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ASTR 5610, Majewski [SPRING 2018]. Lecture Notes

ASTR 5610 (Majewski) Lecture Notes


Spherical (or only slightly elliptical) collections of typically 103-5 stars in tight (but somewhat variable levels of) concentration (a few extreme outliers in this distribution).

For example, the following images spanning 15 arcmin on a side each show four clusters at about the same distance from the Sun (~10 kpc). Note the vast difference in sizes/concentrations/masses of the clusters.

Clusters of different concentrations, marked by the Shapley & Sawyer (1927) concentration class (class I = most concentrated, XII = least concentrated). From Binney & Merrifield.
Recall Baade (1944): Very different from dwarf galaxies.

Classically considered to be close to a Simple Stellar Population (*).

---> Therefore, obviously, very useful to stellar population studies.

(* However, in the very last few years, this idea is undergoing a radical transformation, with the discovery of spreads in detailed chemical abundances, and evidence for multiple stellar populations in an increasing number of clusters --- see below.)

Indeed, globular clusters are useful probes of:

Example of Globular Clusters Use as Stellar Population Probes

Size of the Universe/Milky Way -- the Great Debate.

Color-Magnitude Diagram Features

Binary Sequences:

Blue Stragglers:

Red Bump:

HB types:

Large variety seen.
Usually a reflection of the first parameter effect, dependence on [Fe/H].

Higher metallicity clusters show a red clump.

But there are metal-poor clusters that show a second parameter effect in their HB morphology.

Famous second parameter pair NGC 288 and NGC 362: two clusters with the same metallicity ([Fe/H] ~ -1.3) but different HB morphologies. The cluster NGC 1261 also has a similar metallicity, but an HB morphology that is somewhat intermediate. From a paper by Bolte (1989, AJ, 97, 1688) wherein he argues that the differences are due to an age spread of 3 Gyr between NGC 288 and NGC 362 and 1-2 Gyr between NGC 288 and NGC 1261.
A few anomalous bimodal clusters show both blue HBs typical of metal-poor clusters along with a red clump.

To account for what appears to be multiple populations in a single cluster (one first parameter, the other second parameter), bimodal clusters have been recently postulated to be the cores of highly disrupted dSph galaxies.

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All material copyright © 2003,2006,2008,2010,2012,2014,2016,2018 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 551 and Astronomy 5610 at the University of Virginia.