The Heart of our Milky Way Galaxy
August 2009 :
Ah, the joys of summer observing—let me itemize a few: 1) Mosquitoes. 2) The occasional tick or other problematic insect. 3) Messing with insect repellant, or not having any when you most need some. 4) Mosquitoes. 5) Thunderstorms, frequent and often nearly unpredictable. 6) Generally hazy, humid, “mucky” air—much more the rule than at other times of the year. 7)) Mosquitoes! (Even worse when it’s humid.) 8) Poor transparency of the air, which particularly affects viewing of extended, nebulous deep-sky objects. Dust and other particulates in the atmosphere—enhanced during summer—combine with haze and increased water vapor to reduce contrast and sharpness of certain classes of objects, although not all kinds. (Low altitude objects can be even more adversely affected in this regard; winter is more forgiving when trying to observe things down near the horizon. 9) Humid nights cause optics to dew up with frustrating quickness, sometimes within just minutes after you’ve set all your stuff up. What, you don’t have the right dew-prevention accessories? Break out the hair dryer! (You’ll be using one often.) 10) The shortness of observing hours—it gets dark late and light early; around the June solstice or several weeks thereafter you must pay attention to the clock if you’re an early riser on a normal schedule. 11) Poorer quality air during summer just enhances skyglow from light pollution—light gets spread out through much larger parts of the sky as opposed to fall and winter, when it’s more restricted. The more humid it is, the worse it’ll be. Sometimes it may seem as if you can’t really notice anything close to adequate darkness, even in a rural area after midnight. 12) Speaking of light pollution, how about all these outdoor recreation areas (ballfields, golf, etc.) that are usually lit up like the noonday Sun? Such places tend to be in use often and don’t close until late in the evening at this time of year; many are seasonal and either are not open the other half of the year, or close down much earlier at night outside of summer. This constitutes an additional, summer-related source of light pollution with which we must contend. 13) Mosquitoes—did I mention them already?
In case you’re wondering where I’m going with all these summer observing complaints, let me admit to a few points associated with this time of year that are actually positive and beneficial for amateur astronomers: 1) Comfortable temperatures at night. We don’t have to struggle with outrageously cold, freezing conditions, including snow and ice! (Do you ever envy people who enjoy indoor hobbies, like bowling, for example?) 2) The same atmospheric conditions that make for lousy transparency can sometimes—conversely—actually improve seeing, the ability to resolve finer detail when viewing the Moon, tight double stars, the cores of condensed star clusters, or details of the planets, particularly Jupiter and Mars. Why is this so? Well, oftentimes the presence of greater humidity (water vapor) in the air tends to make it steadier and much more amenable for more sustained viewing, with images that retain their sharpness and steadiness longer than you’d see under the drier air conditions of other seasons. The ever-changing motions of air “cells” along any given sightline through the atmosphere above us tremendously affect the quality of seeing; inconsistencies in speed and direction of the cells’ motions relative to one another throughout the entire length of a sightline are enhanced when less water vapor is present. Incoming light rays from celestial objects are refracted to-and-fro and “disturbed” more in drier air, on average, than is the case while observing through air with greater humidity. If you’ve ever been frustrated with wavering, in-and-out of focus images that don’t seem to stay sharp for more than just a few seconds at a time, and you’re confident that your scope’s optics have equalized to the ambient air temperature (a vitally important factor!), blame it on poor seeing caused by turbulence in the local atmosphere.
Heat rises quickly from buildings or paved areas that can continue radiating their heat to the sky even after nightfall on hot summer days; this can cause a separate mirage condition that will drastically affect image steadiness. Savvy amateurs will try to avoid such problems by not observing directly over houses or pavement, whenever possible.
There are many more globular star clusters to be seen in summer skies than during other seasons of the year, because their orbits around the Galactic center in Sagittarius give the appearance of concentrating them towards that general direction in the sky. Several of the more notable Messier globulars, however, are positioned on the sky well away from the Galactic nucleus. These include M2 in Aquarius, M3 in Canes Venatici, M5 in Serpens Caput (at least as good as M13, in my opinion), M13 and M92 in Hercules, M15 in Pegasus, M30 in Capricornus, and M53 in Coma Berenices. With the exception of the three globulars listed for Serpens Caput and Hercules, most objects I’ve just mentioned are best seen at other times of the year. Although the fantastic Omega Centauri technically just “breaks” a perfect southern horizon when it culminates (transits the Meridian) at our local latitude of 42° north, you can’t really say that it’s visible here—it is just too low in the sky. Observers need to be at least several degrees of latitude further south and have the requisite unobstructed horizon and clear sky conditions in order to even begin to appreciate this magnificent object, arguably the finest of the globular clusters known. 47 Tucanae—equal to Omega Centauri in the considered opinions of many amateurs—is at declination -72° 05’ and is therefore over 24 degrees below our local southern horizon limit of -48° declination.
Now (finally!) we come to the very best that summertime observing has to offer us: the areas around the actual center of our home Galaxy, the Milky Way. (It is correct, by the way, to capitalize “Galaxy” when referring to our own, even when the word is used alone. The word is not capitalized when used to describe another galaxy unless it’s part of a proper noun name, as in “Andromeda Galaxy.”) August is the prime time of year to view this wondrous manifestation of nature’s handiwork, a spectacle rivaling—to the astronomically inclined—all the richness and complexity displayed throughout the natural world here on Earth. Those fortunate enough to have access to truly dark observing sites in rural areas on the best of nights—coupled with the absence of a bright Moon—can see luminous, billowy star clouds that simulate steam issuing from the “spout” of the well-known Teapot asterism comprising the main outline of Sagittarius. The brightest of all star clouds along the Milky Way, the Great Sagittarius Star Cloud, has its center located about 2° to the NW of magnitude 3.0 Gamma Sgr, Al Nasl, the star prominently marking the “spout.” The combined light of uncountable stars populating the dense, central hub of the Galaxy produces the effect of a cloud; these suns are crowded together much more densely here than in any other parts of the sky and the region is so thick with them that the vast majority cannot be distinguished from one another as separate points of light—thus you see actual “clouds” of stars!
Intricate patches of dark nebulae (usually given “B” numbers indicating their designations in the famous catalog developed by American astronomer E. E. Barnard) are also numerous throughout constellations around this part of the sky, namely Ophiuchus, Scorpius, Sagittarius, and Serpens. These seemingly starless “holes” on the sky are areas having so much fine dust and concentrated gas that they obscure the background light of stars behind them, giving the visual impression of dark splotches lacking any features save for the shapes defined by their borders. Dark nebulae are therefore foreground objects lacking visible light of their own and comprise one of the five classes of nebulae defined by astronomers. Light is either emitted from or reflected by those nebulae making up the other four general classes: diffuse/emission (“bright”) nebulae, planetary nebulae, SN’s or supernova remnants, and reflection nebulae.
Somewhat ironically, the Galactic nucleus itself—the very spot from which you’d expect to see the most stars blazing forth—is concealed behind intervening dust that stretches for hundreds of light years outward through the Galactic equatorial (disc) plane from the center. Surprisingly, much of this cosmic dust is thought to be so finely-grained that its average density might approximate that of smoke. Our own position within the Orion Arm lies perhaps 27,000 light years out from the nucleus, so all that dust concentrated along the precise sightline towards the center makes an effective screen of the visual light to which human eyes are attenuated. Fortunately, other wavelengths of energy—comprising a far greater percentage of the electromagnetic spectrum than the very narrow band of visual light we see—do pass through the dust and gas unimpeded. It is due to this fact that we can know exactly the position of the Galaxy’s actual nucleus at a spot called Sagittarius A*, which coincides with the zero degree point of Galactic longitude on the Galactic Equator. On a star atlas you’ll see that this point lies in western Sagittarius in a corner of the constellation close to the borders with both Ophiuchus and Scorpius. Here, at approximately RA 17h 46m and Dec. -28° 55’, lies what I call the “monster in the middle”—the supermassive black hole at the very heart of our Milky Way Galaxy, having a total mass of about 4.1 million times the mass of our own Sun!
Even though there’s nothing you can actually see (in visible light) at this spot, knowledge of the power, energy, and forces at work here can easily impress and fascinate those willing to consider the significance of looking inward in the direction of the Galaxy’s true nucleus. Using binoculars or a rich-field telescope, star-hopping to Sagittarius A* is easily done by first noting Gamma Sgr, the Teapot’s “spout” star. An arc of several much fainter stars will be seen to rise above and curve NW from a point just W of Gamma, finally “hooking” down to what is by far the brightest star nearest to the Galactic center, magnitude 4.5 X Sgr (also known as 3 Sgr), at RA 17h 47m 34s, Dec. -27° 50’. About one degree due W of X is a magnitude 6.4 star; together, these two stars form the base of an inverted, “arrowhead”-shaped triangle. The third point of this triangle must be imagined, because it is not marked by a visible star—this is Sagittarius A*, about 1.1° due S of the middle of a line joining X Sgr to that magnitude 6.4 star. A sprinkling of faint foreground stars will be seen just above Sgr A*. The Galactic center is just over 4° WNW from Gamma Sgr, barely W of the border of the Great Sagittarius Star Cloud—it is lost within the dark murk of the Milky Way’s Great Rift, a continuous band of dark nebulae running basically along the plane of the Galactic Equator that I described in my article on the constellation Scutum in the August 2008 issue of this newsletter.
True enough, you will see a few foreground stars amid the Great Rift but the contrast of this region with the dramatic, intense blaze of concentrated starlight lying just E and NE (the Great Sagittarius Star Cloud) seems to make the Milky Way’s center a visually unimpressive spot. Actually, the entire part of the sky I’ve thus far described—including bright star clouds, but particularly dark nebulae—is impossible to really appreciate from urban areas awash with light pollution spread out through the hazy muck of summer air. Even the best observing sites on good nights in rural locations can’t begin to do justice to all the delicate beauty rendered supremely by good photographs or CCD images, so I’d advise trying “armchair astronomy” by enjoying pictures showing detail that your eyes alone cannot. A huge advantage, too, will be freedom from mosquitoes! Many websites and astronomy books have luscious pictures of the summer Milky Way, but club president Bob Horton has taken what may be the classic, definitive astrophoto of this sky region. No doubt many Skyscraper members have seen or purchased Bob’s splendid photo, described to me by Al Hall as being the best such picture he’d seen anywhere. (Al is known to many of you for his prowess in engineering, machining, and award-winning telescope-making.)
On a trip to California’s White Mountains back in 2002, Bob Horton took advantage of a clear, Moon-free night to capture a long exposure, wide-field image centered about two degrees or so to the NW of Gamma Sgr. He was in a very dark, remote region probably just W of the Nevada border, at a latitude I estimate to be between 37.5° and 38° north. A line of trees cuts across the lower border of Bob’s photo, limiting the southern declination of stars seen to around -39° as opposed to the -52° Dec. that would have been pictured over a hypothetical perfect southern horizon. This doesn’t detract from the impact of all that the photo reveals, though. My words alone would be inadequate to describe all of what registers on the eye in this great picture; looking at it will immediately show what words cannot begin to do justice to, so please treat yourself, if possible.
The July 2008 issue of this newsletter includes an article I wrote containing a listing of 35 various sky objects of note to be seen in the summer Milky Way region, mainly in Ophiuchus, Scorpius, Serpens, and Sagittarius. If you’ve saved that issue or can access it on the Skyscrapers website, you might find many of my choices to be worthwhile—maybe all of them. A star-hop in Scutum is in the August 2008 issue. The Milky Way in Sagittarius is richly described in Burnham’s Celestial Handbook, volume 3, pages 1619-1644, which includes a good finder chart for Sagittarius A*.
