M55 is a globular cluster located in eastern Sagittarius towards its border with Capricornus and Microscopium. At magnitude +6.7, it's beyond naked eye visibility but bright enough to be seen with binoculars. However, it's not an easy globular to locate since there aren't any particular bright stars nearby. With a declination of -30 degrees, M55 is one of the more southerly objects in Messier's catalogue and therefore especially difficult for observers based at northern temperate latitudes. It's best seen from southern or equatorial latitudes during the months of June, July and August.

M55 was discovered by Nicholas Louis de Lacaille on June 16, 1752 while observing from South Africa. Charles Messier then catalogued it on July 24, 1778. From Paris, Messier had difficulty finding M55, it took him 14 years to spot it!

Finding M55 can be challenging. One method is to begin with the "teapot" asterism of Sagittarius. Start by locating stars Kaus Media (δ Sgr - mag. +2.7) and Ascella (ζ Sgr - mag. +2.6). Then imagine a line from Kaus Media moving eastwards towards and passing through Ascella. Curve this line for another 17 degrees to arrive at M55.

M55

Finder Chart for M55 (also shown M22, M25, M28, M54, M69, M70 and M75)

Finder Chart for M55 (also shown M22, M25, M28, M54, M69, M70 and M75) - pdf format

Finder Chart for M54 (also shown M6->M8, M18, M20->M24, M28, M55, M69->M70)

Finder Chart for M54 (also shown M6->M8, M18, M20->M24, M28, M55, M69->M70) - pdf format

When viewed through 7x50 or 10x50 binoculars, M55 appears as a diffuse ball of light, non-stellar without a bright core. Under dark sky conditions, it appears like a round hazy comet. However, M55 has a low surface brightness meaning that only a small amount of light pollution renders it practically invisible to binocular observers. Through a 100mm (4-inch) scope, it appears loose and hints at resolution. M55 in total covers 19 arc minutes of apparent sky but appears much smaller through the eyepiece. A 200mm (8-inch) instrument will resolve many stars with the cluster bursting into life; a magnificent swarm of thousands of pinpoints of light spread across the complete surface.

M55 is located 17,600 light-years distant and has a spatial diameter of about 94 light-years. Only about 55 variable stars have been discovered in the cluster that's estimated to contain 100,000 stars. It's about 12,700 billion years old.

M55 Data Table

Messier55
NGC6809
Object TypeGlobular cluster
ConstellationSagittarius
Distance (kly)17.6
Apparent Mag.6.7
RA (J2000)19h 39m 59s
DEC (J2000)-30d 57m 44s
Apparent Size (arc mins)19 x 19
Radius (light-years)47
Age (years)12,300M
Number of Stars100,000

Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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