NGC 2403
An intermediate spiral galaxy in Camelopardalis, member of the M81 Group.
NGC 2403, also cataloged as Caldwell 7, is an intermediate spiral galaxy located in the constellation Camelopardalis. It sits on the outskirts of the M81 Group of galaxies, about 9.7 million light-years from Earth.
German-British astronomer William Herschel first spotted the galaxy on November 1, 1788. Later, Edwin Hubble used the Hale Telescope to find Cepheid variable stars in NGC 2403, making it the first galaxy beyond the Local Group where such a star was detected. By 1963, astronomers had identified 59 variable stars in the galaxy, 17 of which were confirmed as Cepheids with periods ranging from 20 to 87 days. Hubble initially estimated the galaxy's distance at just under 2 million light-years in 1950, but by 1968, analysis of the Cepheids pushed that figure nearly five times farther, to within 0.2 magnitudes of the current value.
NGC 2403 resembles the Triangulum Galaxy (M33) in many ways, with abundant star-forming H II regions. It is slightly larger, however, spanning about 90,000 light-years in diameter compared to M33's 61,100 light-years. The galaxy is flocculent, meaning its spiral arms are short and patchy. Its northern arm connects to NGC 2404, a massive H II region about 940 light-years across—one of the largest known—and similar in size and location to NGC 604 in M33. NGC 2403 is bright enough to be seen with 10×50 binoculars.
Four notable astronomical transients have been recorded in the galaxy. SN 1954J, initially thought to be a variable star named V12, was observed by Gustav Tammann and Allan Sandage as a major outburst on November 2–3, 1954, reaching magnitude 16. Fritz Zwicky classified it as a Type V supernova in 1972, but it later turned out to be a supernova impostor—a highly luminous, massive eruptive star surrounded by a dusty nebula, similar to the Great Eruption of Eta Carinae in the Milky Way. SN 2002kg, discovered on October 26, 2002, by the Lick Observatory and Tenagra Observatory Supernova Searches, was first labeled a Type IIn supernova or a luminous blue variable outburst; it was reclassified as a Gap transient on August 24, 2021. SN 2004dj, a Type II-P supernova discovered by Kōichi Itagaki on July 31, 2004, reached magnitude 11.2 and was the nearest and brightest supernova seen in the 21st century at the time, later listed as the brightest of 2004.
Quick Facts
- Epoch
- J2000
- Ra
- 07 · 36 · 51.298
- Dec
- +65 · 36 · 09.662
- Dist Ly
- 9.65 Mly
- Z
- 0.000445
- Appmag V
- 8.2
- Constellation Name
- Camelopardalis
Facts from the source article.
Lore & Background
NGC 2403 was discovered by German-British astronomer William Herschel on 1 November 1788. Edwin Hubble later detected Cepheid variables in the galaxy using the Hale Telescope, making it the first galaxy beyond the Local Group within which a Cepheid was discovered. By 1963, 59 variables had been found, 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.
The galaxy bears a similarity to M33, containing numerous star-forming H II regions, but is a little bit larger at approximately 90,000 light-years in diameter compared to the 61,100 light-year diameter of M33. It is a flocculent spiral galaxy, with short, patchy arms. The northern spiral arm connects it to the star-forming region NGC 2404, which is 940 light-years in diameter, making it one of the largest known H II regions. This H II region represents striking similarity with NGC 604 in M33, both in size and location in the galaxy. NGC 2403 can be observed using 10×50 binoculars.
There have been four reported astronomical transients in the galaxy: SN 1954J, initially a bright blue irregular variable star that underwent a major outburst in November 1954, later classified as a Type V supernova and then determined to be a supernova imposter; SN 2002kg, discovered in October 2002 and initially classified as a Type IIn or possibly the outburst of a luminous blue variable, later reclassified as a Gap transient; SN 2004dj, a Type II-P supernova discovered in July 2004, at the time the nearest and brightest supernova observed in the 21st century; and AT2016ccd, a luminous blue variable first discovered in December 2013, with outbursts observed as recently as November 2021.
Reader's Guide
NGC 2403 holds significance as the first galaxy beyond the Local Group in which Cepheid variables were discovered, a milestone that helped calibrate the cosmic distance scale. The analysis of its Cepheids dramatically revised the distance estimate from under 2 million light-years in 1950 to nearly 10 million light-years by 1968, demonstrating the power of variable stars in measuring extragalactic distances. Its flocculent spiral structure and resemblance to M33 make it a valuable object for studying star formation and spiral arm dynamics. The galaxy's large H II region NGC 2404, one of the largest known, provides a nearby analog to similar regions in other galaxies. The recorded transients, including a supernova imposter and a Type II-P supernova that was the nearest and brightest of its time in the 21st century, contribute to understanding stellar evolution and explosive events. Its two known companions, including the disrupted dwarf galaxy DDO 44 and the faint dwarf spheroidal MADCASH-1, offer insights into galaxy interactions and the properties of low-mass galaxies in a group environment. NGC 2403 remains a key target for both amateur observers with binoculars and professional studies of stellar populations and galaxy evolution.
Did You Know?
- NGC 2403 was the first galaxy beyond the Local Group in which a Cepheid variable was discovered.
- The galaxy's H II region NGC 2404 is 940 light-years in diameter, one of the largest known.
- NGC 2403 has a companion dwarf galaxy, DDO 44, which is being disrupted by the larger galaxy.
Frequently Asked Questions
Who is NGC 2403?
NGC 2403, also cataloged as Caldwell 7, is an intermediate spiral galaxy spanning roughly 90,000 light-years and sitting about 9.7 million light-years from Earth in the northern constellation Camelopardalis. It is a member of the M81 Group of galaxies.
What makes NGC 2403 special?
It is famous for being the first galaxy outside the Local Group in which Edwin Hubble identified Cepheid variable stars, using the Hale Telescope. By 1963, observers had logged 59 variable stars in the system, 17 of them confirmed Cepheids.
How was NGC 2403 discovered?
German-British astronomer William Herschel first recorded the galaxy on November 1, 1788, adding it to his growing catalog of deep-sky objects. It has since earned dual listings in both the NGC and Caldwell catalogs.
Why is NGC 2403 important to astronomy?
Hubble's detection of Cepheids there demonstrated that the period-luminosity relationship could be extended to galaxies well beyond the Local Group, reinforcing the cosmic distance ladder. That single observation helped cement our ability to gauge distances to far-off spiral systems.
Where do I look to find NGC 2403?
Aim your scope toward Camelopardalis, the Little Camel constellation high in the northern sky. At nearly 10 million light-years, it is a demanding but rewarding deep-sky target for experienced observers with adequate aperture.
More in NGC Objects, Part 4 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced