The White Dwarf Cooling Sequence of the Globular Cluster Messier 4
  We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I = 28. The white dwarf luminosity function rises sharply for I >25.5, consistent with the behaviour expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 magnitudes fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models (Hansen 1999) to fit each luminosity function yields ages of 7.3 +/- 1.5 Gyr for the disk and 12.7 +/- 0.7 Gyr for M4 (2-sigma statistical errors). 
Brad M. S. Hansen (UCLA/Princeton), James Brewer (UBC), Greg G.   Fahlman (CFHT), Brad K. Gibson (Swinburne Univ.), Rodrigo ...