| The Palomar Globular ClustersDavid Ratledge |
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IntroductionThe Milky Way galaxy has, at the last count, 158 globular clusters. Some of the faintest of these are the so-called Palomar globular clusters which were discovered in the 1950s on the survey plates of the first Palomar Observatory Sky Survey (see reference). Thees 15 globulars make excellent challenging targets for the large telescope owner and astro-imager. BackgroundThe Palomar Clusters were discoverd by some of the most famous astronoers of our time and include Edwin Hubble, Walter Baade, Fritz Zwicky, Halton Arp and George Abell and catalogued by George Abell. However, naming them Palomar should probably be credited to Helen Sawyer Hogg. The initial list included just 13 clusters with Pal14 and Pal15 added later. The reason for their relatively late discovery and requiring the power of the 48-inch Schmidt telescope to see them is that they are either heavily obscured or extremely remote. ReferencesThe catalogue data reproduced here has been extracted from CATALOG OF PARAMETERS FOR MILKY WAY GLOBULAR CLUSTERS: THE DATABASE Compiled by William E. Harris, McMaster University.
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Pal1Discovered in 1954 by George Abell. It belongs to the class of loose globular clusters of the halo system, although not as remote as many of them. Until relatively recently is was uncertain as whether it was a very old open cluster or a very young glubular. This appears to have been finally settled by Rosenberg (Padova, Italy) when it was confirmed that it was indeed a globular. The clusters properties and location in the outer halo, about 55,000 light-years from the Galaxy's center, contradict the possibility of a very old open cluster. However, Pal1 is unusally young (c. 8 billion years) and could therefore have had a different formation process. Whilst most globulars are thought to have formed very early in the Milky Way's history the younger ones, on the other hand, may have formed in three other ways: as gas clouds that survived in the halo after the Milky Way's formation, later to form stars; as captured intergalactic star groups; or as cannibalised dwarf galaxies. Very difficult visually probably requiring a 24-inch Dobsonian.
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Taken from Lancashire using a 400mm F/4.7 Newtonian.
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Pal2Discovered by A.G. Wilson in 1955 Accoding to Harris et al (1998), Pal2 is a distant and heavily obscured globular cluster near the Galactic anticenter. To be recognisable (in my image) as a globular cluster under such circumstances means this cluster must be brighter and more massive than most other clusters in the outer halo. This is precisely what Harris et al determined adding that it probably had a very eccentric orbit and it was likely that it was near its furthest point (perigalacticon). Visually an 18-inch Dobsonian should suffice. |
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Taken from Lancashire using a 400mm F/4.7 Newtonian. |
Pal3Discovered by Baade and Wilson. One of the most remote clusters known (third most distant) - approximately twice as far away as our satellite galaxies, the Magellanic Clouds! According to Gratton it is so remote it has probably never passed through the galactic disk - thus being a true outer halo cluster. It is not surprising therefore that its appearance is somewhat indistinct. For the current theory on how such remote clusters came to be out there see Pal4 below. |
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Taken from Lancashire using a 400mm F/4.7 Newtonian. |
Pal4Discovered by Hubble in 1949 and independently again by AG Wilson in 1950. Second most distant globular cluster known (AM1 is the most distant known). The fact that it looks like a globular cluster given its huge distance gives a clue as to why, when it was first discovered (before the Palomar sky survey), it was catalogued as the Ursa Major Dwarf (galaxy). Current evolution theory (van den Bergh and A. D. Mackey - 2004) suggest remote globulars such as Pal3 and Pal4 were once all associated with dwarf spheroidal-like galaxy
fragments that have since disintegrated. The evidence for this is that these clusters are |
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Taken from Lancashire using a 400mm F/4.7 Newtonian. |
Pal10Discovered 1955 by AG Wilson. Embedded in the Mliky Way this cluster is heavily obscured and reddened and yet it is recognisably a globular cluster which is rarely the case for the Palomar objects. Located just over 2 degrees SW of the Coathangers asterism.
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Taken from Lancashire using a 400mm F/4.7 Newtonian. |
Pal13Discovered by AG Willson in 1955 and named by him the Pegasus Globular Cluster despite Pegasus already having the prominent globular M15 within its boundaries! Pal13 is one of the smallest and faintest globular clusters known. This is a consequence of its eccentric orbit which brings it close to the galactic centre every 1 to 2 billion years. With each dive through the galaxy gravitational tidal forces strip away the member stars. In fact, detailed present day studies offer evidence for a dramatic end to this dwindling cluster's tidal tug of war. Palomar 13's latest close approach was only about 70 million years ago and its next could well turn out to be its last. |
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Taken from Lancashire using a 400mm F/4.7 Newtonian. |