Object Of The Week August 1, 2021 - Messier 55

Messier 55, NGC 6809

Globular Cluster

Constellation: Sagittarius

RA: 19 39 59.71
DEC: -30 57 53.1

Mag: 6.49

Diam: 5.78’

As I am traveling from Chile to Spain this weekend, I am posting my OOTW contribution a few days early.

Messier 55 is a beautiful globular cluster in Sagittarius. With showpieces like M 7, M 8 and M 22 close by, this globular cluster often gets overlooked. However, I recently observed this cluster again and realized that it really should be on everybody’s standard summer (or winter if in the southern hemisphere) observing list.

First a little history. This is one of the Messier objects that were not discovered by Messier himself. Instead, it was discovered in 1752 by Nicolas Louis de Lacaille from the Cape of Good Hope. While observing there, Lacaille catalogued nearly 10,000 stars and introduced 14 new constellations which have become standard since. He also discovered 42 nebulous objects, one of which later became known as Messier 55.

Attachment 4399
Image of Messier 55 taken from https://astropixels.com.au/astrophot...bular_Cluster/

Messier tried to observe this globular cluster from Paris starting in 1754 but didn’t succeed until 1788 due to the low altitude of the object from Paris. I did manage to observe it from the Netherlands, which lie even further north, so that says something about the poor skies and telescope used. My home made 15 cm Newtonian telescope probably gives a much better image quality than Messier’s telescopes.

Physically Messier 55 is a very normal Milky Way globular cluster of condensation type XI which indicates a very loose globular cluster.

I have observed Messier 55 several times. With my 15 cm telescope I noted in 2007 in Southern France that “it is faintly visible in the finder scope. At 146x individual stars are visible which are surrounded by a large glow. M55 appears similar in size as M22 but fainter”.

Three weeks ago I observed this globular cluster with a 12.5” dobsonian telescope from a dark location in Chile. The globular cluster was near the zenith, which of course helps. I recall seeing an impressive and completely resolved globular cluster even at a very low magnification of 45x.

While observing Messier 55, be sure to check out the globular clusters Arp 2, Terzan 7 and Terzan 8. I could spot all three of them with the 12.5” dob as faint, round and unresolved glows.


As always,

"Give it a go and let us know!
Good luck and great viewing!"

Hi here's my observation from 10/25/2014:
Messier 55 globular cluster in Sagittarius; magnitude:6.49 ; size: 19' ; fairly bright; jagged star chains and dark lanes intertwine across the face of the cluster giving it a 3D effect : I used a 10mm Ethos at 198x with my 14.5 Starstructure Dob f/4.3Attachment 4400

Its relative absence of central condensation, combined with its richness, left me with an impression of a fainter Omega Centauri.
Some figures:
Total magnitude: 6.3 (V)
Diameter at mag.22: 9.4'
Diameter at mag.25: 19'
Brightest stars: mag.11.2
Horizontal Branch mag.: 14.4
B-V 1.5 (not seriously reddened)
At nearly -31°, this can be hard to see for those north of 50°N, but it's worth the trouble.
A 6" scope under good conditions can resolve stars across the entire face of the cluster, and even a 4" can see a lot of stars.
It's magnificent with larger apertures.
It is on a line if you extend the line formed by the two outer stars in the Teapot's handle toward the SE.

In my mind, this is the most underrated Messier object, and one of the oft-overlooked gems of the summer sky. I particularly like the "bite" taken out of it on the SF side.

From 48°21’ N it’s not the best for me but seeing as I’m going to the observatory tonight I’ll give it a look. Pretty warm today so I’m not expecting the best seeing looking south across the cold North Pacific.

Bill