NGC 147

Caldwell 17, also known as NGC 147, is a dwarf spheroidal galaxy located in the constellation Cassiopeia. As a satellite galaxy of the Andromeda Galaxy (M31), it is a distant member of our Local Group, residing approximately 2.5 million light-years away. Unlike the massive, gas-rich spiral galaxies that dominate the sky, NGC 147 is composed primarily of older, evolved stars and contains very little interstellar gas or dust, leading to its faint and diffuse appearance. It is a subtle, challenging target for amateur astronomers, requiring dark skies and careful observation to detect. Its physical connection to the Andromeda subgroup makes it a fascinating study in the diverse population of small galaxies that orbit and interact with the major giants of our local universe.

In long-exposure photographs, NGC 147 appears as a faint, nearly circular haze with no obvious nucleus, spanning roughly 13 by 8 arcminutes on the sky. High-resolution imaging by Hubble has resolved individual red giant stars in its outer regions, confirming a uniformly old stellar population with little or no ongoing star formation. It is closely paired on the sky — and gravitationally — with the nearby dwarf galaxy NGC 185 (Caldwell 18), and both are believed to orbit M31 at a projected separation of roughly 300 kiloparsecs from the Andromeda system.

For visual observers, NGC 147 is a demanding object requiring a dark sky, clean optics, and patience. A 6-inch or larger telescope will reveal a faint elliptical glow under good conditions, but the galaxy blends readily into the sky background if transparency is compromised. It rewards observers who have committed to seeking out all the known satellite galaxies of Andromeda — a diverse zoo of dwarf systems that trace the gravitational reach of our larger galactic neighbor. The 2MASS infrared survey image used here cuts through the foreground dust of Cassiopeia more effectively than visible light, revealing the full extent of this elusive galaxy.