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Cosmic Challenge: A Case of Mistaken Identity
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A Case of Mistaken Identity
There once was a mystery in
Lynx. The story opened in 1790 when William Herschel discovered a small, nebulous
glow about 2½° northwest of 27 Lyncis. He later added it as number
That second object was discovered 66 years later by William Parsons, 3rd Earl of Rosse, through his 72-inch "Leviathan" reflector. Both were later incorporated into John Dreyer's New General Catalog. NGC 2474 is described as "faint, pretty small, extended?, brighter middle, very small star?, large star north following." NGC 2475 is simply noted as "makes a double nebula with" NGC 2474.
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
The mystery of NGC 2474 and 2475 was hatched in 1939, when Rebecca Jones and Richard Emberson, astronomers at Harvard Observatory discovered a planetary nebula on patrol photographs that seemed to lie close to the original NGC 2474/2475 coordinates. Their nebula was a large ring-type cloud with two distinctly brighter east-west lobes.
Jones and Emberson announced their discovery in the August 1939 issue of Harvard College Observatory Bulletin ("A Large New Planetary Nebula;" Harvard College Observatory Bulletin No. 911, pp.11-13, August 1939):
On a recent photographic plate, a faint nebular ring has been detected joining two condensations, NGC 2474, observed by Sir John Herschel, and NGC 2475. The latter was discovered by Lord Rosse who described it as forming a double nebula with Herschel's object.
Oops. Not only did they incorrectly credit John Herschel as the discoverer of NGC 2474, they also misidentified NGC 2474 and NGC 2475 as a single planetary nebula.
Their error did not become apparent for more than 40 years, however. That was more than enough time for NGC 2474/75 to be misclassified as a planetary nebula in the original Perek-Kohoutek (PK) catalog of planetary nebulae and several other reliable references.
After more than four decades of confusion, thanks in large part to the research of Nancy and Ronald Buta of McDonald Observatory, University of Texas, we now know that NGC 2474 and NGC 2475 are a close pair of elliptical galaxies discovered by Herschel and Parsons, respectively. Look for them just 2.4' southwest of the 9th-magnitude golden star SAO 26594 (the "large star'' in the NGC description).
While their identity is now certain, there is still some discrepancy between the listed magnitudes of these two objects and their visual appearances. Although both are listed at photographic magnitude 14, NGC 2474 is just bright enough to be seen in 8-inch instruments. NGC 2475, however, requires at least a 12-incher. Look for them just 2.4' southwest of.
So if those are the galaxies, what did Jones and Emberson discover? The answer to the "Mystery of the Missing Lynx" lies half a degree further north. There, we find a planetary nebula, a real planetary nebula that today is identified correctly as Jones-Emberson 1 and cross-listed in the revised Perek-Kohoutek listing as PK 164+31.1.
It's easy for us to criticize Jones and Emberson for their goof, especially since they realized that the planetary did not match the original location of NGC 2474. But one look at the planetary through a giant backyard scope and you will see, well, a double nebula just like the NGC said you would. As luck would have it, PK 164+31.1 is an unusual "ring nebula" with two brighter lobes connected by opposing arcs of faint nebulosity. The visual resemblance to a pair of faint galaxies is undeniable.
Although it is visible in smaller scopes, PK 164+3.1 needs as much aperture as you can throw at it to reveal the full ring. You will find it 21' due west of a distinctive trapezoid of a trapezoid of 9th- to 11th-magnitude stars. Use between 100x and 150x to see the full span, but avoid going too much higher, since more magnification actually works against the planetary. As to filters, a narrowband nebula filter will help improve the odds of spotting the full 360° ring, but an O-III filter will actually smother it. With averted vision, I have seen the full ring through my 18-inch at 121x. The two lobes were seen directly, the southern knot being the brighter of the two. I only caught fleeting glimpses the full ring, however, and again only with averted vision.
Above: PK 164+31.1 as sketched through the author's 18-inch reflector.
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's Cosmic Challenge is copyright 2017 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.
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