M55 is a quite large (about 19', roughly 2/3 of the Moon's apparent diameter) but has such a loose appearence, that the present author had a star cluster impression even in 7x50 binoculars, where most globulars look like round nebulae: This one appeared very grainy. As it is about 20,000 light years distant, this diameter corresponds to a linear of about 110 light years. M55 has only very few known variables, 5 or 6. The published values for M55's magnitude vary from mag 5 to 7. Its total luminosity may be near 100,000 times that of the Sun.
M55 was originally discovered by Lacaille in 1751-1752, when he was observing in South Africa. Messier finally found it in 1778, having probably looked in vain as early as 1764: This is a consequence of this object's southern declination. The present author can confirm that M55 is most difficult from Southern Germany also, but splendid if you go a bit more southward (it was very impressive e.g. in Northern Greece).
Right ascension | 19 : 36.9 (hours : minutes) |
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Declination | -31 : 03 (degrees : minutes) |
Distance | 20.0 (light-years*10^3) |
Visual magnitude | 5.0 |