Corona Australis & the Southern Limits of Sagittarius
August 2010 :
Many astronomers regard Sagittarius not only as the iconic, symbolic king of summer constellations, but also as one of the most important star groups in the entire sky. The rich star clouds of the central Milky Way lie just above the “spout” of the easily identified “Teapot” asterism, near the point denoting the actual nucleus of the Milky Way Galaxy. Some of the best known nebulae-both bright and dark-and numerous star clusters-globular and open (galactic) types-decorate the night sky throughout this region. Open star clusters associated with certain nebulae, as in the Lagoon Nebula (M8) in northern Sagittarius and the Eagle Nebula (M16) in Serpens Cauda, are great examples of what I like to call “clusterosity”-you can see two for the price of one, so to speak. (I hope I’ve coined that word, unless others have used it elsewhere.) I’ve written about this subject in past issues, but it occurs to me now that it might be of interest to mention just the few bright stars that lie way down near the southern border of Sagittarius, only a few degrees above our local horizon limit of -48° in declination. The small constellation of Corona Australis, known as the Southern Crown, lies just west of these stars and is a worthwhile section of sky for those who might wish to become better acquainted with the more southerly parts of the summer Milky Way as presented to our view in August.
I must admit that due to the low southern declination of stars and deep-sky objects described here, your ability to see any of them at all will be predicated on some very important limiting factors. Foremost of these is a condition that’s hard to attain for most amateurs, most of the time. You must be able to view absolutely as low to the true southern horizon as possible, minus any obstructions that might conceal sky objects just above the horizon itself. Trees, buildings, hills, etc. cannot block your southern view. Many Skyscrapers are familiar with East Beach in Charlestown on the coast, near Frosty Drew Observatory in Ninigret Park. This site would be close to perfect for such viewing. Secondly, weather and sky conditions must be optimal. There must be no clouds, haze, or fog to the south; a sightline to the horizon should pass through clear air, all the way down. Obviously, intense skyglow from area light pollution must be absent, as well as a bright Moon. August 9th will be a New Moon, which will be a big help.
Finally, it’s crucial to know the transit times for extreme southern objects. They’re hard enough to see when not at the meridian, so you need to take full advantage of viewing them when they are at their highest altitude above the horizon. This is the time at which an object culminates, meaning it transits (crosses) the celestial meridian. An object furthest to the west would be the first in a group to attempt viewing at or close to its time of transit, followed sequentially in time by other objects in increasing order of right ascension as each, in turn, approaches its transit point. The eastern-most object in a given group would logically be the last you’d try to see. Jim Hendrickson has been kind enough to furnish a handy table listing local transit times for a few objects in August, with times given in Daylight Saving Time (DST). Should you be observing on a date not listed in the table, just add or subtract 4 minutes’ correction per day. A transit time for August 10th, for example, could be easily converted for 2 days prior by adding 8 minutes, meaning a 9:30 transit on 8/10 would occur at 9:38 on 8/8. How about August 12th? In that case you’d subtract the 8 minutes, arriving at a time of 9:22. If you want to figure a transit time for an object not listed in the table, note its right ascension as compared to that of a listed object and interpolate as follows: A listed object has a right ascension of 18h 59m 33s. Another object having no transit time listed lies east by 33 minutes at 19h 32m 28s. (Rounding-off a few seconds won’t matter.) Your object of interest, therefore, will transit 33 minutes later than the more westerly object. The reverse is true for anything west of a listed object; such an object transits sooner than a listed time for something having a greater right ascension value. Just keep in mind that right ascension as a celestial coordinate measurement increases from west-to-east, and this equates to actual time, unlike declination measurement, which is based on angular position expressed in degrees, minutes, and seconds of angle.
Near the southwest corner of Corona Australis, at a hopelessly low declination of -43° 42’ 20”, is a fine globular cluster I’m mentioning here for record only. NGC 6541 is something to try observing if you’re on a trip somewhere at least a few degrees of latitude south of here. Still, if everything’s perfect and you’ve got a quality instrument with which to observe, the plucky among you might just manage to “tease” this cluster out of the background sky. It is magnitude 6.5 approximately and is 13’ in total diameter, but has a bright and compact core with a high surface brightness that seasoned observers find to be rather striking. NGC 6541 lies just 20’ to the SSE of a magnitude 4.9 white star at RA 18h 06m 48s, Dec -43° 25’. This star, h 5014, is a fine double of equal magnitudes but is much too close in separation to be resolved at such a low altitude in our location locally; the components are roughly 1” apart and are suitable for more southerly latitudes.
Two key problems that always accompany observing any sky object at such a low altitude-even when you’re lucky enough to have otherwise perfect sky and weather conditions-are the total amount of air you have to look through (“airmass”) and the small arc of visibility above the horizon that low altitude objects are confined to, which makes knowing a transit time so important. Assume a given object attains an altitude at transit of 4.5° above the true (actual) southern horizon. This value is the radius of the total arc in the sky through which the object is above the horizon. From southwest to southeast, the total diameter of the ½-circle would only be 9.0° from rising to setting points. If you know your site’s latitude you can easily figure an object’s maximum altitude at its transit. Charlestown is approximately 41.4° north, meaning the maximum southern declination of a sky object seen exactly on the true horizon cannot be greater than Dec -48.6°. (Atmospheric refraction can “lift” objects a small amount so as to reveal a false image of something that technically is just below the horizon, but you shouldn’t take this effect into consideration for this purpose.) If an object has a declination of say, -44.8°, just subtract this value from -48.6° to arrive at a transit altitude of 3.8°, much less than the 5.5° of arc separating the 2 “pointer” stars in the bowl of the Big Dipper!
Eminent astronomer James Kaler has written that if we assume a sightline straight overhead to the zenith as passing through a unit value of “airmass” of one, the airmass increases exponentially as we look lower in altitude to a maximum value of 38 if looking precisely horizontal to 90° away from the zenith. This means the total mount of atmosphere-based on a clear sky-we must see through when viewing stars right at a perfect horizon from a sea level site is 38 times that of a sightline overhead to the zenith! The potential inconsistencies in all that air-varying degrees of wind, turbulence, and humidity, plus dust or other particulates-adds to the problem when considering the total length of a sightline through the atmosphere. Even in a fine, clear sky, the dimming of apparent magnitude by atmospheric extinction is dramatic the further from the zenith you look. At an altitude of 32° above the horizon, a star of a given unit magnitude is dimmed by 0.2 magnitude, or 1.2x; at 19° it’s 0.5 magnitude, or 1.6x; at 10° it’s dimmed by 1.0 magnitude, or 2.5x, and lower than that it gets much worse. Stars of precisely the same magnitude value as assumed for my above examples are dimmed by 2.0 whole magnitudes if seen at an altitude of only 4°, or 6.3x dimmer. How about an altitude of just 1° over the true horizon? The answer is 3.0 magnitudes, which equates to a factor of 15.85 times dimmer than if seen overhead at the zenith!
Now we move on to stars seen at higher altitudes in one part of Sagittarius and elsewhere in Corona Australis. To save a little space here, I’ve omitted giving coordinates of RA and Dec for any of the 7 objects listed in Jim’s transit table-it’d be redundant. Lying just over the northern border of Corona Australis is the worthwhile double star Eta Sgr, an orange-reddish star of class M3 with a combined magnitude of 3.1 having contrasting components of magnitudes 3.2 and 7.8 separated by 3.6”, requiring a scope at fairly high power to split. A binocular shows a good color-contrast between Eta and the brightest star in Sagittarius, magnitude 1.8 Epsilon, also known as Kaus Australis. Epsilon is a bluish-white star of class B9 and lies only about 2.5° to the northeast at the southwest corner of the “Teapot”; the contrast is attractive.
Progressing eastward in right ascension, we can note the easy double star Kappa CrA having white components of class B8 and magnitudes of 5.9 and 6.6 separated by 21”, suitable for moderate power in a scope. East of this point you’ll see the semi-bright stars at the northern curve of what I choose to call the “Fish Hook” asterism of the Southern Crown; this “hook” (to me) is the most easily recognizable pattern of stars within Corona Australis and lies mainly at the northeast corner of the constellation. A total of perhaps 8 stars comprise the hook, centered roughly 30’ or so east of Kappa. Corona Australis ranks only 80th in size among the 88 constellations but actually is second in overall brightness due to the number of stars of a certain magnitude level (and brighter) that are within its small area of sky. Still, only 21 stars in total are brighter than magnitude 5.5; just 3 being brighter than 4.4-Alpha CrA is the brightest at magnitude 4.1 and class A0, RA 19h 09m 28s, Dec -37° 54’. The top of the hook-shape I’m describing-featuring Epsilon CrA (RA 18h 58m 43s, Dec -37° 6’; magnitude 4.9 variable, class F3) at the western point of this hook and brighter Gamma to its east by about 1.5°-is key to locating 2 important double stars (Gamma being one) and some deep-sky objects right in this vicinity.
Just a little NNE of Epsilon CrA and almost at the northern border of CrA (but technically in Sgr) is a fairly easy globular cluster, NGC 6723, magnitude 7.1 and size of 12’ total diameter. This is easy to find and not hard at all to make out with a good instrument at low to moderate power; a good binocular might show this globular, if of about 12 power or more and around 60mm in aperture. Due east of Epsilon CrA by about 0.5°at RA 19h 01m 05s, Dec -37° 04’, is a great double star: BrsO) 14 (Brisbane Observatory list) having B8 class components of magnitudes 6.6 and 6.8 at 13” separation. A faint and probably nearly impossible to see reflection nebula, IC 4812, is associated with this star. Go northeast of Brs) 14 by less than 1/4° to a magnitude 7.2 star and you’ve found NGC 6726, a hazy reflection/emission nebula illuminated by this star. Another nebula immediately northeast, NGC 6727, is of the same type and is a separate “lobe” from the associated 6726. NGC 6727 is illuminated chiefly by TY CrA, an irregular variable star with an amplitude from magnitude 8.7 to 12.4. These 2 associated nebulae supposedly have a high surface brightness and might be doable on a good night through quality optics-I haven’t seen them myself. Nebula NGC 6729-a Caldwell object, C68 in Patrick Moore’s catalog-is just southeast of NGC 6726-7, but is far more difficult to see at our latitude and probably is impossible, particularly because it fluctuates in size and brightness. Globular cluster NGC 6541, by the way, is also a Caldwell object-it’s number C78. Gamma CrA is a tight double star of combined magnitude 4.2 involving class F8 components of magnitudes 4.8 and 5.1 at an estimated separation of only 1.0 to 1.4”. If anyone can split this star you deserve a prize!
Sagittarius is a perfect example of a classical constellation that in no way really resembles the mythical centaur-archer for which it is named. We see the Teapot, basically, especially at our latitude. Should you observe at East Beach, though, you can see the southern limit of Sagittarius and the centaur’s hoof (or foot) denoted by 4 stars: Beta #1 and Beta #2 Sgr, a wide optical pair separated by roughly 0.4°, plus Alpha Sgr which is 4° due north, and Iota Sgr at 6.5° ENE, marking the eastern tip of an elongated triangle. (Alpha and the 2 Beta stars form the narrower side of this triangle at its western edge; Alpha Sgr (Rukbat) is noted by looking east a short distance from the middle of the “Fish Hook” shank in Corona Australis.) Beta #1 Sgr (Arkab) is the slightly brighter and more northerly of the wide Beta pair. Through clear air on a good night it isn’t hard to resolve Beta #1 as a generously-separated true double of magnitudes 4.0 and 7.1, both white and at 28” of separation. Beta #2, magnitude 4.3, is SSE of Beta #1 and of class F2. Alpha Sgr, class B8, is magnitude 3.96 and is either the 15th or 16th brightest star in Sagittarius-the most extreme example I know of the Alpha star in a constellation being superseded in brightness by such a host of other stars. The Greek alphabetical naming sequence in Sgr is remarkably “out of whack” for at least a few reasons not entirely clear to me, although one reason regarding J. Bayer’s observations at different latitudes does make sense. Plus, he didn’t always assign Greek letters based strictly on a brightness scale. Sometimes he took relative positions of stars with respect to one another within a constellation into account for his lettering scheme.
By the way, Iota Sgr (remember the east tip of the triangle I described?) is magnitude 4.1 class K0. I almost forgot to mention a nice dark nebula, Be 157, which lies roughly between and below Gamma CrA and BrsO 14.
I hope you enjoy the lesser-known sky region described here, and good luck in your observing attempts.
