The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Θ₀/R₀

We establish an extragalactic, zero-motion frame of reference within the deepest optical image of a globular star cluster, a Hubble Space Telescope (HST) 123-orbit exposure of M4 (GO 8679, cycle 9). The line of sight beyond M4 (l, b = 351 o, 16 o) intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance of 7.6 kpc (for R ◦ = 8 kpc). The main sequence of this population can be clearly seen on the color-magnitude diagram (CMD) below the M4 main sequence. 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA under NASA contract NAS 5-26555. These observations are associated with proposal GO-8679. 2