C22 — Blue Snowball

About

A planetary nebula in Andromeda: the gas shell shed by a dying star, glowing blue-green from doubly ionised oxygen. Sources disagree on its distance, from about 2,500 to about 5,700 light-years. Beyond the bright shell lies a faint halo about 134″ across.

Imaging tips

• Only about 1′ across, so it is too small to frame: use a long focal length and a fine pixel scale.
• The brightness range is huge. One imager called it extremely challenging and stacked exposures from 120 s up to 600 s to cover the bright core and the faint halo.
• The light is mostly blue-green OIII with some red Hα in the inner core, so an OIII filter pays off; one image used 81 ten-minute OIII subs, 13.5 of its 18.3 hours.
• The core is about 0.5′ and the halo about 2.5′ across, one imager reports, so shoot enough frames to show the halo, not only the shell.
• Best around August and September; it peaks near 82° altitude from 50°N.