NGC 6946

Lore & Background

Discovered by William Herschel on 9 September 1798, NGC 6946 was once considered part of the Local Group but is now known to be among the dozen bright spiral galaxies near the Milky Way beyond the Local Group, lying within the Virgo Supercluster. It has been classified as an active starburst galaxy due to its prodigious star formation, and also as a double-barred spiral galaxy, with the inner bar thought to funnel gas into its center. Various unusual objects have been observed within it, including the 'Red Ellipse' along one northern arm, which resembles a super-bubble or very large supernova remnant possibly formed by an open cluster of massive stars. Two regions of unusual dark lanes of nebulosity exist, and several areas within the spiral arms appear devoid of stars and gaseous hydrogen, some spanning up to two kiloparsecs. A third peculiar object, discovered in 1967 and known as 'Hodge's Complex', was once thought to be a young supergiant cluster but was conjectured in 2017 to be an interacting dwarf galaxy superimposed on NGC 6946.

Reader's Guide

NGC 6946 is notable for its exceptionally high supernova rate—ten have been observed since 1917, about ten times the rate of the Milky Way despite the Milky Way having twice as many stars. This prolific activity earned it the nickname 'Fireworks Galaxy'. The supernovae include SN 1917A (discovered by George Willis Ritchey), SN 1939C (Fritz Zwicky), SN 1948B (Nicholas U. Mayall), SN 1968D (Paul Wild and Justus R. Dunlap), SN 1969P (Leonida Rosino), SN 1980K (Paul Wild), SN 2002hh (LOTOSS), SN 2004et (S. Moretti, whose progenitor was identified as a red supergiant of about 15 solar masses in an interacting binary), SN 2008S (Ron Arbour), and SN 2017eaw (Patrick Wiggins, also with a red supergiant progenitor of about 15 solar masses). In 2009, a bright star within the galaxy flared to over a million times the Sun's luminosity and then faded rapidly; Hubble Space Telescope observations suggest it did not survive, with infrared emission possibly from debris falling onto a newly formed black hole. This potential black hole-forming star is designated N6946-BH1, and its progenitor is believed to have been a yellow hypergiant. As of 2017, NGC 6946 held the record for the most supernovae seen in any galaxy, a record later surpassed by NGC 3690.

Did You Know?

• NGC 6946 has produced ten observed supernovae since 1917, about ten times the rate of the Milky Way.
• The galaxy is heavily obscured by interstellar matter because it lies close to the Milky Way's galactic plane.
• In 2009, a star in NGC 6946 flared to over a million times the Sun's brightness and may have collapsed into a black hole (N6946-BH1).

Discovery and Galactic Architecture

William Herschel first recorded this spiral on the night of 9 September 1798, adding it to his expanding catalogue of deep-sky objects. Today we know it as a face-on grand design intermediate spiral with a compact, luminous nucleus, positioned right on the border between the constellations Cepheus and Cygnus. At roughly 25.2 million light-years (7.72 megaparsecs), it sits at a distance comparable to M101 in Ursa Major. For a time both were grouped with the Local Group of galaxies, but modern measurements place them beyond that boundary, among a dozen or so bright spirals that orbit within the broader Virgo Supercluster. The 1991 Third Reference Catalogue of Bright Galaxies estimated its B-band isophotal diameter at 26.77 kiloparsecs, or about 87,300 light-years. Structurally, NGC 6946 is a double-barred system; the smaller inner bar is thought to channel gas toward the galactic core, feeding the vigorous star formation that earns it a starburst classification. Its proximity to the Milky Way's own galactic plane means it is heavily veiled by our own interstellar dust, complicating ground-based study.

The Fireworks Galaxy: A Supernova Record

Few galaxies have earned a nickname so vividly as NGC 6946's moniker, the Fireworks Galaxy. Since 1917, ten distinct supernovae have been detected within its disk—a rate roughly ten times higher than what astronomers observe in the Milky Way, even though our own galaxy contains about twice as many stars. The list reads like a roll call of twentieth- and twenty-first-century observers: American optician George Willis Ritchey spotted SN 1917A, Fritz Zwicky caught SN 1939C, and Paul Wild discovered two of the later events. Perhaps the most scientifically significant was SN 2004et, a Type II-P explosion whose red supergiant progenitor—roughly fifteen solar masses in a binary pair with a blue supergiant—was identified on pre-explosion imagery, making it only the seventh such direct identification ever recorded. SN 2017eaw, discovered by Patrick Wiggins, also traced back to a red supergiant of similar mass. As of 2017, no other galaxy had produced more observed supernovae, a record later broken by NGC 3690.

Peculiar Objects and the 2009 Black Hole Event

Beyond its supernovae, NGC 6946 harbors several odd structures that continue to puzzle astronomers. Along one of its northern arms sits the so-called Red Ellipse, a feature resembling a super-bubble or an enormous supernova remnant, possibly the legacy of an open cluster packed with massive stars. Two regions of unusual dark nebulosity lanes and several starless, hydrogen-free gaps stretching up to two kiloparsecs across further complicate the picture. Discovered in 1967, Hodge's Complex was long interpreted as a young supergiant cluster, yet a 2017 proposal suggested it might actually be a small dwarf galaxy superimposed on the disk. The most dramatic event, however, occurred in 2009, when a star flared to over a million times the Sun's luminosity before fading rapidly. Hubble Space Telescope follow-up indicated the star did not survive; lingering infrared emission at its position is attributed to debris spiraling into a newly formed black hole. The progenitor is thought to have been a yellow hypergiant, and the object is catalogued as N6946-BH1.

A Century-and-a-Half of Discovery

From Herschel's 1798 sketch to Patrick Wiggins's 2017 supernova detection, NGC 6946 has served as a proving ground for observational astronomy across more than two centuries. The galaxy's position near the Milky Way's galactic plane means our own interstellar dust heavily obscures it, a challenge shared by similar objects like IC 342. Yet that very proximity to the galactic plane, combined with its face-on orientation, makes it a rich target for studying star formation, supernova physics, and galactic structure. The supernova record spans a remarkable range of discoverers: American optician Ritchey, Zwicky, Mayall, Wild alongside Dunlap, Rosino, the automated LOTOSS survey system, Moretti, Arbour, and Wiggins. The galaxy's classification shifted over time—once lumped into the Local Group with M101, it is now recognized as one of a dozen bright spirals that lie just outside that family but within the Virgo Supercluster. Each new observation has peeled back another layer, from the double-barred morphology to the 2009 black hole birth, ensuring NGC 6946 remains a galaxy that rewards patient, repeated looking.