Hubble Space Telescope observations of the Draco dwarf spheroidal galaxy

Carl J. Grillmair, Jeremy R. Mould, Jon A. Holtzman, Guy Worthey, Gilda E. Ballester, Christopher J. Burrows, John T. Clarke, David Crisp, Robin W. Evans, John S. Gallagher, Richard E. Griffiths, J. Jeff Hester, John G. Hoessel, Paul A. Scowen, Karl R. Stapelfeldt, John T. Trauger, Alan M. Watson, James A. Westphal

Research output: Contribution to journalArticlepeer-review

64 Scopus citations


We present an F606W - F814W color-magnitude diagram for the Draco dwarf spheroidal galaxy based on Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images. The luminosity function is well sampled to ∼3 mag below the turnoff. We see no evidence for multiple turnoffs and conclude that, at least over the field of view of the WFPC2, star formation was primarily single-epoch. If the observed number of blue stragglers is due to extended star formation, then roughly 6% (upper limit) of the stars could be half as old as the bulk of the galaxy. The color difference between the red giant branch and the turnoff is consistent with an old population and is very similar to that observed in the old, metal-poor Galactic globular clusters M68 and M92. Despite its red horizontal branch, Draco appears to be older than M68 and M92 by 1.6 ± 2.5 Gyr, lending support to the argument that the "second parameter" that governs horizontal-branch morphology must be something other than age. Draco's observed luminosity function is very similar to that of M68, and the derived initial mass function is consistent with that of the solar neighborhood.

Original languageEnglish (US)
Pages (from-to)144-151
Number of pages8
JournalAstronomical Journal
Issue number1
StatePublished - Jan 1998
Externally publishedYes


  • Galaxies: abundances
  • Galaxies: elliptical and lenticular, cD
  • Galaxies: evolution
  • Galaxies: individual (Draco)
  • Local Group

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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