bwilsonchronicle.jpg (24623 bytes)Ranking by Brightest
List of G. Clusters Most distant G. Ranking by Brightest

Ranking by Brightest Core

Milky Way Globulars


Notes and data compiled from various sources by Barbara Wilson 6/27/02. Ref: Harris, W.E. 1996, AJ, 112, 1487

Name Central Surface Brightness V tip V HB Shapley Sawyer Class Core Collapse
V mag per sq. arc sec. Brightest star
NGC 1851 14.15 13.2 16.1 2
7078=M15 14.21 12.6 15.9 4 cc
NGC104=47 Tuc 14.43 11.7 14.1 3
NGC 6388 14.55 14.8 17.2 3
NGC 6715=M54 14.82 15.2 18.2 3
NGC362 14.88 12.7 15.4 3 cc
NGC6441 14.99 15.4 17.1 3


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.

Core Collapse globulars: NGC 362 NGC 5946 NGC 6256 NGC 6266 NGC 6284 NGC 6293 NGC 6325 NGC 6342 NGC 6355 Terzan 2 HP 1 Liller 1 NGC 6380 Terzan 1 NGC 6397 Terzan 6 NGC 6453 UKS 1 Terzan 9 NGC 6522 NGC 6544 NGC 6541 NGC 6624 NGC 6558 NGC 6681 NGC 6717 NGC 6752 NGC 7078 NGC 7099(M30)

Observational notes (Barbara Wilson)

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

Stars are.

Observational questions:

  1. Can you determine if a cluster is undergoing core collapse from visual observations?
  2. Can you accurately determine a cluster’s Shapley-Sawyer Class by observing it?
  3. Is there any correlation between Shapley-Sawyer class and a cluster’s resolvability?

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