Observing Planetary Nebula Nuclei

the Hottest Stars in the Summer Sky

    With the dog days of summer approaching, it is quite difficult to dispute the fiery heat of the Sun and its effect upon our daily lives. On the broad and universal scale of astronomy, however, the 5800 K surface temperature of our star is anything but significant. Although many of the brightest stars in the sky are luminous young giants having temperatures approaching 30,000 K, the very hottest category is reserved for average-mass stars in the process of dying. For these are the blazing cores of planetary nebulae, whose surface temperatures can sometimes exceed 200,000 K!

    After hearing this statement, people often begin to question the where-abouts of these “extreme” stars, and one may wonder why they are not seen glimmering throughout the night sky with other naked eye stars? The answer is actually twofold, although quite simple – first of all, they are very small and, secondly, they are actually too hot to be easily seen! Foremost, many of these planetary nebula nuclei are merely the same size as Earth even though they may contain over half of the mass of our Sun. This fact helps sustain modern stellar evolution theories that rely upon such stars eventually evolving into very compact objects known as white dwarves. A scientific principal known as Wein’s Law explains the second part of the answer. As a result of this rule, hotter stars should ideally radiate more strongly towards the blue end of the spectrum, while cooler stars should appear more red. Generally speaking, this rule is quite handy in describing to newbies the reasoning behind Betelgeuse appearing obviously ruddy in comparison to its bluish neighbor Rigel. It just happens that the fossil suns found within planetary nebulae are often so hot that much of their copious radiation is released in the “ultraviolet” part of the spectrum. Therefore, they offer very little detectable light to the visual observer.

    Furthermore, such extreme temperatures and peculiar energy distributions make these stars difficult for professionals as well as amateurs. Because these tiny stellar tombs are embedded within a fluorescing shroud of gas and dust, it is often impossible to completely distinguish the spectrum of the star from that of the surrounding nebula. Therefore, accurately determining even the simplest of physical parameters can prove to be quite challenging for researchers. For this reason, all physical quantities quoted hereafter must be regarded as researched estimates that will likely change as our understanding of such stars evolves.

    Let’s begin our nocturnal journey with NGC 6026 located in the southern constellation of Lupus. Although not very exciting as for as planetary nebulae go, this diffuse oval is home to a nice 13.2 magnitude central star. At 357x, my 10-inch scope had no problem pulling the tiny star out of the nebula’s 30 arcsecond diameter glow. I actually managed to glimpse the faint star at much lower powers when I was merely attempting to identify the nebula. I personally suspect that this star is somewhat brighter than the published estimates.

    Slewing across the zenith into the northern sky brings us to NGC 6058 in Hercules. Glowing at about the same magnitude as NGC 6026, this roundish nebula resides within a small triangle of 10th and 11th magnitude stars. The stellar nucleus is quite conspicuous and easily seen centered within the nebula at moderate powers. Despite its published magnitude of 13.9, I estimated the star’s visual magnitude to be closer to 12.5. Such discrepancies can obviously be associated with the star’s estimated surface temperature of ~90,000 K! Near the Hercules/Serpens border, we find the high surface brightness planetary IC 4593. At 71x, this nebula appeared as a nearly star-like object having a vivid blue color and containing a definite central brightening. Since smaller nebulae sometimes display central concentrations of nebulosity, higher power is often needed to distinguish the tiny star from the nebular disc. At 357x, the central star was easily recognizable within the irregular “box-like” disc of the planetary. This is a very pleasing nebula at such high powers, because it suprisingly retains its pretty bluish-green hue.

    Another bright planetary in Hercules is the 9th magnitude NGC 6210. Despite its reported magnitude of 12.6, the blazing core of this peculiar nebula is anything but easy to identify. Although some observers have reported seeing the central star in relatively small scopes, it is quite likely that they are actually spotting a prominent “knot” of nebulosity that resides within the nebula just NE of the hot core. Indeed, the tiny nebulous patch definitely resembles a slightly offset central star at low to moderate powers, and it is most certainly easier to spot than the elusive star itself. However, it was only after careful inspection of the area at over 600x under steady skies that I was able to resolve the star responsible for the spectacular nebula. During moments of good seeing, the tiny furnace glimmered in and out of view through the 10-inch scope.

    Another challenging central star can be found within NGC 6309, an outstanding planetary nebula located in the constellation Ophiuchus. Aptly named the Box Nebula, this 10th magnitude planetary is easily spotted residing very closely to the south of a 12th magnitude star. At low to moderate powers, the nebula seems rectangular in shape and is oriented nearly north-south. At 357x, however, two distinct lobes could be seen along the nebula’s major axis as well as the reportedly 13.7 magnitude central star. Just as many other fainter stars in the background, the tiny stellaring occasionally disappeared from view when the seeing became unsteady at this high power. Larger scopes can often provide a fairly steady view of this nebula’s estimated 60,000 K core even in less than optimum conditions.

    If your particular skies require you to go for something slightly easier, then simply point your scope toward the northern tip of the Sagittarius “teapot”. Within the nearby crowded starfields of the southern Milky Way, one can find NGC 6629. Though quite small at a diameter of merely 15 arcseconds, this 11th magnitude planetary sports a fairly easy 12.9 magnitude stellar core. Although, I was able to occasionally glimpse the quarry at 125x using averted vision, powers of 357x and more rendered the tiny star quite nicely even when viewed directly. This object is certain testimony to the usage of higher powers when observing the deep-sky. At lower powers, the nebula itself appears as nothing more than a bloated star!

    Venturing back northward brings us to Aquila, a constellation that is virtually a stellar graveyard and home of many planetary nebulae such as NGC 6751. Low power reveals this fuzzy oval lying within a field very rich with faint stars. At over 300x, the nebula displayed an obvious annular structure centered upon a faint, twinkling star. Further scrutiny revealed very complex structure within the nebula, as it seemed to contain several dark features immediately surrounding the central star. The mottled appearance was actually reminiscent of Messier 97, the Owl nebula, as it appears through smaller scopes. Located just across the Aquila/Delphinus border is NGC 6891, a small but bright planetary nebula containing a fairly conspicuous central star. At 356x, the 12.6 magnitude core could be seen lying within a bright round disc that appeared to be enveloped by a fuzzy glow extending to about 20 arcseconds in diameter. As with IC 4593, however, don’t let the nebula’s brighter center fool you at lower power without confirming the sighting of the inner star with at least 200x.

    Also located in the often-overlooked constellation of Delphinus is the remarkable NGC 6905, also known as the Blue Flash. Located directly between a pair of 11th and 12th magnitude stars, this intriguing planetary vaguely resembles a face-on galaxy when viewed at higher powers! Two obvious lobes appear oriented much like a pair of spiral arms situated around a faint stellar nucleus. Like many others previously mentioned, the central star tends to sparkle in and out of view with the steadiness of the sky. Just to the north lies Vulpecula, home of Messier 27, the famous Dumbell Nebula. Truly a spectacle through nearly any form of optical aid, the Dumbell with its mottled disc is certainly one of the most prominent of all planetary nebulae in the entire sky. Its 13.8 magnitude central furnace, however, can present quite a challenge for small scopes. At 227x, I found the tiny star to be quite easy with a 10-inch aperture. Considering recent estimates of nearly 150,000 K for its surface temperature, this star is likely only visible due to the Dumbell’s relatively close proximity.

    Cygnus is home to many planetaries, but two of them certainly stand-alone. NGC 6826, the Blinking Planetary, is one of my personal favorites. Not only because of its wonderful multiple-shell structure, but this nebula’s central star is literally a blazing 10.7 magnitude beacon! At higher powers, the nebula displays an amount of detail that can only be described as the arch-typical planetary nebula. I’ve even spotted this nebula’s central star in a 4-inch refractor under favorable observing conditions. A truly awesome sight! Another one of the Swan’s grandest quarry is NGC 7008, a relatively large planetary that is richly detailed in backyard scopes. Situated just to the northwest of a bright pair of close stars, the nebula’s 1.5 armintue elongated disc appears very mottled and ill-defined at moderate powers. Higher powers reveal at least 3 stars apparently involved with the nebulosity that form a triangle. The southernmost stellaring appears to be at the nebula’s estimated center and is supposedly the responsible party. This is certainly an often-overlooked showpiece planetary for larger scopes.

    Last but certainly not least is NGC 6543, the mysterious Cat’s Eye, in Draco. Though its central star is considered fairly easy with larger scopes, this 11.3 magnitude target requires pretty steady skies to be viewed in scopes with less than 10-inches of aperture. I found the most comfortable view of the star at 625x, where I could actually hold the star with direct vision at nearly all times. Because of the high surface brightness of the surrounding nebulosity, lower powers could only reveal the star for fleeting glimpses. The higher power, however, spread the nebulosity across a larger portion of my viewing field, as the elusive star remained relatively concentrated in the center. Technique is certainly everything when looking for such challenging targets, no matter what size instrument is used. With that in mind, this adventure has hopefully inspired everyone to view planetary nebulae as something more than just faint fuzzy glows amongst the myriad of other deep-sky objects. After all, the fate of our very own sun is likely to be one of these faint central stars – a tiny cinder of the fiery globe that once heated our planet.