Before it was seen, it was heard. Brainstorming

Star Clusters Chang, Seo-Won

QUIZ = ? 2 = ?. Galaxy = ? smooth distribution of stars + agglomerations of stars +....

Key role Development of our understanding of the Universe! Globular Clusters and Open Clusters - compactness - luminosity - metallicity

6.1 Globular Clusters Roughly spherical feature Positions ranging from near the GC out to remote regions in the halo Metal-deficiency and coeval with our Galaxy Research Method: Photometry + Spectroscopy + Astrometry -> Age, Metallicity, and Kinematics

6.1 Historical Background Optical Messier catalog ( 2MASS Atlas Image Gallery: The Messier Catalog ( Abell(1955), "Globular Clusters and Planetary Nebulae Discovered on the National Geographic Society-Palomar Observatory Sky Survey“ (Pal I, …, Pal 13 series) Harris(1996), “A Catalog of Parameters for Globular Clusters in the Milky Way“ (

6.1 Historical Background

6.1 Some Properties Morphology 1. Approximately circular in projection (presumably toughly spherical) 2. Purely stellar systems since no diffuse gas or dust is apparent in them 3. They are slightly elliptical. Why? King says, "....Galactic interaction effect" Van den Bergh says, "....Galactic absorption effect” See the following paper, White and Shawl (1987), "Axial ratios and orientations for 100 Galactic globular star clusters"

6.1 Some Properties Luminosity ~ P gussian (M v = -7, 1.25)

6.1 Some Properties Number Density and Radial distribution (mail to or Spectrum similar to those of individual stars 1. About F3 to G5 (Monella 1985) 2. Spectral variation ~ F ([Fe/H], α-elements, …. ) 3. [Fe/H] ranging from -2.5(?) up to solar or even greater 4. Age and formation history

6.1.1 GC Photometry Color Magnitude Diagram 1. age and mass 2. formation and subsequent evolution Advantages – observation, distance, reddening Difficulties – compactness, missing some stars especially faint ones/ overlap information and misidentify adjacent stars (source confusion) Solutions – attention to the outer regions of clusters; contamination of field stars, – high-quality CCD and computing power; software – improvements in image quality; money

6.1.1 Solution Stetson (1987), "DAOPHOT - A computer program for crowded-field stellar photometry " Completeness test (McClure et al. 1985) Guhathakurta (1996), "Globular cluster photometry with the HST. V. WFPC2 study of M15's central density cusp"

(e.g.) DAOPHOT – Ho II

(e.g.) Hybrid method – M15 P PSF + Aperture Photometry

All we can say about this is, “globular cluster” P Durrell(1993)