Fireworks Galaxy (NGC 6946)

The Fireworks Galaxy (NGC 6946) is a barred spiral galaxy located approximately 22 million light-years away in the constellation Cygnus. It is about 40,000 light-years across (for comparison, our own Milky Way galaxy is about 100,000 light-years across). NGC 6946 is known for its rich cloudscape of gas and dust, which is home to intense star formation. From 1917 to 2008, this galaxy hosted nine supernova explosions, the most for any galaxy. NGC 6946, also known as the Fireworks Galaxy, is a face-on spiral galaxy with a small, bright core region. Its location on the celestial sphere is on the border between the constellations Cepheus and Cygnus. The distance to Earth is about 25.2 million light-years or 7.72 megaparsecs - about the same as the galaxy M101 (NGC 5457) in the constellation Ursa Major. Both of these galaxies were once considered part of the Local Group, but are now known to be among the bright spiral galaxies located outside this group, but within the Virgo Supercluster. The galaxy was discovered by William Herschel on September 9, 1798. According to the Third Handbook of Bright Galaxies (RC3) of 1991, the galaxy has an isophote diameter in B-band of about 26.77 kiloparsecs (87,300 light-years). NGC 6946 is heavily obscured by interstellar material due to its location close to the plane of the Milky Way galaxy. Due to its intense star formation, the galaxy is classified as a starburst galaxy. NGC 6946 is also classified as a double spiral galaxy with a small inner bar, which is thought to be feeding gas into the center of the galaxy.

Several unusual celestial objects have been discovered in NGC 6946. Among them is the so-called "Red Ellipse" along one of the northern arms, which appears to be a superbubble or very large supernova remnant. It may have been formed by an open star cluster containing massive stars. Two regions with unusual dark lanes of nebulosity have also been observed, and several starless and hydrogen-free regions in the spiral arms, in some places extending up to two kiloparsecs. The third unusual object, discovered in 1967, is now known as the "Khodge Complex". This object was previously thought to be a young supergrant cluster, but in 2017 it was proposed that it may be an interacting dwarf galaxy superimposed on NGC 6946.

In the 20th and 21st centuries, NGC 6946 has hosted ten supernovae:

• SN 1917A
• SN 1939C
• SN 1948B
• SN 1968D
• SN 1969P
• SN 1980K
• SN 2002hh
• SN 2004et
• SN 2008S
• SN 2017eaw

Because of this, the galaxy is often called the Fireworks Galaxy. This is about ten times the rate of supernova explosions observed in the Milky Way, even though the Milky Way contains twice as many stars as NGC 6946.

A particular highlight was supernova SN 2004et, which was detected on September 27, 2004, with a visual magnitude of 15.2 and later peaked at 12.7. The supernova exploded on September 22, and its precursor was identified in previous images - only the seventh time a supernova has been positively linked to its parent star. The supernova precursor was a red supergiant star with a mass of about 15 solar masses in a system interacting with a blue supergiant.

In 2009, a bright star in NGC 6946 flared up to a million times brighter than the Sun within a few months. It quickly faded after that, and observations from the Hubble Telescope showed that the star did not survive. A black hole may have formed in this location, which would also indicate N6946-BH1. The progenitor was likely a yellow hypergiant star.

In May 2017, supernova SN 2017eaw was discovered in the northwestern region of the galaxy. Light curves obtained over the next 600 days showed that it was a II-P type supernova, and its progenitor was a red supergiant star, with a mass of about 15 solar masses.

Up until 2017, NGC 6946 had hosted more supernovae than any other galaxy, a record that was later surpassed by NGC 3690.

The galaxy's nickname, "Fireworks," comes from its abundance of bright regions of Hydrogen Ionization (HII), which appear red in photographs. This indicates a large number of massive stars, whose radiation not only explodes stars as supernovae, but also ionizes nearby hydrogen clouds.