July 2024 - Picture of the Month
IC 4628 (Prawn Nebula) in Scorpius
I had hoped to bring you this rather splendid image of IC 4628 by Josep Drudis, but I wasn't able to secure permission in time. So you'll have to make do with the smaller field in the version above by ESO, with some amateur data from Martin Pugh, that's still rather impressive. There's another spectacular close-up of the region in the top left of this field taken with the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile.
IC 4628 lies 6,000 light-years away from us in the Sagittarius Arm of the Milky Way, and it's about 250 light-years across, with an apparent size of roughly 90 arc-minutes. Despite varying reports of its apparent magnitude of 7.3–10.0, the fact that it covers an area of the sky about four times the size of the Moon means that this nebula is not particularly bright, and as such few people image it, which seems a pity.
The majority of the colour in this image is the result of light emitted by the glowing gas and dust surrounding the hot young stars in this HII region. But there's some blue there too from reflected light. Of course, against such a background there's a fair amount of dark nebulosity on show.
The majority of the stars in this image are part of open cluster Trumpler 24 (Tr 24), which so happens to be a subset of Collinder 316 (Cr 316). The Night Sky Observer's Guide (NSOG) Volume 4 reports that …the two groups appear as one huge object…
in binoculars, and that the star field is rich. Archinal and Hynes in Star Clusters provide Tr 24 with a IV2p n classification, noting that Cr 316 is superimposed on top and might even be just the brighter members of Tr 24. How many members either of these open clusters might have, they're not saying.
NSOG suggests that with a modest scope, but good skies, an OIII filter starts to bring out IC 4628 itself, arcing across the NE perimeter of Tr 24 and earning that 'n' in its classification.
If you are foolish enough to try superimposing the SIMBAD data on this image it will disappear beneath a sea of Young Stellar Object (YSO) candidates, perhaps unsurprisingly.
James Whinfrey - Website Administrator.
