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Cosmic Challenge: NGC 2363 and NGC 2366

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Cosmic Challenge:

NGC 2363 and NGC 2366


March 2019


Phil Harrington


This month's suggested aperture range

15-inch (38cm) and larger telescopes









NGC 2363


07 28.5

+69 11.6




NGC 2366


07 28.9

+69 12.7




Will the real NGC 2363 please stand up?  For years, there has been an ongoing debate over the true identity of the 2,363rd entry in the New General Catalog.  Many references cite it as a huge area of ionized hydrogen (an H-II region) within NGC 2366, a dim irregular galaxy.


Above: Winter star map. Credit: Map adapted from Star Watch by Phil Harrington



Above: Finder chart for this month's Cosmic Challenge.

Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window

That was the explanation in the press release that accompanied a magnificent shot of the galaxy taken with the Hubble Space Telescope in 1996.  In part, the release said, "Clusters of stars and a fishhook-shaped cloud of luminescent gases glow brilliantly in NGC 2363, a giant star-forming region in the galaxy NGC 2366."  The press release went on to describe how the Hubble image revealed that the brightest individual star in the region is a rare example of an erupting Luminous Blue Variable.  This star is thought to be between 30 and 60 times more massive than our Sun and is currently enduring a very unstable, eruptive phase of its life.  The same Hubble image also shows two dense clusters of massive stars.  The older cluster is about a tenth of the age as our solar system, while the other is probably less than half as old, judging by how much remnant gas and dust remains.

Recently, however, some historians have suggested that William Herschel, who is credited with discovering both NGC 2363 and NGC 2366, was describing the H-II region and the galaxy collectively when he recorded a circular patch of light with a dim protruding extension. According to Dr. Harold G. Corwin, Jr., the catalog number NGC 2366 refers to both the brighter H-II region as well as its faint home galaxy.  He writes on his web site, in part:

"Well, folks, it's bad news for those of us who have always identified NGC 2363 as the giant HII region in the low surface brightness irregular galaxy NGC 2366.  WH's original description clearly refers to the HII region as the principal object with the bit of fuzz to the north as an incidental appendage. This view was further solidified by Ralph Copeland, observing with Lord Rosse's 72-inch reflector.  Copeland identified the HII region as the center of a greatly extended object, stretching 9 or 10 arcmin to the northeast."

If that's the case, then what is NGC 2363?  Corwin's research points to an even fainter galaxy just to the southwest, which is identified as UGC 3847 in the Uppsala General Catalog of Galaxies.  He contends that this second galaxy is actually NGC 2363.  UGC 3847, also an irregular galaxy, shines at 13th magnitude.

Fortunately, these targets are not terribly difficult to pinpoint, as they lie 4° due north of the bright galaxy NGC 2403.  To get there, begin at Omicron (
ο) Ursae Majoris, the 3rd-magnitude star marking the tip of the Great Bear's nose.  Head 4° north to a triangular asterism creating by Pi-1 (π-1), Pi-2 (π-2), and 2 Ursae Majoris, and then due west 5° to 51 Camelopardalis.  NGC 2403 is just a degree west of the star and is always worth a stopover.  Then, it's off to the north for 4° to a 6th-magnitude field sun and our targets, which lay just a bit farther north still.

Whenever I've turned my 18-inch (46cm) reflector toward this area in the past, I could see the extragalactic H-II cloud directly using a 12-mm eyepiece (171x).  I estimated it to be about 2 arc-minutes across, perhaps 12th magnitude, and with a fairly bright stellar core.  Spotting the faint disk of its home galaxy, however, proved more difficult.  I had to use averted vision to catch even a passing glimpse of its extended disk, which measures about 4'x2'.  Together, they reminded me of a faint comet, with the H-II region serving as the coma and the disk of the irregular galaxy forming a dim tail extending toward the north.

Above: NGC 2366 (and NGC 2363?) through the author's 18-inch (46cm) reflector at 171x

That same night, try as I might, I saw no sign of the second, smaller galaxy to the south of the hydrogen-II region.  If the former is indeed NGC 2363, then it's a challenge that may only be met with the largest backyard telescopes.

Post your own observations and sketches in this article's discussion forum.

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test.  Contact me through my
web site or post to this month's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2019 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


  • okiestarman56, John O'Hara, Sasa and 1 other like this


Observed the pair January 4, 2019; SQM-L 21.38, clear and still.


Large H-II region in SW NGC2366 dominated the view. Triangulation using that feature and faint optical dbl *'s NW of N2366 core located N2363 S of and ~in-line with the dbl *'s. N2363 is due W of the bright HII knot in N2366 and is faint and diffuse though not small. N2366 was large, not so faint, V elongated, not concentrated and dominated by the bright H-II region (Mrk 71).


This observation was a lot of fun.

    • PhilH and Knasal like this

Nice writeup. I observed NGC2366 in 110mm refractor. NGC2363 was out of reach, however NGC2366 was quite interesting. In the beginning, I saw only the bright H-II region, which looked from time to time like a misty double star. The faint body of NGC2366 was quite a challenge. The elongated misty thin line started to appear with more time spent at the eyepiece and with quite concentrated averted vision,



    • PhilH, Astrojensen and jonalmada like this

Interesting, and prompted me to check out the Hubble ST image of NGC2366:



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