Caldwell 7 (NGC 2403) is a prominent intermediate spiral galaxy and one of the brightest external galaxies in the northern sky outside the Messier catalog. It belongs to the M81 Group of galaxies and is often compared in structure and appearance to the Triangulum Galaxy (M33). The galaxy contains numerous bright HII regions and young star-forming complexes that stand out well in deep astrophotography. Because of its relatively large apparent size and high surface brightness, it is a rewarding target for medium focal lengths and detailed long-exposure imaging.
Names and Catalog numbers
NGC 2403
Caldwell 7
UGC 3918
PGC 21396
Position and the cosmic neighborhood
NGC 2403 lies in the faint northern constellation Camelopardalis, not far from the border with Ursa Major and Lynx. Although Camelopardalis contains few bright guide stars, the galaxy can be located roughly between the well-known galaxies of the M81 Group and the head of the constellation Lynx. It is considered an outlying member of the M81 galaxy group, sharing the same general region of space as the famous galaxies M81 and M82. Several smaller galaxies are located nearby in the same area of the sky, including NGC 2366 and various faint dwarf galaxies associated with the group.
Nice to Know
NGC 2403 hosts a very prominent star-forming complex known as NGC 2404, a bright HII region that often appears clearly in deep RGB and narrowband astrophotography. The galaxy has hosted multiple recorded supernovae and supernova-like events, including SN 2004dj and the famous luminous blue variable eruption SN 1954J (V12). Its structure and star-formation activity make it visually similar to the Triangulum Galaxy (M33), offering comparable detail in long-exposure images. Because of its high surface brightness and large angular size, the galaxy is an excellent imaging target even under moderately light-polluted skies.
Brightnes, distance and size
NGC 2403 has an apparent magnitude of about 8.9 and lies roughly 8 million light-years from Earth. The galaxy spans approximately 70,000 light-years in diameter, somewhat smaller than the Milky Way but still a substantial spiral system. In the night sky it appears quite extended, measuring about 22 by 12 arcminutes, which makes it well suited for astrophotography with medium focal lengths where its spiral structure and bright star-forming regions can be captured in detail.
