By Jim King
We are playing a little catch-up this month and will finish the Messier late-summer observations.
I apologize for allowing my feeble ole body to interfere with the production of this column last month, but sometimes the bad bugs win for a while. I hope, after 5 weeks of antibiotics, that I am well on the way to being my normal, fun-loving, feisty self…I will know in a couple of weeks when the docs resume their poking and prodding.
(An important jargon jogger: Messier uses the descriptive term “nebula” frequently since he was using a small telescope which had difficulty resolving dim stars. It appears that he thought “nebula” were not gas clouds, but simply unresolved star clouds. Therefore, in our descriptions, we must mentally separate Messier’s “nebulas” from the real thing. He does differentiate nebulosity from luminosity.)
M24 Small Sagittarius Star Cloud: (6) very challenging *
Between the Lagoon Nebula (M8) and the Swan Nebula (M17) lies one of the most impressive stellar cities visible in small telescopes. M24 is not a true galactic cluster but a small, rectangular-shaped star cloud 2 X 1 degrees. Of all the Messier Objects, M24 is second only to the Andromeda Galaxy (M31) in apparent size. Commonly called the Sagittarius Star cloud, M24 is a virtual carpet of stellar jewels, laid out across 350 light-years of space.
Messier note: (Observed June 20, 1764). Cluster above the tip of the bow of Sagittarius, in the Milky Way. A large nebula, within which are several stars of various magnitudes.
NGC note: none
Data: Messier 49 aka none
Con: Sagittarius Mag: 8.4
RA: 18h17.4m Dec: -18.36
Dist: ~10,000 ly
M28 Globular Cluster: (2) easy
Like M23, M28 is another lost gem in the glittering Sagittarius Milky Way. This tiny, but charming globular cluster suffers the misfortune of being too close to the bigger and overpoweringly beautiful globular cluster M22. M28 lies only 1 degree northwest of magnitude 2.8 Lambda Sagittarii, the golden K2 star marking the top of the Sagittarius teapot, and can be seen in binoculars shining at 7th magnitude.
Messier notes: Observed July 27, 1764) Nebula discovered in the upper part of the bow of Sagittarius, about one degree from Lambda Sagittarii, and not far from the beautiful nebula (M22) that lies between the head and bow. It does not contain any stars; it is circular and visible only with difficulty in a simple three and one-half foot refractor.
NGC notes: Remarkable globular cluster, very bright, large, round, gradually extremely compressed in the middle, well-resolved stars from 14th to 16th magnitude.
Data: Messier 28 aka NGC 6626
Con: Sagittarius Mag 6.9
RA: 18h24.5m Dec: -24.52
Dist.: ~17,900 ly
Located about 2.5 degrees west- northwest of the star magnitude 4.7 Gamma Scuti, in the spine of the Milky Way, M16 is a most tantalizing sight – a fan of nebulosity (about 60 light-years in true physical extent) centered on a loosely scattered cluster of stars that spans 13 light-years.
Messier notes: (Observed June 3, 1764) Cluster of faint stars, mingled with faint luminosity, close to the tail of Serpens. With a small telescope this cluster appears to be a nebula.
NGC note: Cluster, at least 100 bright and faint stars.
Data: Messier 16 aka the Ghost aka Eagle Nebula aka Star Queen Nebula NGC: 6611 aka IC4703
Con: Serpens Mag: 6.0
RA: 18h18.75m Dec: -13.48
Dist: ~5,700 ly
M25 Open Cluster (2) easy*
This bright open cluster in Sagittarius makes an easy naked-eye target and is finely resolved in small telescopes. It lies about 3.5 degrees east-southeast of the Small Sagittarius Star Cloud (M24) and just south of a group of binocular stars. Its estimated 600 stars stretch across 15 light-years of space, and the 30 or so bright ones that we can see cover ½ degree of sky.
Messier notes: (Observed June 20, 1764) Cluster of faint stars near {M23 and M24}, between the head and the tip of the bow of Sagittarius. The know star closest to this cluster is sixth-magnitude Flamsteed 21. The stars in this cluster are difficult to see with a simple 3 foot refractor. No nebulosity is visible.
NGC note: none. IC note: Cluster, pretty compressed
Data: Messier 25 aka IC 4725
Con: Sagittarius Mag: 4.6
RA: 18h31.75m Dec: -19.07
Dist: ~1,950 ly
M22 Globular Cluster (2) easy*
Without question, M22 should be called the “Great Sagittarius Cluster”. It is a bonfire of a half-million stars that blazes at magnitude 5.2 and measures 32’ across – about the apparent size of a full Moon. Among globulars, it ranks third only to Omega Centauri and 47 Tucanae in brightness and apparent size. In The Hobbit, J.R.R. Tolkien described M22 as the fabulous jewel called the Arkenstone of Thrain: “It was as if the globe had been filled with moonlight and hung before them in a net woven of the glint of frosty stars.”
Messier notes: (Observed June 5, 1764) Nebula, below the ecliptic, between the head and bow of Sagittarius, close to the seventh-magnitude star Flamsteed 25 Sagittaraii. This nebula is circular, does not contain any stars, and is clearly visible in a simple three-and-a-half foot refractor. The star lambda Sagittarii was used to determine its position.
NGC notes: Very remarkable, globular cluster, very bright, very large, round, very rich, very much compressed, stars from 11th to 15th magnitude.
Data: Messier 22 aka NGC 6656 aka Sagittarius Cluster aka Crackerjack Cluster
Con: Sagittarius Mag: 5.2
RA: 18h36.4m Dec: -23.54
Dist: ~10,400 ly
M26 Open Cluster (3) detectible
From brighter skies, this object might well be overlooked because its tantalizing gleam of background stars would be all but washed out. M26 is an innocuous knot of stars that spans an area of about 15 light-years and contains 120 telescopic stars in a disk 10 degrees in apparent diameter, though only about two dozen of the stars are readily seen in a 4-inch. It is pretty condensed when compared to other Messier open clusters.
Messier notes: (Observed June 20, 1764) …with a three-foot refractor they cannot be detected; a good instrument must be used. This cluster contains no nebulosity.
NGC note: Cluster, quite large, pretty rich, pretty compressed, stars from 12th to 15th magnitude.
Data: Messier 26 aka NGC 6694
Con: Scutum Mag: 8.0
RA: 18h45.25m Dec: -09.23
Dist: ~5,200 ly
M11 Open Cluster (2) easy*
M11 is a small but extremely rich open cluster, bristling with the light of at least 680 stars; of these, about 400 stars shine brighter than 14th magnitude. The cluster measures about 20 light-years in diameter, and its core is very dense. If one lived at the center of M11, you would see several hundred 1st-magnitude stars in the sky and possibly 40 or so with an apparent brightness of 3 to 50 times the brightness of Sirius.
Messier notes: (Observed May 30, 1764) Cluster of a large number of faint stars…which may be seen only with good instruments. With a simple 3-foot refractor, it resembles a comet. This cluster is suffused with luminosity.
NGC note: Remarkable, cluster, very bright, large, irregularly round, rich, one star of 9th magnitude among stars of 11th magnitude and fainter.
Data: Messier 11 aka NGC 6705 aka Wild Duck Cluster
Con: Scutum Mag: 5.8
RA: 18h51.1m Dec: -6.16.2
Dist: ~5,460 ly
M54 Globular cluster (3) detectible*
Beaming from a distance of 86,400 light-years from the Sun, and 61,000 light-years from the Galactic center, M54 is the farthest of all the Messier globulars and the second most massive known. It is an impressive system 300 light-years wide that is approaching us at 460 miles per second. (Collision not eminent)
Despite its distance, M54 is no problem for 7 x 35 binoculars, appearing as a bright, star-like object. But, being at such a distance, M54 does not easily reveal its secrets to telescopic viewers. That is no wonder, given that the cluster’s stars shine at a dim magnitude of 15.2, while its horizontal-branch magnitude is a dim 18.2. Therefore, one must be able to see beyond 18th magnitude to fully resolve the cluster.
Messier notes: (Observed July 24, 1778) Very faint nebula discovered in Sagittarius. The center is bright, and it does not contain any stars.
NGC note: Globular cluster, very bright , large, round, gradually, then suddenly much brighter in the middle, well resolved with 15th magnitude stars.
Data: Messier 54 aka NGC 6715
Con: Sagittarius Mag: 7.7
RA: 18h55.1m Dec: -30.29
Dist: ~86,400 ly
M75 Globular Cluster (3) detectible*
Charlie’s 75th deep-sky curiosity is also one of the more challenging to find, because it lies in a southern region of sky devoid of bright guidepost stars. Despite its great distance, M75’s highly compact 7 degree disk shines at a respectable magnitude of 8.6. That is because it is a sizable system spanning a whopping 140 light-years of space.
Messier notes: (Observed October 18, 1780) Nebula without a star between Sagittarius and the head of Capricornus. Seen by Messier as only containing very faint stars with faint nebulosity.
NGC note: Globular cluster, bright, pretty large, round, very much brighter in the middle to a brighter nucleus, partially resolved.
Data: Messier 75 aka NGC 6864
Con: Sagittarius Mag: 8.6
RA: 20h06.1m Dec: -21.55
Dist: ~68,200 ly
* Sky Tools offers an observability scale of 1 – 6, with 1 being “Obvious” and 6 being “Very Challenging”. This particular rating scale I use is based on a Celestron SCT 8 Evolution telescope at the HAS dark site on a moonless night.
Finito!
Ex astris, scientia, y’all!
Jim King
Field Trips and Observing Chair
Want more? Check out the HAS website under “Programs”/Messier Challenge/HAS 45
