The Milky Way Project: What are Yellowballs?
  Yellowballs are a collection of approximately 900 compact, infrared sources
identified and named by volunteers participating in the Milky Way Project
(MWP), a citizen-science project that uses GLIMPSE/MIPSGAL images from Spitzer
to explore topics related to Galactic star formation. In this paper, through a
combination of catalog cross-matching and infrared color analysis, we show that
yellowballs are a mix of compact star-forming regions, including ultra-compact
and compact HII regions, as well as analogous regions for less massive B-type
stars. The resulting MWP yellowball catalog provides a useful complement to the
Red MSX Source (RMS) survey. It similarly highlights regions of massive star
formation, but the selection of objects purely on the basis of their infrared
morphology and color in Spitzer images identifies a signature of compact
star-forming regions shared across a broad range of luminosities, and by
inference, masses. We discuss the origin of their striking mid-infrared
appearance, and suggest that future studies of the yellowball sample will
improve our understanding of how massive and intermediate-mass star-forming
regions transition from compact to more extended bubble-like structures.
