Wednesday, December 12, 2012

Globular Cluster Photo Series (Part 28): M55

Today I have another globular cluster picture for you, and this one just happens to be the next in the Messier catalog: Messier 55, in Sagittarius. This globular cluster is much closer than M54, at a moderately distant 17,600 light-years. It appears almost twice as large on the sky at 19.0 arc-minutes, but is a mere third its actual size at 96 light-years in diameter. It's also a lot less compact than M54 (class XI out of XII), and really looks quite nice.

Messier 55 in Sagittarius.
Not every object in Charles Messier's catalog was discovered by him (and he gave credit where it was due), and M54 is one such object. It was discovered by an astronomer named Nicholas Louis de Lacaille from an observatory in South Africa in 1752. Messier, having heard of this discovery, tried several times to locate the cluster starting in 1764, but was stymied by its low apparent height from his location in Paris (it is located 30 degrees south of the celestial equator, which makes it rather difficult to see from mid-northerly latitudes). In fact, it wasn't until 1778 – 14 years later – that Messier was actually able to find it, after which he included it in his famous catalog of objects.

All in all, M55 is a rather nice looking cluster, if I say so myself.


  1. I think I've asked you this before but I forgot: How are globular clusters different from a nebula/galaxy?

    Yes, I can probably Google this up but I like the way you explain stuff. Hehe.

    1. Well, a nebula is basically a cloud of gas and dust in space, while a globular cluster is a large (fairly spherical) bunch of stars held together by their own gravity, with little to no gas or dust in it.

      Globular clusters are fairly similar to both elliptical galaxies and their little brothers, dwarf spheroidal galaxies. All of them are made up primarily of older stars with very little star formation happening (thus appearing more red/yellow than blue), they all have little to no gas or dust in them, and they all have fairly spherical (or cigar-shaped) forms.

      On the other hand, globular clusters (and elliptical galaxies) are very different from spiral galaxies such as the Milky Way. Spiral galaxies have mixed populations of young (blue) and old (red/yellow) stars, a fairly disk-shaped appearance, active star formation (ranging from very little to OH MY GOODNESS SO MANY STARS), and lots of gas and dust distributed as nebulae throughout the disk. Spirals do tend to contain a spherical bulge at their centers that is similar to an elliptical galaxy or globular cluster, composed of old stars with little gas mixed in.

      Both elliptical and spiral galaxies can have globular clusters orbiting around them. The largest known galaxies are ellipticals many times larger than the Milky Way, and they can have larger globular clusters orbiting them that blur the line between large cluster and small galaxy. On characteristic sometimes used to distinguish between them is that most galaxies appear to have a black hole at their center, while few (or no) globular clusters do. This classification scheme would probably make Omega Centauri a small galaxy rather than the largest Milky Way globular cluster, as there is evidence for a black hole near or at its center.


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