Messier 55
A low-metallicity globular cluster in southern Sagittarius.
Messier 55 (cataloged as M55, NGC 6809, and also called the Specter Cluster) is a globular cluster located in the southern part of the constellation Sagittarius. Nicolas Louis de Lacaille first spotted it in 1752 while observing from what is now South Africa. Charles Messier tried repeatedly to find it from Paris starting in 1754, but its low declination meant it barely rose above the horizon there, making it difficult to see; he finally observed and added it to his catalog in 1778. With 50 mm binoculars, the cluster is visible, but resolving individual stars requires a medium-sized telescope.
The cluster lies about 17,600 light-years from Earth and contains roughly 269,000 times the mass of the Sun. Like most globular clusters in the Milky Way, it has a low abundance of elements heavier than hydrogen and helium compared to the Sun—a property called low metallicity. This is expressed as [Fe/H] = −1.94 dex, meaning the cluster has only 1.1% of the Sun’s iron content relative to hydrogen and helium (a value of −2 would indicate 100 times less iron than the Sun). In the central region of M55, only about 55 variable stars have been identified.
- Discovered by
- Nicolas Louis de Lacaille
- Discovery year
- 1752
- Discovery location
- what today is South Africa
- Catalogued by
- Charles Messier
- Catalogued year
- 1778
- Distance from earth
- about 17,600 light-years
- Mass
- about 269,000 solar masses
- Metallicity
- [Fe/H] = −1.94 dex (1.1% of the Sun's iron proportion)
- Variable stars found
- about 55
Lore & Background
Messier 55 was discovered by Nicolas Louis de Lacaille in 1752 while he was observing from what today is South Africa. Starting in 1754, Charles Messier made several attempts to find this object from Paris, but its low declination meant that from there it rises daily very little above the horizon, hampering observation. He eventually observed and catalogued it in 1778. The cluster can be seen with 50 mm binoculars; resolving individual stars needs a medium-sized telescope. It is about 17,600 light-years away from Earth and contains about 269,000 solar masses. As with other Milky Way globular clusters, it has few elements other than hydrogen and helium compared to the Sun, giving it a low metallicity of [Fe/H] = −1.94 dex, meaning it has 1.1% of the proportion of the Sun's iron compared to hydrogen and helium. Only about 55 variable stars have been found in the central part of M55.
Reader's Guide
Messier 55 is significant as a globular cluster in the southern constellation Sagittarius, discovered by Nicolas Louis de Lacaille in 1752 from what is now South Africa. Its low declination made it challenging for Charles Messier to locate from Paris, despite attempts starting in 1754; he finally catalogued it in 1778. The cluster is notable for its low metallicity, with an iron abundance only 1.1% that of the Sun, typical of Milky Way globular clusters. It lies about 17,600 light-years away and contains roughly 269,000 solar masses. Observers can spot it with 50 mm binoculars, though a medium-sized telescope is needed to resolve individual stars. Only about 55 variable stars have been identified in its central region. Its legacy includes being a well-studied example of a metal-poor globular cluster, with data available through various astronomical databases and imagery.
Frequently Asked Questions
What kind of object is Messier 55?
M55 is a globular cluster—a tight, spherical swarm of ancient stars—located in the southern portion of Sagittarius. It carries the NGC designation 6809 and is also nicknamed the Specter Cluster.
Who first discovered Messier 55 and when?
Nicolas Louis de Lacaille first spotted the cluster in 1752 while observing from what is now South Africa. Charles Messier did not add it to his catalog until 1778, after years of failed attempts to locate it from Paris.
How far is Messier 55 from Earth?
The cluster lies roughly 17,600 light-years away. It is classified as a low-metallicity globular cluster, meaning its stars formed from gas that was comparatively poor in elements heavier than hydrogen and helium.
Can I see Messier 55 with binoculars?
A pair of 50 mm binoculars is sufficient to pick out the cluster as a faint, fuzzy patch. Resolving it into individual stars, however, calls for a medium-sized telescope or larger.
Why did Charles Messier struggle so long to catalog M55?
The cluster's very low declination meant it barely rose above the horizon as seen from Paris, making clear observation extremely difficult. Messier kept attempting to find it from 1754 onward and finally secured a firm observation in 1778.
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