When he accidentally found Uranus among the stars of Gemini in March 1781, William Herschel opened up our solar system.  Until that instant, it was thought that our Sun's planetary family contained only six members, ending at Saturn.  But with Uranus suddenly joining the group, the prospect for even more planets -- perhaps many more -- took the astronomical world by storm.  Herschel, subsequently enjoying the fruits of his unexpected discovery as the new Astronomer Royal at the Royal Greenwich Observatory in England, led the charge to find them.

Within a month of commencing his systematic effort of scanning the skies, Herschel found another planet, this time in Aquarius.  No doubt he judiciously marked its position on his charts so that he could return again a few nights later to see just far his new world had moved against the starry backdrop.  But when he returned, he immediately realized that it hadn't moved at all; instead, it had stayed anchored in place.  Other discoveries of similar objects that looked like tiny versions of Uranus soon followed, but all also remained stationary among the stars.  Because their small disks looked like distant planetary spheres, Herschel coined the phrase "planetary nebula."  Even though it is one of the greatest astronomical misnomers of all times, since it says nothing of the true nature of these ghostly objects, the term stuck.

Herschel went on to discover 33 planetary nebulae scattered across the sky. (Historical trivia: William Herschel originally classified 79 objects as planetary nebulae, but only 20 proved to be correct.  The others were subsequently reclassified as other types of objects.  However, 13 other objects that he had categorized as other types of objects were actually planetary nebulae.  Therefore, whether or not he knew it, Herschel discovered 33 planetary nebulae.)

One of the more interesting, and at the same time, challenging of Herschel's planetaries to view through 3- to 5-inch (7.6- to 12.7-cm) instruments is NGC 6369 in southern Ophiuchus.  Nicknamed the "Little Ghost Nebula," NGC 6369 is an example of a ring-type planetary nebula, a faint version of M57.  That is, if you can find it.

Zeroing in on NGC 6369's exact location isn't too difficult, since it lies just half a degree northwest of 51 Ophiuchi.  To get there, first locate 3rd-magnitude Theta (θ) Ophiuchi, about 12° east of Antares.  Hop a little more than a degree northeast from Theta to 44 Ophiuchi, and then another degree east-northeast to 5th-magnitude 51 Ophiuchi.

You should take that final step from 51 Oph to NGC 6369 using a low-power eyepiece, but then to switch to at least 120x to see the nebula's disk.  NGC 6369 measures about half the apparent diameter of Jupiter.  The Little Ghost looks like as a small, wraithlike disk of grayish light (see sketch below) through my own 4-inch (10.2-cm) refractor at 142x, as if floating in the ether once thought to flood the cosmos.  When the air is especially transparent, the nebula's pale turquoise hue becomes clearer, but resolving its distinctive ring shape remains difficult even under the best conditions.  The nebula's delicate color results from the ionizing radiation that is generated by the progenitor star and streams through the tenuous cloud.  In the process, electrons are ripped away from their parent hydrogen and oxygen atoms, causing the cloud to glow colorfully.

Like its more famous kinfolk, M57, the remnant of the star that spawned the nebula remains invisible in all but the largest backyard telescopes.  NGC 6369's central star, a white dwarf measuring perhaps no larger than our planet in diameter, glows weakly at 15th magnitude and is a challenge to spot through far larger instruments than those here.  Those who successfully spot the star will notice that it is not exactly centered, but rather is offset slightly.

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 website 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!