NGC 2403, Galaxie
This beautiful galaxy is located at a distance of about 10 million light years in the constellation "Camelopardalis" and was found by Wilhelm Herschel in 1788.
Despite its large apparent magnitude of 8.2mag it is not included in the Messier catalog. This could be due to the fact, that the galaxy is rather large with an
apparent size of 23.4x11.8' - and this is associated with a rather low surface brightness of 14.2mag/'².
The barred spiral type galaxy (SBc) has a diameter of 50-70,000 light years and is thus only about 60% as large as our Milky Way. The mass has been
determined to be 95 billion solar masses. There are also two known companion galaxies, the larger of which, designated "DDO 40", has a mass of 20-60
million solar masses.
In the spiral arms there are many HII regions, some of them even emitting soft X-rays. These are generated when a star explodes as a supernova. Then the
outward racing shock front heats up the gas of the surrounding HII region. Hubble images show that many of the HII regions have a "hole" in the center
because the gas has been driven outward by supernovae and the stellar winds of massive O stars with temperatures up to 40,000 Kelvin. The largest HII
region even has its own entry in the New General Catalogue with the number "2404". It has a diameter of an impressive 940 lightyears. For comparison: the
Orion Nebula has an extension of only 30 light years.
So far, even two supernovae have been observed in NGC 2403: "SN 1954J" and "SN 2004dj". The latter has been one of the brightest supernovae of the 21st
century. Its maximum brightness was 11.2mag - so it could be seen even with small telescopes. The progenitor star was a supermassive hot O-star with a
mass of 8-40 solar masses called "Sandage 96".
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In a 20" telescope at an intermediate magnification of 210x NGC 2403 is a beautiful sight. The galaxy core appears bright and extended, while the spiral arms are
rather faint but clearly structured and can be traced quite far out. In the northwestern part of the galaxy there is a very faint star cloud, which is only visible
with averted vision as a very faint patch of light.
The HII regions are all quite bright and even show a little bit of structure. Brightest is NGC 2404 (blue arrow), which stands out quite easy when looking
through the eyepiece - similar to NGC 604 in Messier 33. Here it would be worth to make a detailed drawing with high magnification.
