The Fireworks Galaxy has that nickname for a good reason: more supernovas have popped off there than in any other known island universe. Counting the latest, the supernova discovered by Patrick Wiggins at 8:28 p.m. on May 13, a total of 10 of these largest known explosions have occurred in the galaxy since 1917.
The Fireworks Galaxy, technically called NGC 6946, is a beautiful looping structure with odd arms, only 22 million light-years away. An ordinary galaxy like ours, which is at least twice its size, is reckoned to experience supernovas about once a century.
Wiggins, a long-time leader of the Salt Lake Astronomical Society and a resident of Tooele County, has discovered three supernovas. The latest is by far the most interesting, as it’s taking place close enough for the blast to be visible with a good amateur telescope. It has generated scores of photographs since that Saturday evening, according to the webpage at rochesterastronomy.org dedicated to this supernova, maintained by the Rochester Academy of Science.
NGC 6946 is counted among the deep-space objects in the constellation Cepheus the king, and is located on the border between Cepheus and Cygnus. A moderate-sized galaxy in the northern sky, it's visible all year but especially well in October. It is a face-on barred spiral galaxy, with a bridge of stars across the center and that confusing set of spiral arms.
Robert Bernham Jr., author of the magnificent — though somewhat outdated — set of astronomy books, Burnham’s Celestial Handbook: An Observer’s Guide to the Universe Beyond the Solar System, comments on the strange arms. "Multiple branching of the arms makes it difficult to trace the course of any one arm from its point of origin in the central mass to its gradual disappearance on the galaxy’s outer rim. There are, however, at least four well-defined arm segments, and several further branches or 'sub arms,'" he wrote in the 1978 three-volume edition published by Dover Publications.
Bernham writes that the astronomer and telescope designer George Willis Ritchey was the first to notice a supernova in NGC 6946. Designated SN 1917a, the explosion was discovered on July 19, 1917, a century ago.
According to NASA, the orbiting observatory Chandra checked NGC 6946 and found evidence of "three of the oldest supernovas ever detected in X-rays, giving more credence to its nickname of the 'Fireworks Galaxy.'" Although they had faded out optically, their energy ghosts showed up in X-ray studies. Chandra made X-ray exposures totaling 48 hours in five observations from September 2001 to December 2004.
The galaxy’s history of frequent supernovas makes amateur astronomers hopeful of catching one there. To speculate, one explanation of the many gigantic explosions might be that the complex, contradictory-looking winding arms might be triggering the formation of great stars; ultimately, the stars must die.
Astronomers studying the new supernova's spectra classified it as a Type II/P. Type II indicates it resulted from the violent and enormously bright demolition of a red supergiant star at the end of its life. The star exploded at billions of degrees, shining 10 billion times as luminous as the sun, spewing material throughout its region. Assuming the remaining core is massive enough, as seems likely, it collapsed and formed a black hole.
The "P" designation means "plateau," so the explosion probably will remain bright for weeks or months. The Rochester Academy of Sciences log shows little fluctuation in luminosity from the discovery in mid-May through July 24 —magnitude 12.8 versus 13, only slightly dimmer.
Earlier the accepted idea about the death of supermassive stars was that they die in supernovas and collapse into black holes. A more recent theory was that the very largest might collapse directly into black holes without going supernova. Today that seems no longer theory but proven fact.
The evidence is recorded in a scientific paper published April 1, only two and a half weeks after the supernova flared into Wiggins' telescope.
"The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star" by astronomers S.M. Adams, C.S. Kochanek, J.R. Gerke, K.Z. Stanek and X. Dai, was published in the Monthly Notices of the Royal Astronomical Society, based in London. Adams is with Caltech and Ohio State University, Dai with the University of Oklahoma and the others are with Ohio State University.
Earlier, Gerke, Kochanek and Stanek had noticed a possible star disappearance, through observations with the large binocular telescope on Mount Graham, Arizona. A monstrous red giant star, estimated about 25 times the mass of the sun, "experienced a weak … optical outburst in 2009" and then dimmed tremendously.
The latest report is the confirmation of the star’s vanishing, as shown by the Hubble Space Telescope. "The event is consistent with the ejection of the envelope (outer atmosphere) of a red supergiant in a failed SN (supernova) and the late-time emission could be powered by fallback accretion on to a newly formed black hole," says the paper's abstract online at academic.oup.com/mnras. It calls for future infrared and X-ray studies to confirm this first-ever-known vanishing star.
And where did the amazing disappearing act happen? In NGC 6946, the Fireworks Galaxy.