Challenge Objects - June 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 - M83 (Southern Pinwheel)
NGC 5236 is the 83rd member of Messier’s list and is also known as the Southern Pinwheel (M33 being the other Pinwheel Galaxy). It is a face on spiral galaxy and is an easy object for small apertures. Scotty Houston described the galaxy as a beautiful object in a 4” refractor at 20x. Long exposure photographs reveal its spiral structure, but visually it would take an aperture of around 6” to start to recognise parts of the spiral arms. Naturally, increasing aperture makes the spiral arms more apparent.
It has also been reported that there is significant detail available to the skilled observer in and around the core of this galaxy – even with small apertures. As part of your observing of this object you could try and spend 10 minutes staring at the core and see what you can detect. Compare this to widely available photos on the web and see what features you were able to identify.
The galaxy should be glimpsed in binoculars from a dark sky and it would be great to see some reports of this.
Open Cluster - NGC3532 (Football Cluster)
NGC 3532 is also known as the “Football Cluster” as its visual appearance to the casual observer, is that of a large oval shaped football. It is a large cluster covering an area twice the size of the full moon.
NGC 3532 is visible to the naked eye from urban skies and is a magnificent sight in binoculars and any telescope at low to medium power. X Carinae appears at the edge of the cluster and could easily be mistaken as being a member, however the Mag 3.9 Orange Supergiant is 5 times farther off at a distance of almost 7,000 ly. The cluster contains 150 members of magnitude 7 to 12 and has almost 400 members in total. The cluster contains a large number of orange and yellow stars.
Observers can see the cluster and interpret its shape and structure in many different ways. Some see it as having an oval shape with a “star free lane” down the centre, while others see it as being made up of dense straight and curved arms of stars. NGC 3532 is one of the finest visual or photographic targets in the entire sky regardless of the size of your telescope.
Globular Cluster - NGC6397 (Ara Glob)
NGC6397 has no other common names other than a colloquial designation of the “Ara Glob”, in a similar vein to the “Pavo Glob” (NGC 6752). NGC6397 is the 2nd closest globular cluster to earth, the closest being M4 in Scorpius. NGC6397 is one of over 20 Milky Way Globular Clusters which have undergone a “core collapse”. It is the closest “core collapsed” globular cluster to earth.
NGC 6397 is not only an interesting target visually it is a very interesting scientific target. Normally globular clusters contain only, older low mass stars which have evolved off the main sequence. NGC 6397 is somewhat different; Hubble and spectroscopic results indicate the presence of a large number of massive and luminous blue main sequence stars which basically “should not be there”. Astronomers have concluded that these stars have formed long after the original formation of the cluster as a result of “stellar collisions”. Astronomers believe “several” collisions have occurred within NGC6397. Consequently NGC6397 contains some of the most luminous stars of any globular cluster, due to its close proximity to us; these stars appear as the brightest individual stars of any globular cluster.
As a visual target, NGC6397 is a wonderful target in any sized telescope, the bigger the better however, as with any globular. It has a very bright condensed core as a result of its collapse but outliers stretch out over almost ½ a degree and appear to trace wispy arms as they extend from the core. NGC 6397 is visible naked eye by keen eyed observers under clear dark skies and makes a nice binocular target in medium to larger binoculars.
Nebula - M8 (Lagoon Nebula)
M8 is popularly known as the Lagoon Nebula and is assigned the designation of NCG 6523 (for the nebula). It is one of the brightest nebulae in the sky and is easily visible to the naked eye from a dark site. The full extent of 90x40 arcmins is the size of the extended nebula that shows up in long exposure photographs.
The cluster at the heart of the nebula (NGC6530) was the initial discovery made in this part of the sky (Flamsteed, 1680), with Le Gentil later (1747) discovering the nebula.
The brightest part of M8 contains a remarkable feature that is called the "Hourglass Nebula" – due to its shape. This feature was discovered by John Herschel and occurs in a region where star formation processes appear to be taking place. The hot star Herschel 36 causes the illumination of the hourglass nebula. Close by this feature is the brightest of the stars associated with the Lagoon Nebula, 9 Sagittarii that is likely to contribute some of the high energy radiation which also excites the nebula to shine.
One of the features of the nebula is the existence of dark nebulae known as 'globules' throughout its extent. Some of these have been catalogued in E.E. Barnard's catalogue of dark nebulae - namely B88, B89, and B296. The nebula's faint extension to the east has an own IC number: IC 4678.
Binoculars and rich field scopes are ideally suited to this extended object. Naturally the larger the aperture the more of the faint nebula can be glimpsed. The Hourglass nebula will be best seen with higher magnifications, and if you don’t have an eyepiece to frame the entire nebula, enjoy “cruising” around on the Lagoon and watching the ever changing vista. Heaps to see in this one – spend some time and explore with different magnifications. Being an emission nebula, experiment with various narrowband filters to see how the view changes.
Planetary Nebula - NGC3132 (Eight Burst Nebula)
NGC 3132 also goes by the name of the Southern Ring Nebula and the Eight Burst Nebula. The later name comes from the concentric ring effect of the visible in photos and to some extent through the eyepiece.
When viewing the nebula the “central star” is quite prominent, however it has been discovered that it is this star’s companion – a mag 16 bluish star that is unusually hot and is powering the nebula. It is 1.65” away from its brighter partner (mag 10) and is more closely aligned to the nebula’s symmetry axis. The Hubble Space Telescope photo of this nebula shows the small companion easily, however it would be impossible in most amateur equipment.
Larger apertures will show the inner nebula to be unevenly illuminated, much the same as is displayed in the HST photo. The outer ring is also unevenly illuminated, though brighter than the inner nebula. With smaller aperture and with low power in larger apertures the nebula can appear to blink on and off between direct and averted vision - due mostly to the bright central star. A narrowband filter (OIII, UHC, etc) will help, and don’t be scared of a bit of power as the nebula can take it. There is also a faint star (~mag 13) that borders the nebula to the south west that those with larger instruments may want to search for.
Multiple Star - 24 Comae
24 Comae is not known by any other common names, it is a lovely colour contrast double star consisting of a Mag 5.1 Orange giant primary accompanied by a Mag 6.3 Blue/White Secondary.
Separation is currently 20.1” making it easy to separate in the smallest telescopes and it basically has remained unchanged since the 1st measures in 1781. The position angle (PA) of the secondary is 270º. The stars have similar proper motions and are believed to form a long period system.
24 Comae like X Velorum is somewhat reminiscent of Albireo (B Cygni) only not quite as bright. It makes a lovely target in any sized telescope. The Hipparcos Star Catalogue Number is HIP 61418.