NGC 147, also known as Caldwell 17, is a dwarf spheroidal galaxy about 2.5 Million light-years away in the constellation of Cassiopeia. Together with neighbouring galaxy, NGC 185, it can be found in the southern part of the constellation, close to the Andromeda boundary. Both galaxies are satellites of M31 (Andromeda galaxy) and therefore members of the Local Group. They are best seen from Northern Hemisphere latitudes during the months of October, November and December.
Dwarf spheroidal galaxies are similar to dwarf elliptical galaxies except they are spheroidal in shape, generally of a lower luminosity and recognised only as Local Group satellite galaxies. They are also believed to contain large amounts of dark matter.
John Herschel discovered NGC 147 on September 8, 1829. It was Walter Baade, in 1944, who first resolved the galaxy into individual stars and proved it was a member of the Local Group. At the time he was using the largest telescope in the World, the 100-inch (2.5 m) Hooker telescope, at Mount Wilson near Los Angeles.
NGC 147 shines at magnitude +9.3 and is within the range of small and medium size amateur scopes. To locate the galaxy, bright star Schedar (α Cas - mag. +2.2) in the "W" asterism of Cassiopeia is a good starting point. Imagine a line extending 8 degrees south from this star and you will arrive at NGC 185. Omicron Cas (ο Cas - mag. +4.5) is a degree east of NGC 185 with NGC 147 a degree west of NGC 185.
NGC 147 has quite a large apparent size of 13 x 8 arc minutes. It can be spotted in 100mm (4-inch) scopes on good nights, but generally larger instruments are required. Through a 250mm (10-inch) reflector, it appears large and elongated but faint. The galaxy does brighten gradually towards the centre. However, it does suffer from a low surface brightness. For comparison, NGC 185 is brighter, closer, compact and easier to observe.
NGC 147 lies 300,000 light-years from M31, which is roughly twice the distance between the Large Magellanic Cloud and our Milky Way galaxy.
NGC 147 Data Table
|Object Type||Dwarf Spheroidal Galaxy|
|Distance (light-years)||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)|