Challenge Objects - November 2005
The November 2005 DSO challenge objects were written up by Rob Charteris (xrekcor) and John Bambury (ausastronomer). The CDC charts were compiled by Andrew Durick (astro_south). Sorry, no lunar challenge this month.
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 - NGC55 (String of Pearls)
Usually regarded as a member of the Sculptor Group (or South Polar Group because of its situation near the South Galactic Pole), this galaxy is listed as a Local Group member by Mateo (1998) who gives a distance of only 5.4 million light-years, compared to the value of 7 million ly given elsewhere. Stephen O’Meara in his book Deep Sky Companions – The Caldwell Objects puts it at 4.2 million light-years and a magnitude of 7.5. Discovered by James Dunlop on August 4, 1826 (Dunlop 507). Catalogued as h 2315 by John Herschel in 1847, and as GC 27 in the 1864 General Catalogue. It is also listed as Caldwell 72.
Open Cluster - M41
M41 lays about 4 degrees nearly exactly south of Sirius, the brightest star in the sky. It contains about 100 stars, including several red (or orange) giants, the brightest being of spectral type K3 and mag 6.9, and situated near the cluster's center. This star is about 700 times more luminous than our Sun. The stars are distributed over a volume about 25 or 26 light years across, and all receding from us at 34 km/sec. As they are at a distance of 2,300 light years, they appear scattered over an area of 38 arc minutes diameter.
The age of M41 was estimated at 190 million years (Sky Catalog 2000) and 240 million years (G. Meynet's Geneva Team). The hottest star has been found to be of spectral type A0. All sources agree that it is to be typized as of Trumpler class I,3,r. This stellar swarm is receding from us at 34 km/sec.
Perhaps known to Aristotle about 325 B.C. It was Hodierna who was first to catalogue it before 1654, and it got generally known after John Flamsteed's independent rediscovery of February 16, 1702, who remarks (No. 965 in his catalogue): "Near this star (12 CMa), there is a cluster." It was independently found again by Le Gentil in 1749, and apparently by Charles Messier, who added it to his catalogue on January 16, 1765.
Globular Cluster - M30
Globular cluster M30 can be found in Capricornus, at about 26,000 light years distance from our Sun. It’s physical size is estimated to be about 90 light years across, and appears to us under an angular diameter of about 12.0 arc minutes. It is fairly dense, as its concentration class V indicates. Its brightest red giant stars are about of apparent visual magnitude 12.1, its horizontal branch giants at magnitude 15.1. Only about 12 variable stars have been found in this globular cluster. Its overall spectral type has been determined as F3, and its colour index was given as B-V=0.60 mag. It is approaching us at 181.9 km/s.
M30 was one of the original discoveries of Charles Messier, who catalogued it on August 3, 1764 and like most of his globulars, described it as round nebula, containing no stars. It was first resolved by William Herschel around 1784.
A fine object in even small telescopes M30 will be well place and high in the sky for most of November.
Nebula - NGC2032 (Seagull Nebula)
The Seagull Nebula is a nebula complex that contains four separate NGC designations: NGC 2029, NGC 2032, NGC 2035 and NGC 2040 with NGC 2035 being the brightest and largest of the group. Collectively the area is known as N59A and is located at the boundary of the supershell LMC4 in the Large Magellanic Cloud. NGC 2032 and 2035, which form the bright core of the H II region N59A (B053530- 6736), belong to a single H II region which looks divided due to the presence of a heavy dust lane. This bright core presents an expansion motion of 24 km s-1.
The kinetic energy involved in this motion is of about 1:5x10 (49) erg. This value is compatible either with a supernova explosion origin or with a formation by the winds of interior massive stars. Since no clear traces of a SN explosion have been found in this nebula and since the stellar content of N59A (B053530-6736) is rich in blue stars, we conclude that these stars, mainly the very massive star HDE 269810 (R122), and probably other stars hidden by the dust lane, are sufficient in providing the wind power to drive the expansion motion. The dust lane seems to be mixed in with the nebular gas and the stars, suggesting a site where star formation may still take place.
Planetary Nebula - NGC7009 (Saturn Nebula)
This remarkable planetary nebula gets it’s name from its appearance, which resembles a faint suggestion of the planet Saturn with rings nearly edge-on (Lord Rosse named it in the 1840s). According to Admiral Smyth, the Saturn Nebula was one of Struve's nine "Rare Celestial Objects." While the Saturn nebula measures 36" in diameter, it has an extended halo of about 100". Its central star is rather bright with mag 11.5. The distance of the Saturn Nebula is not known very accurately. Hynes gives 2,400 light-years, the Sky Catalogue 2000.0 about 2,900, and Burnham adopts O'Dell's (1963) estimate of 3,900 ly and Stephen O’Meara in his book Deep Sky Companions – The Caldwell Objects lists it at a distance of 1,400lys. N7009’s progenitor was probably a main-sequence star of about 2 solar masses. It is now a hot, bluish dwarf with a surface temperature of about 55,000 degrees Kelvin. Strong ultra-violet radiation from the central star causes the nebula to fluoresce. N7009 is also listed as number 55 in the Caldwell Objects List. Discovered by William Herschel on September 7, 1782
Multiple Star - Theta Eridani (Acamar)
Theta Eridani is more commonly known as Acamar. It was described by Hartung as one of the gems of the Southern Sky; I tend to agree with him, it is one of my all time favourites. The primary is a pale blue/white star of magnitude 3.2, spectral type A4III (pale blue to white giant), with the secondary another pale blue/white star magnitude 4.1 of spectral type A1V (pale blue dwarf) at position angle 90° with an angular separation of 8.4”.
I see both stars as pale blue/white in my 10” reflector which is consistent with their spectral types, this does not always occur with visual observations of double stars. Some observers see the colours as being more white than blue while some observers have described both stars as having a yellowish tinge. The apparent visual colour of stars can of course be aperture dependant.
This is a truly magnificent double star that is easily observed and enjoyed in all apertures over 60mm.
The Hipparcos Catalogue Number for Acamar is HIP 13647.