…continuedA Collection of Bipolar Planetary Nebulae
Two Classics to Start With
Visually, M76 is a wonder object in almost any telescope, given a dark sky. In small instruments only the bright central bar shows readily. But an observer equipped with a 6- or 8-inch scope and a UHC or O III nebula filter will be able to make out Lord Rosse's outer "spiral loops." In my 17.5-inch Newtonian reflector, M76 offers rich texture to explore at medium to high magnifications. The main bar is bright and mottled, while the dim lobes extending from each side display delicate, almost filamentary structure. It's a shame that most images of this fine object are exposed only enough to show the central bar. Not until recently have I seen CCD imagers taking the plunge and making deep color exposures that capture the outer extensions.
A classic butterfly-shaped bipolar is NGC 2346, located 2/3° southwest of Delta (δ) Monocerotis. It has a bright, irregularly oval central bar with faint extensions protruding northwest and southeast. You may or may not see the star at its center, V651 Monocerotis; it's normally bright at magnitude 11.4 but has gone through spells of dipping to 14.5 (Sky & Telescope: September 1992, page 249). The actual source of the nebula is a hot, very faint companion to this star.
Using the Atlanta Astronomy Club's 20-inch reflector at 176x, I could detect an entire faint halo surrounding a bright inner disk. At much higher magnification, other observers have detected considerably more detail. Virginia deep-sky observer M. Eric Honeycutt writes, "Through my 22-inch at 500x and no filter, the two separate lobes were distinct yet were attached in the central region." He also notes that the central region "had a criss-crossing pattern."