Going Deeper in Sagittarius

Steve Crouch

The purpose of this article is to highlight some of the less well-known sights in Sagittarius. Last month's Constellation of the Month dealt with the Sagittarius objects that everyone knows, such as the Lagoon and Trifid, but what I'm going to try and do this month is to draw attention to objects that may not have previously been on your observing list. Some of these are quite challenging but most should be in reach of a 20-25 cm telescope under dark and transparent skies.

This list is by no means exhaustive and there is some bias toward faint globular clusters (my favourite type of object). I also didn't include any double stars as they were adequately covered last time. This article is structured somewhat differently than I usually do, and I'm just going to go through the whole constellation from west to east listing each object in order of increasing right ascension.

The transparency wasn't outstanding on the nights that I performed these observations and the second night had a five-day-old crescent moon. Nevertheless, I was able to snare practically everything I looked for using the trusty C14 with 22mm and 27mm Panoptic eyepieces.

Let's start with NGC 6440 (17h 48m 52.6s, -20° 21' 34"), which is a small faint globular cluster that has a strong central condensation. There was no hint of resolution. What makes this rather unremarkable globular more interesting though is the presence of the rather strangely shaped planetary nebula NGC 6445 only 22' to the north-east at (17 49 14.9, -20 00 36). This planetary appears to me as an oblong shaped ring and is certainly different from your usual fuzzy ball. It's not particularly bright but is still quite easy. One observer's description is that it looks like a tiny dumbbell nebula with a hole in the middle.

The next object of note is the compact dark nebula B86 (18 03 01.5, -27 52 00), which shows up very nicely as a sharply-defined roughly triangular patch in the C14 and is about 5' in diameter. A rather bright orange star is on the northwestern edge. B86 is a convenient jumping-off point for some quite interesting, if occasionally faint objects. Just slightly to the east, the first of these is NGC 6520 (18 03 25.0, -27 53 30), which is a very pretty open cluster that contains a prominent orange star, and where the other stars seem to be mainly arranged in loops around the centre.

A little to the west is Djorgovski 2 (18 01 49.1, -27 49 33) which is also known as ESO 456-38. This is a surprisingly bright globular that was only discovered in 1987. I saw a faint but prominent approximately round granular object. It's easy to find, being enclosed in a trapezoidal asterism of four stars that reminds the well-known Texan globular cluster hunter, Barbara Wilson, of the keystone in Hercules.

Moving to the east again brings us to NGC 6540 (18 06 08.6, -27 45 55), which is one of the faintest NGC globulars that I have seen. It actually looks more like a distant open cluster and is just a tiny wisp in the C14. This whole area is also dotted with planetary nebulae, one example being IC 4673 (18 03 18.5, -27 06 22) a little under one degree to the north of B86. This one is quite faint but I was able to see it with direct vision as a slightly elliptical grey ball. The contrast is considerably improved with an O III filter. An even fainter planetary is M2-26 (18 03 11.9, -26 58 31) roughly 8' to the NNW of IC 4673. I could just pick this one out using the O III filter. A considerably brighter planetary nebula than the previous examples is NGC 6563 (18 12 02.5, -33 52 06), which is easily visible as a prominent grey fuzzy ball of even illumination.

Next we go to two small reflection nebulae, NGC 6589 (18 16 54.0, -19 47 00) and NGC 6590 (18 17 06.0, -19 52 00). NGC 6589 seemed to be slightly more noticeable and surrounds two 10th magnitude stars. IC 1283/84 (18 17 48.0, -19 40 00) is a large faint area of emission nebulosity to the northeast of these. Using an O III filter I could just make some of this out but it certainly wasn't easy.

Now we come to two of the famous Palomar series of globular clusters, so called because they were identified by George Abel on Palomar Sky survey plates. Palomar 8 (18 41 29.9, -19 49 33) and Palomar 9 (18 55 06.2, -22 42 03) are the two brightest and Palomar 9 even has a NGC designation (NGC 6717). Palomar 8 is a reasonably large round grainy globular that is very easy and has even been glimpsed in 10 cm telescopes, which always makes me wonder how it managed to avoid a NGC number. Palomar 9 is even easier and is quite easy to find as a small fuzzy object 2.5' south of one component of the double star Sgr. Some of its component stars are easily visible.

Jumping a fair bit north, we come to NGC 6818 (19 43 57.8, -14 09 10) which is another attractive planetary nebula sometimes known as the "little gem". The pale blue colour is certainly very pretty and I could see a definite ring shape with the O III filter.

Galaxies are certainly not a type of object that you expect to find in Sagittarius, given the proximity of the Milky Way, but there are a few to hunt down. The best known of these is NGC 6822 (19 44 58.3, -14 48 03) or Barnard's galaxy. This is not a particularly easy object, as it is quite large and therefore of low surface brightness. It was only dimly visible on the night that I observed it. Two even fainter galaxies are NGC 6835 (19 54 32.8, -12 34 08) and NGC 6836 (19 54 40.6, - 12 41 17). Both were elliptical in shape with NGC 6835 being somewhat easier and rather more elongated.

The most difficult object is the last. This is the globular cluster Terzan 7 (19 17 43.7, -34 39 27). The Terzan series of globular clusters are an even fainter group than the Palomar clusters and were discovered by infrared means. They were once dismissed as being totally impossible visually, but the previously noted Barbara Wilson has seen them all using 20-36 inch telescopes. Terzan 7 is one of the easiest but I still had to stare at the field and make extensive use of averted vision before I could convince myself that it was there. All that was apparent was a very dim spot and I would think that 30 cm would be the minimum aperture needed.

I hope that everyone enjoys this selection of lesser-known Sagittarius deep-sky objects.

The Double Star I 1467

Ross Gould

Over the last few years, I've been contributing material for a book on double stars, to be published (one day) by Sky & Telescope. Readers of S&T may have noticed a letter by Sissy Haas asking for southern hemisphere double star observations, to be included in an observer's guide she was putting together. I replied to the letter, and have since contributed notes on hundreds of pairs, as well as other material. The following discussion was sent to help decide whether this particular pair (I 1467) might be retained for the book or left out. Some doubles, especially when there are long intervals between measures, are problematic.

The double in question is one of RTA Innes' many discoveries. It was found and first measured in 1926. The few measures show an odd pattern, which is the problem.

First, the data, from the Washington Double Star Catalog (WDS)-

I 1467   23133-4937  1991  348  0.3   7.01  9.94   G8/K0III

The 1991 measure is from Hipparcos. The pair is located in Grus.

The magnitudes listed here make B much fainter than the older magnitude measures, given as mags 6.8 and 8.4 in the 1984 edition of WDS. This is unexpected, as Hipparcos usually finds close companions to be brighter than visual observers thought.

My only observation of I 1467 was in 1996, with a 14" telescope. The seeing was only fair, not allowing magnification higher than 240x. The star appeared single at that time. This is a changing binary, and Hartung's notes suggest it is sometimes accessible to amateur scopes.

There have not been many measures. Four were made from 1926 to 1936; a fifth in 1991.

Measures:

1926  042  1.4"
1936  009  0.5"
1991  348  0.3"

It's difficult to assess the likely orbit, as there appear to be no measures between 1936 and 1991. Hartung lists the pair, and gives a description of its appearance in 1960 and 1961, which fall into a long measuring gap. His estimates suggest PA ca230, and a separation of more than 1 arc second (given that he saw it double with 6-inch aperture). If Hartung is right, the pair had widened at that time, after closing previously, then closed again to the 1991 measure. However Hartung's estimated PA of 230 - and visibility with a 6-inch - doesn't fit the pattern otherwise suggested by the measures, of a gradually closing pair.

The PA may be a clue here. It reduced 33 degrees in 10 years between 1926 and 1936, along with a fast reduction in separation (1.4" to 0.5"). This suggests a short-period binary, which has been under-measured later, making it hard to establish the likely orbit if it is short-period.

I find it a puzzle. Plotting the three measures (1926, 1936, 1991) suggests a fairly long period binary, that will slowly widen (perhaps) in the near future. But the slow change suggested by the 1936 and 1991 measures is a puzzle. A solution would be to treat the PA of the Hipparcos measure as erroneous - reverse it 180 degrees, and it would then fit for a relatively short-period binary, and make sense of Hartung's observations, which otherwise can't be made to fit. I'm suspicious of the Hipparcos measure because it suggests an odd rate of change from 1936 to 1991 compared to the previous 10 years.

It's also interesting that the pair wasn't discovered until 1926. Given the measure in that year, the pair should sometimes be accessible to moderate size scopes (vide Hartung). There were surveys of the brighter stars in the south in earlier times to discover new pairs - by Russell and others at Sydney, in the 1880s and 1890s, often with the 11.3" refractor; and by Innes himself, with various telescopes, including an 18" refractor from 1898. These would certainly have shown the pair if it were near the separation listed for 1926. Again, this suggests a short period, with close separation being around the times it happened to be observed previously - hence no discovery.

The distance measures to I 1467 (both by Hipparcos, and by spectroscopic parallax) suggest it's around 200 parsecs - c. 650 ly. That fairly large distance in turn makes me wonder about the difference between the 1926 and 1936 measures. At 650 ly 1.4" represents more than 300 AU - a very wide pair, and unlikely to move very fast, therefore probably long period, not short.

This in turn suggests an error or errors in the early measures - difficult to check now. The 1984 edition of the WDS had dropped the 1926 figures - quoting PA 009 and 0.5" separation for 1926-36. That's one solution, and leaves Hartung out of the picture. The pair would then be a slow-changing pair, of gradually decreasing separation and angle. Hartung's estimate would then be wrong - perhaps he (twice) observed another pair which he recorded as I 1467. A surprising conclusion.

Obviously we need new observations to help work out what's happening with this pair. Measures at regular intervals in the future will be helpful in establishing the parameters for a possible long-period orbit. Overall it points to the problem of neglected doubles, more common in the southern sky.

See also: A Problem Double: I 1467 in Grus