Starship Asterisk*

Comet PanSTARRS in the Southern Fish

Explanation: Now approaching our fair planet this Comet PanSTARRS (C/2013 X1) will come closest on June 21-22, a mere 5.3 light-minutes away. By then its appearance low in northern hemisphere predawn skies (high in the south), will be affected by the light of a nearly Full Moon, though. Still the comet's pretty green coma is about the apparent size of the Full Moon in this telescopic portrait, captured on June 12 from the southern hemisphere's Siding Spring Observatory. The deep image also follows a broad, whitish dust tail up and toward the left in the frame, sweeping away from the Sun and trailing behind the comet's orbit. Buffeted by the solar wind, a fainter, narrow ion tail extends horizontally toward the right. On the left edge, the brightest star is bluish Iota Picis Austrini. Shining at about fourth magnitude, that star is visible to the unaided eye in the constellation of the Southern Fish.

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Cool....

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I was actually surprised to see today's APOD image of Comet PanSTARRS, because lately there have been many fine images of this comet passing by the Helix Nebula posted in the Recent Submissions forum. I think Sebastian Voltmer's picture at left is particularly lovely.

But I certainly don't complain about seeing today's Helix-less APOD! I always love the chance to compare the colors of stars with the colors of comets. The bright-looking fourth magnitude blue star in today's APOD, Iota Piscis Austrini, is intrinsically a bit brighter than Vega (59 times solar versus 48 for Vega) and a little bit bluer. In today's APOD, Iota PsA is just as blue as the coma of the comet is green. Actually the HEX designation I used to make that greenish color corresponds to OIII emission at 500.7 nm. But in the picture of Comet PanSTARRS and the Helix Nebula at left, the inner, OIII-dominated part of the Helix Nebula is clearly bluer than the green coma of the comet. All the other PanSTARRS/Helix images in the Recent Submissions forum also show the inner part of the nebula to be bluer than the coma of the comet. The color of the coma must be due to the emission of specific green wavelengths by the comet, most of them longer than the 500.7 nm wavelength of the OIII emission of so many planetary nebulas, among them the Helix.

Interesting! And thanks to everyone who has contributed photogenic images of Comet PanSTARRS here recently.

Ann