The University of California - Berkeley announced precovery observations of supernova SN 2014J last week. This is the supernova in the M82 galaxy that college students discovered in late January. The press release and accompanying arXiv preprint describes how astrophysicists at the Lick Observatory combined data from "a Japanese amateur" with archived observations to estimate when the supernova's first light appeared.
That amateur is Koichi Itagaki, the most prolific supernova hunter in Japan with over 80 discoveries to his name. The CEO of his family's snack food company, Itagaki-san searches for supernovae from his personal observatory in the forested mountains outside Yamagata City. Itagaki-san describes how chance led him into amateur astronomy and the search for comets and then supernovae in an interview with Mitsubishi Motor's Starry Sky Project.
When I went on to junior high school, my father said I could get either a telescope or a microscope. I couldn’t make up my mind about which I wanted, so I stuck out my finger, gave it an eenie meenie miney mo, and ‘it’ turned out to be the telescope.
His prolific production of high-quality supernovae observations makes Itagaki-san a frequent collaborator in professional research. A quick search on the Astrophysical Data System finds 271 articles listing Itagaki-san as a co-author. Observations Itagaki-san made of two stellar explosions in 2004 and 2006, for example, turned out to be a double explosion of the same star. The team led by Queen's University Belfast that studied this first-ever event listed Itagaki-san as a co-author in the arXiv preprint and the published Nature paper.
His latest contribution to professional research is important because SN 2014J is a Type 1a supernova. These are relatively rare supernovae that occur in a galaxy once every hundred years. They are rare because they start as stars like our Sun that don't explode at the end of their lives. When the Sun's core consumes its hydrogen and helium fuel gravity will collapse the carbon and oxygen-rich core into a dense white dwarf star. Fusion reactions stop leaving the white dwarf -- the dying ember of the once-burning star -- to cool and fade over billions of years. Sometimes, though, a white dwarf gets a last burst of life. If it is part of a closely-orbiting binary system, the white dwarf will rip material away from its companion star. The accreting plasma from the companion increases the white dwarfs mass and compresses the core. At a critical point that pressure becomes strong enough to trigger an uncontrolled carbon-carbon fusion reaction that blasts the star our of existence. For a few weeks the star outshines its host galaxy.
Type 1a supernovae are so similar that astronomers can measure the distance to one of these supernova based on the brightness of the explosion. Research into the evolution of galaxies, the distribution dark matter, dark energy, and the expansion of the Universe all start with galactic maps based on supernova measurements. That makes the brief stellar outbursts a subject of intense study by astronomers around the world. Several sky surveys such as the intermediate Palomar Transient Factory, the Lick Observatory Supernova Search, and the newly-launched Gaia space telescope scan the skies every night searching for changes in stellar brightness that could be a supernova. Software algorithms processes the data and broadcasts alerts when it detects a new supernova. The world's professional observatories then turn their telescopes to collect data on the exploding star.
But no algorithm is perfect. Clouds and moonlight can ruin the quality of observations. In the case of the Lick Observatory Supernova Search, the M82 galaxy is so close and bright that the algorithm couldn't recognize SN 2014J among all of the other stars in the galaxy. Fortunately SN 2014J may be the closest Type 1a supernova to occur in decades making it easy for amateur astronomers around the world to see through their telescopes. The credit for discovering the supernova went to students at University College London who were learning how to use the university's observatory on January 21. The Lick astronomers searched their archives and found normal images of M82 recorded on Jan 14.365* followed by images with the supernova on Jan 16.381 -- a two-day span during which the supernova's light first appeared. Itagaki-san's observations on Jan 14.559 and 15.571 narrowed that to a single day. His observations let the Lick team calculate the time of the supernova's first light -- only twenty hours before amateur astronomer Itagaki made his observations.
*In this format days start at noon with fractions representing hours, minutes, and seconds. Jan 14.365 would be January 14 ~20:45 while Jan 14.559 would be January 15 ~01:25.