AAS 201st Meeting, January, 2003
Session 64. Russell Prize Lecture
Invited, Tuesday, January 7, 2003, 11:40am-12:30pm, 6AB

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[64.01] The Origin and Composition of the Galactic Halo

G. Wallerstein (University of Washington)

There are presently two viable suggestions for modeling the formation of the Galactic Halo. They are known as the Eggen, Lynden-Bell, and Sandage (ELS) approach and the Searle and Zinn (SZ) model. According to ELS the Halo formed in a general and singular collapse of very metal-poor gas which condenced into stars. The most massive of these stars evolved and ejected material that was enhanced in elements heavier than helium. Thus the interstellar medium in the collapsing gas was enhanced in metals as further stars formed closer to the Galactic Center. The SZ model envisions the formation first of many small systems like the presently known dwarf spheroidals which were then captured by the forming Galaxy.

I will discus the observational evidence that bears on these models. Two types of evidence will be considered. The first is the chemical composition of stars in the halo field, the globular clusters and the nearby dwarf spheroidal systems. The second is the populations of variable stars of different types in the same three environments.

My research was supported by the National Science Foundation from 1959 until about 1988. In addition I have been a guest observer at the Mt. Wilson and Palomar Observatories, The Lick Observatory, the Kitt Peak and Cerro Tololo Observatories, The Dominion Astrophysical Observatory and the VLT on Cerro Paranal.

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