Looking like a lone headlight on a rainy night or a glowing lime in the sky, the bright green ball is actually the coma of Comet Lemmon, caught on camera by Australian astronomer Peter Ward from his Barden Ridge Observatory on Feb. 4 (view the hi-res version here). The comet's steadily-growing tail can be seen extending to the lower right.
Comet Lemmon (C/2012 S1) is currently traveling across the sky in the southern hemisphere and has brightened to a visible magnitude of about 6.2 - just at the limit of what can be seen with the naked eye under very dark, clear skies. Peter used a 14.25″ F7.9 Ritchey-Chretien telescope to capture the image of Lemmon, set against long-exposure trails from background stars. See how you can find Comet Lemmon here.
PICTURES: Close Encounters With Comets
The green color comes from outgassed ethane surrounding the comet's nucleus interacting with radiation from the sun, causing it to glow - in very much the same way that neon signs and fluorescent lights work.