Ottawa Centre Monthly Observing Challenges

Beginning in 2007, Ottawa Centre has introduced a new item to its monthly meetings: an observing challenge object. Each month the centre will post a description of an interesting object. Members are invited to observe, sketch, photograph, or otherwise learn about this object. At the subsequent monthly meeting, an agenda item will provide an opportunity to share your thoughts, photos, sketches, or comments.

July 2007

Lunar: Illuminated Objects Beyond the Terminator

This month's Lunar challenge was inspired by member Peter Hayman's presentation of lunar photos at the July meeting.

Observe the moon during its waxing and waning phases, and look for times when objects in shadow near the terminator are high enough that they stick up into the sunlight, shining mysteriously out of the darkness. Photograph, sketch, or describe these objects and use maps to identify them.

". . . there were two . . . in the presentation. The first, of course, was the one with Sinus Iridum. The second is the one labeled "Crescent." Its beyond terminator detail is near north and the only true example is the Walled Plain Alexander, at the bottom of the neat row of things at the top. To understand its true nature, you have to try not to picture it as a crater, and see it as a brightly illuminated object instead. The other two, which are craters, are Aristoteles and Eudoxus, in descending order. " Images and comments by Peter Hayman.

Deep Sky: The Upper Scorpius OB Association and its Dust Clouds

By Glenn LeDrew

For the beginner

• The Upper Scorpius OB association (naked eye and binocular)
• Reflection nebula IC4603 (10X50 binocular)
• Dark nebula B44 (binocular, and even naked eye on the best nights)
• Dark nebula B45 (binocular)

For the experienced observer

• Reflection nebulae IC4601 and IC4592 (70-100mm binocular)
• Reflection nebulae IC4604 and IC4605??
• Dark Nebulae B41, B43 and B229 (100mm binocular)

The Hipparcos catalogue contains some 120 stars which have been accepted as members of Upper Scorpius. (The next generation of astrometric satellites will surely increase this membership list.) The only O-type star isn't actually within the association; it's the runaway star zeta Ophiuchi, displaced some 15 degrees from the association's centre to the north. Among the other spectral classes there are 49 B-types, 34 A-types, 22 F-types, 9 G-types, 4 K-types and 2 M-types (of which Antares is one.)

The luminary of Upper Scorpius is the strongly tinted, M-type red supergiant Antares, which is now in the final stages of its life as a star. Within no more than perhaps 100,000 years it will explode as a supernova--possibly even tomorrow! Antares is also a binary, with a B-type main-sequence star a not-too-distant 2.9 arc seconds distant. But what makes this double a challenging object is the large brightness difference between the two members. Antares is more than 4 magnitudes, or 50 times brighter, and so its brilliant glare dominates its much feebler companion. If you do succeed in splitting this interesting pair, note the strong colour contrast. While the fainter companion is inherently bluish, many observers report its hue as green, a not uncommon impression when such a star is adjacent to an orange K- or M-type star.

Much easier to resolve is the wide triple, rho Ophiuchi, the illuminating system of the nebula IC4604; even 7X binoculars will split it. Another easy binocular double is nu Scorpii, the star which illuminates the nebula IC4592. Beta Scorpii is somewhat more difficult, requiring at least 12X.

The Dust Clouds

Because Upper Scorpius is only some 5 million years old, there remains a significant amount of dust-laden gas clouds in its vicinity. The denser, and thus generally more conspicuous clouds were catalogued by E.E. Barnard nearly a hundred years ago, and today bear the "B" designation. Many of them are oriented such that they point more or less away from the heart of the stellar association, toward the east and northeast. Moreover, some clearly resemble "dark comets", with denser heads and gauzy tails (B44 being a good example). This no coincidence. The combination of strong stellar winds and one or more supernova blasts have sculpted the clouds and tended to stretch them radially.

While the majority of stars in Upper Scorpius were minted some 5 million years ago, star formation has not yet ceased here. Herschel's "hole in the heavens" near rho Ophiuchi is currently forming hundreds of stars which will be somewhat similar to our sun in size (as opposed to the much more massive and brilliant stars which we so easily see now with the unaided eye). The dense cocoon of this stellar nursery hides the birthing process from our prying eyes, but at infrared wavelengths these stars-in-the-making are revealed.

Happily, most if not all of the involved dark clouds are located on the near side of the association. This results in numerous and sizeable reflection nebulae. Dust grains scatter light more strongly in the original, or forward direction of travel, while back scattering is very much less efficient. Therefore a reflection nebula is generally brighter when the dust lies between us and its illuminating star, as is the case here. Of course, if the intervening cloud is sufficiently dense or deep, it can absorb the scattered light to the point where a reflection nebula is completely invisible.

Besides dimming the light of background objects, interstellar dust preferentially scatters shorter (bluer) wavelengths, while affecting longer (redder) wavelengths to a lesser extent. Therefore light that passes through a dust cloud--or even the much more rarefied ISM in general--is reddened, or more properly "de-blued." And this region provides a good example of this effect, which you can see with a humble binocular. The two bright stars which closely flank Antares are sigma and tau Scorpii, both having very similar surface temperatures. Normally their colours would be utterly indistinguishable to even an experienced eye. But sigma is shining through a thicker haze than does tau. The result? Sigma is clearly white or cream coloured (somewhat "de-blued"), while less-affected tau is still clearly bluish.

The preference for scattering blue light makes almost all reflection nebulae appear blue in colour photographs, and moreover rather bluer than their illuminating stars. In fact, the vast majority of visually observable reflection nebulae are seen to be illuminated by bluish stars, mainly because these nebulae mostly occur where star formation is ongoing or has occurred recently and therefore nearly all the brightest nearby stars will be young, blue objects. But a reflection nebula is subject to the same reddening as happens to stars if they are seen through dust lying along the line of sight. Deep colour images of the Rho Ophiuchi region reveal this very powerfully. The thinner cloud through which rho Ophiuchi shines results in the strongly blue reflection nebula IC4604. But to the immediate south, IC4603 is seen through a substantial intervening cloud which clearly reddens the nebula to the point where its colour is for the most part rather neutral, or white-ish.

Not many red stars are observed to have attendant reflection nebulae, but Antares is the most famous, illuminating as it does the photographically obvious IC4606. It has been commonly thought the the Antares nebula results from dust that the star itself expelled during its swollen red supergiant phase. I have always doubted this. If that were the case, at least some other red supergiants should have similarly obvious optical nebulae. There are none that I know of, although some evolved red (super) giants do exhibit excess infrared emission in their immediate vicinity, due to expelled material. But in the case of Antares, it's quite obvious the nebula is simply the illuminated edge of the cloud complex to its immediate north.

Observing the Reflection Nebulae of Upper Scorpius

First off, the association does contain some emission nebulosity.The two brightest examples are illuminated by sigma and tau Scorpii, but their surface brightnesses are quite low and probably beyond the ability of the human eye to detect. Of the kinds of bright nebulae, it would seem that only the reflection type can be seen in Upper Scorpius. And there's more of it here than can be found in any other similarly sized piece of sky.

Deep photographs reveal vast sheets of illuminated dust covering a considerable fraction of Upper Scorpius on the sky. Visually, however, only the brightest portions near several illuminating stars can be seen. But in one case, the visible extent is astounding.

To observe these low contrast objects requires a sky sufficiently dark that the milky way is very easy to see and presents lots of structure. For larger nebulae, a rich field telescope which can provide at least a 2 degree field is very desirable. And any instrument should allow an exit pupil diameter of at least 4mm. But better than the traditional telescope is a giant binocular. Using two eyes results in a 41% increase in signal-to-noise. This can make the difference between seeing a faint nebula and not. I can't extol the virtues of binocular observing enough. And today's decently-made Chinese 70mm to 100mm binos offer excellent value. In all cases I highly recommend the technique of slowly "nodding" the instrument so that image motion across the retina will more easily reveal low contrast brightness variations. Nebula filters, whether broad- or narrow-band, must not be used because they're designed to pass a fairly small slice of the visible spectrum centred around specific spectral lines found in emission nebulae. Even a broader bandpass LPR (light pollution rejection) filter might be a detriment, but do try it out if you have one.

Your first reflection nebula target should be IC4603, specifically the brightest part immediately surrounding its illuminating star. Even though it's significantly dimmed by intervening dust, it is inherently quite bright because a 10X binocular will easily reveal a small hazy fuzz surrounding the star. I first stumbled across it with a 10X50 about 10 years ago, in no way expecting to see nebulosity, and moreover not even looking for any.

If you succeed with IC4603, try for IC4601 next. But I suspect at least a 70mm aperture will be required; I saw it in a 22X100 binocular in a fairly good--but not excellent--sky. It's easy to locate, surrounding as it does a distinctive triangle of stars just to the east of nu Scorpii.

A big surprise for me was the detection of IC4592. Being already familiar with the area because of my involvement with the Desktop Universe project, I knew of its existence but assumed it would be of too-low surface brightness for visual observation, at least from mid-northern latitudes. But the 22X100 binocular revealed half of its 3-degree extent. The most obvious portion was near and to the northwest of nu Scorpii, and particularly along the fairly sharp southern edge where the brightness changes most abruptly.

I have yet to see IC4604, the nebulosity surrounding rho Ophiuchi, or IC4605, but I'll keep trying whenever sky transparency is good. You should too!

Previous Challenge Objects

Header Image Credit

The icon leading to this page and image in the header are images of globular cluster M3 produced by Ottawa Centre member Albert Saikaley. This was chosen as the icon of the "Challenge Objects" section because the globulars are within the range of beginners with modest telescopes, yet advanced amateurs are still challenged by teasing out the spectacular colours and details such as those in Albert's image.

Modified: August 8, 2007