to 20 stars, very close to each other, in the form of a square.” He registered M 7 as his number 14 in the category of star clusters.

Charles Messier made his observation of M 7 on the 23rd of May 1764, and noted: “Cluster of stars more considerable than the previous [M 6]; this cluster appears to the naked eye like a nebula; 30' diameter.” Of the later observers, John Herschel gave M 7 some attention and characterized it as “cluster; very bright; pretty rich; little compressed; stars from 7th to 12th magnitude.”

Astrophysics With a total magnitude of 3.3, M 7 is one of the very brightest Messier objects; it takes third place closely behind M 44. At just under 1000 light-years distance, this open cluster gains much of its apparent brightness from its proximity. It still belongs to the local spiral arm, located on its inner side with respect to the galactic center. The rich star fields in the background, however, are made by the galactic bulge, almost 30,000 light-years away.

The Great Hercules Cluster

Degree of difficulty 1 (of 5)

Minimum aperture Naked eye

Designation NGC 6205

Type Globular cluster

Class V

Distance 25,890 ly (R2005) 24,530 ly (CMD, 1998)

Size 160 ly

Constellation Hercules

R.A. 16h 41.7min

Decl. +36° 28′

Magnitude 5.7

Surface brightness –

Apparent diameter 21′

Discoverer Halley, 1714

History In 1714, Edmond Halley noticed, more or less by accident, a “nebulous patch” in the constellation of Hercules. A year later, he wrote up a list of the six nebulae then known to him (M 42, M 31, M 22, ω Cen, M 11, and M 13) and commented about the latter: “This is but a little patch, but it shows itself to the naked eye, when the sky is serene and the moon absent.” Charles Messier observed M 13 on the 1st of June 1764, and noted: “Nebula without star, discovered in the girdle of Hercules; it is round & brilliant, the center brighter than the borders; it is near two stars, both of 8th magnitude, one of them above & the other below; 6′ diameter.”

It was left to William Herschel to recognize the true nature of this star cluster in 1784: “A most beautiful cluster of stars. It is exceedingly compressed in the middle and very rich. About 14,000 stars can be counted.” 40 years later, John Herschel commented: “Very rich cluster; irregular figure; very large; very gradually much brighter toward the middle; stars from 10th to 15th magnitude, of which there must be thousands; does not come up to a nucleus; has hairy-looking curvilinear branches.” This impression was shared by Lord Rosse, who pointed his large 72-inch mirror towards M 13: “singularly fringed appendages to the globular figure out into the surrounding space.” His drawing shows a Y-shaped dark structure over the whole central region of the cluster. D'Arrest, too, saw “arm-shaped outliers” with a magnification of 95×.

When Charles Messier observed the objects that eventually made up his famous catalog, his visual impressions only allowed him to distinguish between “amas d'étoiles” (star clusters) and “nebuleuse” (nebulae). By our modern methods of astrophysics, we can finally classify Messier's objects according to their physical categories. His catalog contains:

1. • 6 Galactic nebulae

2. • 28 Open clusters

3. • 4 Planetary nebulae

4. • 29 Globular clusters

5. • 40 Galaxies

6. • 3 other objects

In this list, M 8 has been counted as an open (star) cluster, because Messier noted the star cluster as the main object. M 16, of which Messier could not see the surrounding nebulosity, is registered as a star cluster, while the nebula has its separate catalog designation (IC 4703). M 20, regarded by Messier as a star cluster like the other two objects, is classified as a galactic nebula today. But in reality, modern classification is still a bit ad hoc. For example, the galactic nebulae M 17, M 20, and M 42 are best known for their dust-enshrouded, young star clusters in the process of formation, only hidden in visual light.

M 45, the Pleiades, is the brightest Messier object with a total magnitude of 1.2. This bright star cluster is visible to the naked eye even under a light-polluted sky. The status of the faintest Messier object is shared by M 76, M 91, and M 98, all at a total magnitude of only 10.1. The largest in angular size of all Messier objects is undoubtedly the Andromeda Galaxy (M 31) at 4° × 1°. Under a very dark, transparent sky, a visual size of as much as 5° has been reported. However, in absolute terms, the galaxy M 101 is the physically largest object of the Messier catalog. With a diameter of 184,000 light-years, it reaches 1½ times the size of our neighbor galaxy M 31 and almost twice the size of our own galaxy.

Degree of difficulty 3 (of 5)

Minimum aperture Naked eye

Designation NGC 2437

Type Open cluster

Class II2r

Distance 4480 ly (K2005) 4490 ly (proper motion, 2002)

Size 26 ly

Constellation Puppis

R.A. 7h 41.8min

Decl. –14° 49′

Magnitude 6.1

Surface brightness –

Apparent diameter 20′

Discoverer Messier, 1771

History M 46 was discovered by Charles Messier on the 19th of February 1771, as the first object after the completion of his initial catalog of 45 objects. He described this object as “cluster of very small stars, the stars are not visible except in a good refractor, the cluster contains a bit of nebulosity.” In 1786, William Herschel found the planetary nebula NGC 2438 in this cluster. His son John later described the view: “A very fine rich cluster which fills the field. Within the cluster at its northern edge is a fine planetary nebula.” He characterized the latter as “exactly round, of a faint equable light. Has a very minute star north of center. It is not brighter to the middle nor fading away, but a little velvety at the edges.”

Leo Brenner recommended M 46 to amateur observers, about 100 years ago, with the words: “Splendid object, even for smaller telescopes. 30′ diameter, very rich and bright. Stars of 10m and lower. The cluster contains a planetary or ring-shaped nebula of 3.75′ diameter and of considerable brightness.”

Curtis, based on the photographic images, gave a characterization of M 46 which does not quite agree with its visual appearance: “a very large, bright, sparse cluster about 25′ in diameter, in which is involved the planetary nebula 2438.”

Astrophysics M 46 and M 47 form a very appealing cluster pair in the winter sky. In real space, though, they are by no means close. Rather, M 46 is supposed to be two to three times farther away than M 47. M 46 has a position 400 light-years above the galactic plane and is among the richest Messier clusters.