Challenge Objects - July 2005
Scroll down to see each of the objects, and click on the links to download the finder chart or view the forum threads related to each object.
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Galaxy - NGC5128 (Centaurus A)
NGC5128, also known by the names “Centaurus A” and “Hamburger Galaxy”, is a type E0 peculiar elliptical galaxy in the constellation Centaurus. It is one of the most luminous and massive galaxies known and is a strong source of both radio and X-ray radiation. Centaurus A has a distinctive dark belt cutting through it, which is approximately 15,000 light years wide. Some theories suggest that the nucleus is experiencing giant explosions involving millions of stars and that the dark band across the galactic disk is material being ejected outward: others suggest the band is debris from a smaller dusty galaxy that is being absorbed by Centaurus A.
Together with M83 (Southern Pinwheel), NGC253 (Sculptor Galaxy) and M104 (Sombrero Galaxy), Centaurus A is among the most popular galaxies visible from Southern skies because it is so bright, with a striking yet peculiar appearance. These characteristics also make it a must-bag object for astrophotographers.
Under good, dark skies with reasonable aperture and good dark adaptation, the visual appearance starts to resemble what can be seen in long exposure images. It can be seen as roughly elliptical in shape with 2 bright bulges separated by the dark lane. Larger apertures even begin to reveal detail in the dark lane. Small aperture telescopes and binoculars under dark skies will also be able to make out the shape.
The galaxy is setting earlier as July progresses, so I’d recommend observing it earlier in the month under the dark skies of the new moon (between the 2nd and 10th of July).
Open Cluster - M6 (Butterfly Cluster)
M6, also known as NGC6405 or the “Butterfly cluster”, and is visible to the naked eye in the constellation Scorpius. It is described by Burnham as a "charming group whose arrangement suggests the outline of a butterfly with open wings". The cluster is made up of approximately 80 stars spread over a region about 54 arcmin in diameter, and the visually most conspicuous star is a rather reddish slow, semiregular variable BM Scorpii (HD 160371), a yellow or orange supergiant (spectral type K0-K3 Ib).
M6 can be seen naked eye as a bright patch of sky below the tail of the scorpion. Even small binoculars resolve the cluster, and telescopes of any aperture using low to medium power views will show the cluster in all its glory. Against the backdrop of the Milky Way, M6 and its close neighbour M7 provide a magnificent target for widefield astrophotographers. I’d love to see your sketches of this object to see how (and if) you interpret the butterfly shape.
M6 is rising to overhead most of the month, so can be observed at any time throughout July.
Globular Cluster - M22 (..)
NGC 6656 is also the 22nd object in Messier’s list. It is a globular cluster and is one of the closest to us, and contains over 100,000 stars. It is the third brightest globular in the sky outshone by only Omega Cent. And 47Tuc, and from dark skies is visible to some people with the unaided eye. It is very conspicuous in a finder as a round smudge.
It is one of my favourite globulars and is easy to find. Those with only binoculars will find this object easily using the accompanying charts. The globular is resolvable to the core in a telescope of moderate aperture.
Nebula - M20 (Trifid Nebula)
M20 is commonly known as the Trifid Nebula, which was the name first given to it by John Herschel, due to its appearance caused by the three dissecting dark dust lanes. M20 is identified as a “diffuse nebula” which is a widespread conglomeration of interstellar gas and dust. Diffuse nebula often contain three types of nebula, Emission, Reflection and Dark and M20 is no different to most others in this regard. The dark nebula which is superimposed over the emission nebula causing the three laned appearance was originally catalogued as Barnard 85. In long exposure photographs the emission and reflection components are clearly evident. The Emission nebula at the heart of the trifid is fired by a wonderful star forming region called HN40 (see multiple star challenge for July) and appears as the red area on photographs due to the ionised oxygen, the reflection nebulae appears as the pale blue area on photographs.
Visually, the trifid is a very interesting target and can be viewed in the same low power field as M8 (Lagoon Nebula) which is located 1.2 degrees to the South. At low power they make a wonderful wide field target. The three dark lanes in M20 are very evident in larger apertures but can prove a little difficult to resolve in smaller apertures under less than ideal skies. M20 is a target that takes high magnification well and at high power with good optics and reasonable aperture you “can” experience the effect of flying a spaceship through the dust lanes. If you have a scope over 8” aperture make sure you try to observe the target at 200X and above as well as at low power, with smaller scopes anything approaching 200X will give you a wonderful sight.
Planetary Nebula - NGC3918 (Blue Planetary)
NGC 3918 is a planetary nebula located in Centaurus, but is probably easier to find via star-hopping from Crux. Discovered by John Herschel in 1834, the planetary nebula presents as a striking blue disk in the eyepiece – hence its more common name as the “Blue Planetary”.
Hubble has imaged this nebula and has shown significant structure internal to the disk as well as around the fringe of the disk. With a 12.5” dob I have noticed some faint halo structure around the disk, but no detail on it. I would be interested if any bigger scopes have seen detail on the disk, and also how low an aperture is required to glimpse the faint halo around the disk. It will also be interesting to hear how various filters enhance the image, and possibly make the faint halo structure stand out more.
Multiple Star - HN40 (..)
HN40 is a multiple star system tucked within the dark rifts of M20, the Trifid Nebula (the July Nebula Challenge object). It is believed to be the chief source of illumination of the Trifid nebula although there may be other hot stars not visible, also illuminating the field. There are 7 known components of HN40 although not all are visible in the average amateur instrument. The 2 main components “A” and “C” are of magnitudes 7.6 and 8.6 respectively with a separation of 11.0” and are easily resolved in small telescopes. The 3rd component, “B” is mag 10.4 and located 6.1” from “A” and the 3 stars almost form a straight line. A 4” telescope will easily show these 3 stars at medium power. The 4th component “D” is mag 10.5 and lies 2.3” from “C”, it is infinitely more difficult to resolve than “B” in smaller telescopes and will usually require a sharp 8” telescope at high power under good conditions to resolve. The “E” component is mag 12.4 and located 6.2” from “C”. The “E” component will require a telescope over 12” aperture to resolve.
This is a very difficult photographic target; obviously the level of equipment used by the photographer will greatly influence the final result. Let’s have a go and see how many stars can be shown in the final result.
Here is a link to a photograph of M20 which also shows HN40, which you can take as the benchmark. Four stars are clearly visible in three of the photographs, particularly the two taken with the 12” Takahashi Mewlon, although four stars are also visible in the 2nd photograph taken with the 10” Takahashi E250. These photographs were taken by Arnie Rosner who is an exceptionally talented astrophotographer, as are many of the members of this forum.
The purpose of posting this link is also to show those visual observers the relative positions of the four main stars.