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R. Michael Rich (UCLA), Bruce Margon (U. Washington)
Although many view Next Generation Space Telescope (NGST) as a project primarily aimed at studies of the high redshift Universe, it will also make a variety of fundamental contributions to stellar astrophysics. These observations will be invaluable not only for inferences they yield on stars, but also to fill tremendous gaps in our knowledge of stellar populations and star formation processes at z=0. Until very recently, most direct observations relevant to these problems are available only in our Galaxy and the Magellanic Clouds; HST extends the reach for certain issues to M31, but is only just capable of studying the brightest giants in 2 nearby ellipticals. Thus without a program of NGST stellar observations, there will be little data relevant to the present epoch for comparison with NGST results on star formation at very high redshift.
The NGST Ad Hoc Science Working Group has considered a variety of possible observations in stellar astrophysics, and we present a sample of such problems here. As many of these problems require precision photometry at very faint (m~0) flux levels, we have where appropriate used several of the point spread functions of likely NGST primary mirrors for detailed simulation of the observations. Examples of interesting programs are:
a) Determination of globular cluster ages independent of most previous theoretical frameworks, via detection of the end of the white dwarf cooling sequence at MH~2;\ b) derivation of very deep stellar luminosity functions, reaching the faintest H-burning stars at MH~0, in the galactic bulge, dwarf spheroidals, and the Magellanic Clouds;\ c) direct measurement of the star formation history in irregular and dwarf galaxies, via observations reaching several magnitudes below the turnoff point;\ d) accurate constraints on the metallicity distribution and ages of old halo Pop I stars throughout the Local Group, yielding strong inferences on whether star formation was coeval.
Some of the above goals require an extension of the NGST baseline 1-5\mu wavelength range shortward to 0.5-0.6\mu, a stretch goal also desired by additional, unrelated NGST scientific programs. A primary mirror diffraction limited at 2\mu still provides superb performance in the optical, superior to HST.