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Object Name: Messier 27
Alternative Designations: M27, NGC 6853, The Dumbbell Nebula
Object Type: Planetary Nebula
Right Ascension: 19 : 59.6 (h:m)
Declination: +22 : 43 (deg:m)
Distance: 1.25 (kly)
Visual Brightness: 7.4 (mag)
Apparent Dimension: 8.0×5.7 (arc min)
Locating Messier 27: When you first begin, Messier 27 will seem like such an elusive target – but with a few simple sky “tricks”, it won’t be long until you’ll be finding this spectacular planetary nebula under just about any sky conditions. The hardest part is simply sorting out all the stars in the area to know the right ones to aim at!
The way I found easiest to teach others was to start BIG. The cruciform patterns of Cygnus and Aquila are easy to recognize and can be seen from even urban locations. Once you’ve identified these two constellations, you’re going smaller by locating Lyra and the tiny kite-shape of Delphinus. Now you’ve circled the area and the hunt for Vulpecula the Fox begins! What’s that you say? You can’t distinguish Vulpecula’s primary stars from the rest of the field? You’re right. They don’t stand out like they should… And being tempted to simply aim halfway between Albeireo (Beta Cygni) and Alpha Delphini is too much of a span to be accurate. So what are we going to do? Here’s where some patience comes into play.
If you give yourself time, you’ll begin to notice the stars of Sagitta are ever so slightly brighter than the rest of the field stars around it and it won’t be long until you pick out that arrow pattern. In your mind, measure the distance between Delta and Gamma (the 8 and Y shape) and then just aim your binoculars or finderscope exactly that same distance due north of Gamma. You’ll find M27 every time! In average binoculars it will appear as a fuzzy, out of focus large star in a stellar field. In the finderscope, it may not appear at all… But in a telescope? Be prepared to be blown away…
What You Are Looking At: As chance would have it, we happen to be looking at this planetary nebula from the viewpoint of its equatorial plane. Hanging out around 1200 light years from our solar system, this dying star is ejecting a shell of hot gas into space for about the last 48,000 years. The star responsible is an extremely hot blueish subdwarf dwarf – which emits primarily highly energetic radiation of the non-visible part of the electro-magnetic spectrum, which is absorbed by exciting the nebula’s gas, and re-emitted by the nebula. What’s that spell? Try the forbidden zone of doubly ionized oxygen… one spectral line only, in the green light at 5007 Angstroms! That’s why Messier 27 glows like a green apple core.
For many years I quested to understand the 850 light-year distant M27, but no one could answer my questions. I researched and learned it was made up of doubly ionized oxygen. I had hoped that perhaps there was a spectral reason to what I viewed year after year – but still no answer. Like all amateurs, I became the victim of “aperture fever” and I continued to study M27 with a 12″ telescope, never realizing the answer was right there – I just hadn’t powered up enough. Several years later while studying at the Observatory, I was viewing through a friend’s identical 12″ telescope and, as chance would have it, he was using about twice the magnification that I normally used on the “Dumbbell.” Imagine my total astonishment as I realized for the very first time that the faint central star had an even fainter companion that made it seem to wink! At smaller apertures or low power, this was not revealed. Still, the eye could “see” a movement within the nebula – the central, radiating star and its companion.
“As the gas at the inner edge begins to ionize, the pressure throughout the nebula is equalized by a shock which moves outward through the neutral gas. Later, when about 1/10 of the nebular mass is ionized, a second shock is released from the ionized front, and this shock moves through the neutral shell reaching the outer edge.” says W.G. Mathews, ” The density of the HI gas just behind the shock is quite large and the outward gas velocity increases within until it reaches a maximum of 40-80 km per second just behind the shock front. The projected appearance of the nebula during this stage has a double ring structure similar to many observed planetaries.” R.E. Lupu of John Hopkins has also made studies of motion as well, this time of Ly Alpha Pumped Molecular Hydrogen Emissions and found them to “have low surface brightness signatures in the visible and near infrared.”
But, movement or no movement, Messier 27 is known as one of the top “polluters” of the interstellar medium. “The high mass loss rates of stars in their asymptotic giant branch (AGB) stage of evolution is one of the most important pathways for mass return from stars to the ISM. In the planetary nebulae (PNe) phase, the ejected material is illuminated and can be altered by the UV radiation from the central star. PNe therefore play a significant role in the ISM recycling process and in changing the environment around them.” says Joseph L. Hora of Harvard-Smithsonian Center for Astrophysics, “A key link in the recycling of material to the Interstellar Medium (ISM) is the phase of stellar evolution from Asymptotic Giant Branch (AGB) to white dwarf star. When stars are on the AGB, they begin to lose mass at a prodigious rate. The stars on the AGB are relatively cool, and their atmospheres are a fertile environment for the formation of dust and molecules. The material can include molecular hydrogen (H2), silicates, and carbon-rich dust. The star is fouling its immediate neighborhood with these noxious emissions. The star is burning clean hydrogen fuel, but unlike a “green” hydrogen vehicle that outputs nothing except water, the star produces ejecta of various types, some of which have properties similar to that of soot from a gas-burning automobile. A significant fraction of the material returned to the ISM goes through the AGB – PNe pathway, making these stars one of the major sources of pollution of the ISM. However, these stars are not done with their stellar ejecta yet. Before the slow, massive AGB wind can escape, the star begins a rapid evolution where it contracts and its surface temperature increases. The star starts ejecting a less massive but high velocity wind that crashes into the existing circumstellar material, which can create a shock and a higher density shell. As the stellar temperature increases, the UV flux increases and it ionizes the gas surrounding the central star, and can excite emission from molecules, heat the dust, and even begin to break apart the molecules and dust grains. The objects are then visible as planetary nebulae, exposing their long history of spewing material into the ISM, and further processing the ejecta. There are even reports that the central stars of some PNe may be engaging in nucleosynthesis for purposes of self-enrichment, which can be traced by monitoring the elemental abundances in the nebulae. Clearly, we must assess and understand the processes going on in these objects in order to understand their impact on the ISM, and their influence on future generations of stars.”
History: So, chances are on July 12, 1764, when Charles Messier discovered this new and fascinating class of objects, he didn’t really have a clue has to how important his observation would be. From his notes of that night, he reports: “I have worked on the research of the nebulae, and I have discovered one in the constellation Vulpecula, between the two forepaws, and very near the star of fifth magnitude, the fourteenth of that constellation, according to the catalog of Flamsteed: One sees it well in an ordinary refractor of three feet and a half. I have examined it with a Gregorian telescope which magnified 104 times: it appears in an oval shape; it doesn’t contain any star; its diameter is about 4 minutes of arc. I have compared that nebula with the neighboring star which I have mentioned above [14 Vul]; its right ascension has been concluded at 297d 21′ 41″, and its declination 22d 4′ 0″ north.”
Of course, Sir William Herschel’s own curiosity would get the better of him and although he would never publish his own findings on an object previously cataloged by Messier, he did keep his own private notes. Here is an excerpt from just one of his many observations: “1782, Sept. 30. My sister discovered this nebula this evening in sweeping for comets; on comparing its place with Messier’s nebulae we find it is his 27. It is very curious with a compound piece; the shape of it though oval as M. [Messier] calls it, is rather divided in two; it is situated among a number of small [faint] stars, but with this compound piece no star is visible in it. I can only make it bear 278. It vanishes with higher powers on account of its feeble light. With 278 the division between the two patches is stronger, because the intermediate faint light vanishes more.”
So where did Messier 27 get its famous moniker? Try Sir John Herschel, who wrote: “A most extraordinary object; very bright; an unresolved nebula, shaped something like an hour-glass, filled into an oval outline with a much less dense nebulosity. The central mass may be compared to a vertebra or a dumb-bell. The southern head is denser than the northern. One or two stars seen in it.”
It would be several years, and several more historical astronomers, before the true nature of Messier 27 would even be hinted at. At one level, they understood it to be a nebula – but it wasn’t until 1864 when William Huggins came along and began to decode the mystery: “It is obvious that the nebulae 37 H IV (NGC 3242), Struve 6 (NGC 6572), 73 H IV (NGC 6826), 1 H IV (NGC 7009), 57 M, 18 H. IV (NGC 7662) and 27 M. can no longer be regarded as aggregations of suns after the order to which our own sun and the fixed stars belong. We have with these objects to do no longer with a special modification only of our own type of suns, but find ourselves in the presence of objects possessing a distinct and peculiar plan of structure. In place of an incandescent solid or liquid body transmitting light of all refrangibilities through an atmosphere which intercepts by absorption a certain number of them, such as our sun appears to be, we must probably regard these objects, or at least their photo-surfaces, as enormous masses of luminous gas or vapour. For it is alone from matter in the gaseous state that light consisting of certain definite refrangibilities only, as is the case with the light of these nebulae, is known to be emitted.”
Whether or not you enjoy M27 as one of the most superb planetary nebula in the night sky – or as a science object, you will 100% agree with the words of of Burnham, “The observer who spends a few moments in quiet contemplation of this nebula will be made aware of direct contact with cosmic things; even the radiation reaching us from the celestial depths is of a type unknown on Earth…” Enjoy!
Top image thanks to Palomar Observatory, courtesy of Caltech and color images thanks to REU program/NOAO/AURA/NSF and Bill Schoening, Nigel Sharp/NOAO/AURA/NSF.