We present distributions of colors of stars along the horizontal branch of
the globular cluster NGC 2808, from Hubble Space Telescope WFPC2 imaging in B,
V, and an ultraviolet filter (F218W). This cluster's HB is already known to be
strongly bimodal, with approximately equal-sized HB populations widely
separated in the color-magnitude diagram. Our images reveal a long blue tail
with two gaps, for a total of four nearly distinct HB groups. These gaps are
very narrow, corresponding to envelope-mass differences of only \sim 0.01 Msun.
This remarkable multimodality may be a signature of mass-loss processes, subtle
composition variations, or dynamical effects; we briefly summarize the
possibilities. The existence of narrow gaps between distinct clumps on the HB
presents a challenge for models that attempt to explain HB bimodality or other
peculiar HB structures.Comment: LaTeX, including compressed figures. To appear in ApJL. Larger (851k)
PostScript version, including high-quality figures, available from
http://astro.berkeley.edu/~csosin/pub
