Notes:

All these clusters are strongly concentrated objects. M 15 is the prototypical example of a core collapsed globular. NGC 362 is also undergoing core collapse. The tell tale signature of core collapse is when a cluster’s surface brightness (a measure of the number of magnitudes of brightness per square arc second) rises continually from the cluster’s outskirts all the way to the its very center. In normal uncollapsed clusters, the surface brightness increases until about 3 to 5 light years from the center and then remains constant. To put simply, the higher density of stars in collapsed core globulars means they will have much brighter/denser centers than normal clusters.

Core collapse occurs when the orbits of stars in a cluster change from circular to radial. As the orbits change, the stars gravitationally interact, transferring energy from the core to the halo.

This energy transfer forces stars in the core to migrate closer to the center causing the core to contract, while the halo expands. This in turn accelerates the transfer of energy causing the center of the cluster to collapse, and become up to ten billion times more densely populated with stars than our normal solar neighborhood. This process occurs rapidly, and stars in the cluster’s core are on average only as far apart as the distance between the Sun and Pluto.

NGC 1851- center mottled with 5mm resolves in outer portions. Intense bright tight center. Very loose outer halo.

M 15- Very condensed core, very bright and dense. Resolves almost to core with 7mm. Looks like shimmering sugar around periphery.

NGC104- The core is a brilliant crowded mass of stars, it resolves to the core, some stars are reddish in the center, and the center is sharply brighter than the outer portions. The cluster has an almost delicate appearance with spiraling lines of stars emanating in all directions. Truly breathtaking.

NGC 6388 – This tiny globular is very bright The change in light from the center to its outer regions is very abrupt. It has no resolution in my 20".

M54- small, round, and condensed. Only 2 stars are resolved in 8" f/6 with 9 mm.

NGC 362 – Very bright core. This cluster is just north of the SMC, it appears close to the Cloud.

The core is near stellar, but I can see about 20-30 stars near the core. The cluster’s core is not resolved in the 7".

NGC 6441 – This globular is extremely concentrated. It is tiny with one star at east edge.

Shapley-Sawyer concentration class 1 – 12 The higher the number the less concentrated the

Observational questions: