When most people hear the word supernova, they envision a massive star reaching the end of its life and exploding outwards to leave a ghostly remnant in its place. This is called a Type II supernova — the spectacular Supernova 1987A, which recently celebrated its 30th anniversary, was a Type II. Alternatively, a Type Ia supernova occurs when a white dwarf, the remnant of a Sun-like star, grows too massive after stripping a binary companion star of its outer layers. When the white dwarf reaches a critical mass, a runaway fusion reaction occurs in its core and the star explodes in a Type Ia supernova. Such a supernova has just been spotted occurring in a galaxy about 55 million light-years away.
The supernova which was officially announced via Astronomer's Telegram after an excited tweet by Rachael Beaton at the the Observatories of the Carnegie Institution for Science in Pasadena, CA, and known as 2017cbv (though Beaton has nicknamed it Bob), the explosion was spotted in NGC 5643, a spiral galaxy in the constellation Lupus. The area of the sky it inhabits is also part of the area covered by the Carnegie-Irvine Galaxy Survey, a project aimed at gathering optical and near-infrared images of bright Southern Hemisphere galaxies. NGC 5643 was also the home galaxy of SN 2013aa, which occurred in early 2013.
Type Ia supernovae play an extremely important role as rungs on the astronomical distance ladder that allows astronomers to measure the distance to faraway galaxies. They’ve also played a critical role in measuring the accelerating expansion of the universe. Because they occur in white dwarfs of exactly the same mass every time (that critical mass mentioned earlier: about 1.4 times the mass of the Sun), Type Ia supernovae are always the same brightness, which means astronomers can use them as standard candles. Knowing how bright the explosion is in terms of absolute luminosity allows astronomers to then work backwards to calculate the distance to the object based on how bright it appears.
But the word “exactly” is perhaps a bit misleading. Not every star system in which a Type Ia supernova occurs can be exactly the same. Moreover, events in the real world do not always reflect the precise nature of theoretical calculations — as in, some white dwarfs might explode at a mass slightly under 1.4 solar masses, while others might grow a little heavier than this limit before exploding. The fact that 2017cbv is the second recorded Type Ia supernova to occur in NGC 5643 is thus extremely valuable. By comparing the distance to the galaxy as calculated from each supernova, astronomers can better characterize the real-world variance in supernova Type Ia magnitudes that occur, which in turn will improve the accuracy of using these events to measure distance.