The galaxy was discovered by German-British astronomerWilliam Herschel on 1 November 1788.[4]Edwin Hubble detectedCepheid variables in NGC 2403 using theHale Telescope, making it the first galaxy beyond theLocal Group within which a Cepheid was discovered.[3] By 1963, 59 variables had been found in NGC 2403, of which 17 were eventually confirmed as Cepheids, with periods between 20 and 87 days. As late as 1950 Hubble was using a distance of just under 2 million light years for the galaxy's distance, but by 1968 the analysis of the Cepheids increased this by almost a factor of five, to within 0.2 magnitudes of the current value.
NGC 2403 bears a similarity toM33, containing numerous star-formingH II regions, but being a little bit larger at approximately 90,000 light-years in diameter compared to the 61,100 light-year diameter of M33.[5] It is aflocculent spiral galaxy, with short, patchy arms.[6] The northern spiral arm connects it to the star forming regionNGC 2404.[3] NGC 2403 can be observed using 10×50binoculars.[3]NGC 2404 is 940 light-years in diameter, making it one of the largest known H II regions. This H II region represents striking similarity withNGC 604 in M33, both in size and location in galaxy.
SN 1954J was first noticed byGustav Tammann andAllan Sandage as a "bright blue irregular variable"star, which they named V12. They noted it underwent a major outburst on 2/3 November 1954, which attained a magnitude of 16 at its brightest.[7] In 1972,Fritz Zwicky classified this event as a Type Vsupernova.[8] It was later determined to be asupernova imposter: a highly luminous, very massive eruptive star, surrounded by a dustynebula, similar to the 1843 Great Eruption ofη Carinae in the Milky Way.[9]
SN 2004dj (Type II-P, mag. 11.2) was discovered byKōichi Itagaki on 31 July 2004.[12][13] At the time of its discovery, it was the nearest and brightest supernova observed in the 21st century, and is now listed as the brightest supernova of 2004.[14]
AT2016ccd, initially designated as SNhunt225, is a luminous blue variable, first discovered byCatalina Real-time Transient Survey (CRTS) and Stan Howerton in December 2013. Outbursts from this star have been observed as recently as November 2021.[15][16]
NGC 2403 has two known companions. One is the relatively massivedwarf galaxyDDO 44. It is currently being disrupted by NGC 2403, as evidenced by a tidal stream extending 82 kly (25 kpc) on both sides of DDO 44. DDO 44 is approaching NGC 2403 at a distance much closer than typical for dwarf galaxy interactions. It currently has a V-band absolute magnitude of −12.9, but its progenitor was even more luminous.[2]
The other known companion is officially named MADCASH J074238+652501-dw, although it is nicknamed MADCASH-1. The name refers to the MADCASH (Magellanic Analog Dwarf Companions and Stellar Halos) project. MADCASH-1 is similar to typicaldwarf spheroidal galaxies in the Local Group; it is quite faint, with an absolute V-band magnitude of −7.81, and has only an ancient, metal-poor population ofred giant stars.[17]
^Ho, Luis C.; Filippenko, Alexei V.; Sargent, Wallace L. W. (October 1997). "A Search for "Dwarf" Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies".Astrophysical Journal Supplement.112 (2):315–390.arXiv:astro-ph/9704107.Bibcode:1997ApJS..112..315H.doi:10.1086/313041.S2CID17086638.
^Van Dyk, Schuyler D.; Filippenko, Alexei V.; Chornock, Ryan; Li, Weidong; Challis, Peter M. (2005). "Supernova 1954J (Variable 12) in NGC 2403 Unmasked".Publications of the Astronomical Society of the Pacific.117 (832):553–562.arXiv:astro-ph/0503324.Bibcode:2005PASP..117..553V.doi:10.1086/430238.
