... There are more than a few stars in our Galaxy. ... A typical place is an area of dense nebular gas common to arms in spiral galaxies. Sometimes, however, a burst of star formation can occur with unusual geometry. Nearby galaxy NGC 1317 shows such an unusual ring of star formation surrounding its barred nucleus. Comet Halley's Nucleus . ... For all active comets except Halley, it was only possible to see the surrounding opaque gas cloud called the coma. ... Callisto's surface shows its age. ...