NGC 147 (also known as Caldwell 17) is a dwarf spheroidal galaxy about 2.5 Million light years away in the constellation Cassiopeia. Along with neighbouring galaxy NGC 185 (Caldwell 18) it's located at the southern end of Cassiopeia, close to the Andromeda boundary. Both galaxies are satellites of the Andromeda galaxy (M31) and therefore members of the Local group.
Dwarf spheroidal galaxies are similar to dwarf elliptical galaxies only that they are approximately spheroidal in shape, generally of a lower luminosity and recognised only as satellite galaxies in the Local Group. They are also believed to contain large amounts of dark matter.
NGC 147 was discovered by John Herschel on September 8, 1829. It was Walter Baade in 1944 who first resolved the galaxy into individual stars and proved it was indeed a member of the Local group. To do this he used the largest telescope in the World at the time, the 100-inch (2.5 m) telescope at Mount Wilson near Los Angeles.
The galaxy shines at magnitude +9.3 and therefore within the range of small/medium size scopes. The bright star Schedar (α Cas - mag. +2.2) in the "W" of Cassiopeia can be used to locate it. Imagine a line extending 8 degrees south of the star and it takes you almost to exactly the area of sky where neighbouring galaxy NGC 185 is positioned. The star omicron Cas (ο Cas - mag. +4.5) is a degree east of NGC 185 with NGC 147 a further degree west of NGC 185 and therefore 2 degrees west of ο Cas.
NGC 147 has quite a large apparent size of 13 x 8 arc minutes. It can be spotted in a 100mm (4-inch) scope on good nights but generally larger scopes are required. Through a 250mm (10-inch) reflector it appears as a faint, large, elongated smudge of light with a gradual brightening towards the centre. However, this is not an easy object to observe due to its low surface brightness. For comparison, NGC 185 is brighter, closer and more compact than NGC 147 and therefore much simpler to detect.
NGC 147 is 300,000 light-years distant from M31, which is about twice the distance between the Large Magellanic Cloud and our Milky Way galaxy. It's best seen from the Northern Hemisphere latitudes during the months of October, November and December. The apparent diameter is 9,500 light-years.
NGC 147 Data Table
|Object Type||Dwarf Spheroidal Galaxy|
|Distance (ly)||2.5 Million|
|RA (J2000)||00h 33m 12s|
|DEC (J2000)||48d 30m 31s|
|Apparent Size (arc mins)||13.2 x 7.8|
|Notable Feature||Satellite galaxy of the Andromeda Galaxy (M31)|