Challenge Objects - August 2005
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Galaxy - NGC4945 (..)
NGC4945 (not known by any other name) is a spiral galaxy in the constellation Centaurus. This remarkable galaxy is seen almost edge-on, and the spiral structure is chaotic and difficult to discern. In part this is because the galaxy is itself extremely dusty, and it is seen through dust in the Milky Way. These effects together account for its yellowish hue, though patches of bluish stars can be seen. The dust in NGC 4945 absorbs most of the visible light, especially the blue light from the numerous bright stars known present within the galaxy. This energy is re-radiated at infrared wavelengths, which penetrate dust more readily, which is why the galaxy is found to be unusually bright in the infrared.
NGC4945 is part of the Centaurus Group of galaxies, and is located 6 times further away than the popular Northern target, the Andromeda Galaxy (M31). X-Ray observations reveal that NGC4945 has an unusual, energetic, Seyfert 2 nucleus that might house a large black hole.
Because of the shape and edge-on appearance from Earth, it is difficult to observe much structure in the galaxy, however the elongated shape is clear to see and should be able to be seen in 4-6" of aperture and above. With large binoculars, the galaxy might resemble a fuzzy grey patch - I'd be interested in reading reports on what can be seen with suitably sized binoculars.
The galaxy is setting earlier as August progresses, so I'd recommend observing it earlier in the month under the dark skies of the new moon (between the 1st and 10th of August).
Open Cluster - NGC6025 (..)
NGC 6025 is an open cluster in Triangulum Australe. It is also known by other identifiers, including Caldwell 95, Melotte 139, Collinder 296 and Dunlop 304. It is a grouping of about 30 bright white and yellow suns within 5 arcmins of the clusters centre, although up to 139 members have been attributed to this cluster in total out to about 15 arcmins.
The open cluster in easy to find by taking the line formed by Alpha and Beta Centauri and heading back east about twice their spacing from Alpha Centauri. Under a dark sky the cluster appears as a "puff of soft light" (O'Meara,2002) to the naked eye. It is an excellent binocular object and is certainly more suited to rich field scopes. Higher power does reveal some of the nice pairings of cluster members.
The stars form gentle curved and straight lines and O'Meara noted the brighter members forming the rough outline of the Greek letter c. At the centre of this is a grouping of four stars forming a "southern cross" asterism. What else can you see in the alignment of the stars?
Globular Cluster - M55 (..)
M55 is also known as NGC 6809 it was discovered by Lacaille in 1752 who described it as "shadowy nucleus of a large comet". Messier confirmed its existence in 1778. M55 is one of the closer galactic globular clusters to us at 20,000 light years.
M55 is somewhat reminiscent of a compressed open cluster, yet it is a globular. Unlike most other globulars M55 is an open type of globular cluster with a slightly higher concentration of stars in the central area. This lack of a brighter core area causes M55 to have a low surface brightness and it can easily be passed over by an inexperienced observer. It also has advantages, in that it can be easily resolved in small telescopes. A 4" telescope under good conditions will usually resolve most of the stars. The individual stars in M55 appear to be bluish white in colour making it a very pretty globular in small and large telescopes.
Nebula - M17 (Omega/Swan Nebula)
M17 (NGC6618) is known by many names, including the Omega Nebula, Swan Nebula, Horseshoe Nebula and Lobsta Nebula, is a region of star formation and shines by excited emission, caused by the higher energy radiation of young stars. Unlike in many other emission nebulae, however, these stars are not obvious in optical images, but hidden in the nebula. Star formation is either still active in this nebula, or ceased very recently. A small cluster of about 35 bright but obscurred stars seems to be imbedded in the nebulosity.
The Swan Nebula is quite large and massive as it contains roughly 1000 times the mass of our Sun. The bright central region is about 15 light years across lies about 5000 light years away toward the constellation of Sagittarius.
At magnitude 6.0, M17 can be observed naked eye under dark skies with good conditions, and makes a wonderful target for binoculars while doing a tour of the objects in Sagittarius. In a telescope of any size, it is best viewed with moderate power (~80x or higher), and nebula filters like the UHC and OIII will help to increase the contrast and show structure within the nebulosity.
Planetary Nebula - NGC5189 (..)
NGC 5189 is a planetary nebula in Musca (near the border of Circinus and Centaurus). It is unusual due to its irregular shape, which has given rise to the common names of “The Spiral Planetary” and “The Leafy Sea Dragon” to name just two. Discovered in 1835 by John Herschel, it was described by him as a very strange object. It has been catalogued earlier on as an emission nebula, which came about due to its unusual shape. As described by Hartung, it has a “knot of bluish light east, from which a bright curved bar passes axially west”. It is this curved bar that gave the nebula the name of the Spiral Planetary – appearing similar in appearance to a barred spiral galaxy.
Visible in apertures of 75mm and larger, the nebula shows considerable structure with large apertures. Within the nebula are four stars (though these may not be visible in smaller apertures) of which the brightest is around mag 11. This brightest star is not the source of the nebula, rather that honour goes to the fourth brightest of the stars imposed on the nebula. Magnitude estimates of this star range from 12.9 to 14th mag. It is also thought that this star is a close binary leading to the unusual structure of the expanding nebula. This source star is slightly off centre in relation to the extents of the nebula. Even given its unusual appearance, the nebula still has a surprising amount of symmetry, which shows up well in photography. A UHC (and equivalent) filter enhances the appearance of the nebula and an OIII does an even better job.
Multiple Star - Gamma Delphinus
Gamma Delphinus is not known by any other common names. Like some of the earlier Monthly Observing Challenge (MOC) double stars it presents a lovely colour contrast, with a Mag 4.36 Yellow primary accompanied by a Mag 5.03 Pale Yellow/White Secondary. Separation is currently 9.1" making it easy to separate in the smallest telescopes. Separation has decreased slightly from 12.0" since the first measures in 1755 to its current separation of 9.1". The position angle (PA) of the secondary has also decreased slightly over the same time period from 280º to its current position of 266º. The stars have similar proper motions and are believed to form a long period system.
Both stars are of a similar apparent size with the primary being slightly larger and brighter than the secondary. Different observers see the secondary as different colours. The colours I have mentioned previously, Yellow and Pale Yellow/White are consistent with the spectral types, sometimes I see the secondary as having a pale green or emerald tone to it depending on conditions, what colours do you see. This is another showpiece double star in small and large telescopes.
The Hipparcos Star Catalogue Number is HIP 102532.