Supernovae—the final explosive efforts of dying stars—are the most beautiful of astronomical phenomena. Usually occurring in higher-mass stars than our sun, a supernova produces an explosion so bright that its luminosity can rival that of the galaxy.

In astronomy, supernovae are used as standardized candles to understand dark energy and the expansion of the universe. Scientists can predict the distance of a supernova from earth, given average supernova brightness. If a supernova is dimmer than expected, it is farther away. When a supernova is observed moving farther away, it is said to be redshifting. This is evidence of the acceleration of the universe. However, astronomers at the University of Toronto have recently put the validity of the supernova average brightness into question.

Supernovae occur in stars with masses great enough to turn hydrogen and helium into heavier elements. Bigger stars burn more rapidly and their hydrogen fuel eventually runs out. At this point, the shell of the star begins to contract, increasing the star’s pressure and temperature and starting a reaction in which helium produces carbon. The same chain of events occurs until carbon turns into heavier elements. Each time this chain of events takes place, the star becomes more and more unstable, eventually leading to the implosion of its core. The implosion has enough force to create an outward explosion, which can be brighter than the light of ten million suns. These kinds of explosions provide an environment for the birth of new stars.

U of T researcher Andrew Howell explained in a recent interview that there are two recognized kinds of supernovae, one brighter than the other. The brighter kind has a greater rate of occurrence in galaxies where more stars are created, whereas the dimmer supernovae are more prevalent in older galaxies.

Howell and other U of T researchers from the Department of Astronomy and Astrophysics measured the brightness of a supernova and quantified it by constructing a stretch distribution.

A stretch distribution is a graph that compares the brightness of a supernova over the amount of time it takes for the explosion to subside. The greater the width of the graph, the brighter the light, because it takes longer for that light to subside. Using this technique, Howell and his team made a key discovery in their 2007 paper: they found the stretch was getting larger with greater redshift, as one moves further away from the earth.

Since light takes time to travel to Earth from far-away supernovae, the light that we see now is, in reality, from a time far in the past—closer to the beginnings of the universe and the big bang. The further we look, the further back in time we are seeing, but supernovae from earlier in time bear different characteristics from more recent ones. Supernovae have evolved so that the average ratio of bright to dim supernovae today is different from that in the past. In fact, the average supernova now is becoming dimmer as compared to a supernova from earlier in the universe’s history, when there was a much higher rate of star formation.

Howell concluded, therefore, that the averages used by cosmologists for predicting brightness will change over time. This discovery is important for the study of dark energy and the expanding universe, as a change in the average opens up possible inaccuracies in former conclusions.

Howell chose to study astrophysics because of dark energy, a substance that composes over 70 per cent of the entire universe. The nature of this substance is largely unknown. In a field with very little a priori knowledge, Howell’s discovery is important, though the bias noticed in the stretch distribution has always been corrected for, despite not being understood. However, Howell explains that in future research in this mysterious area of astronomy, even the slightest changes are sure to make a difference.

Fast Facts: Supernovae

  • Chinese astronomers observed the first recorded supernova in AD 185

  • After the invention of powerful telescopes, it became easier to observe supernovae. The 1885 discovery of S Andromedae (in the Andromeda galaxy) heralded this new age of astronomy

  • The core of a supernova collapses in on itself at a velocity close to 70,000 km/s

  • After a supernova is discovered, it is reported to the International Astronomical Union’s Central Bureau for Astronomical Telegrams and named

  • Amateur and professional astronomers discover hundreds of supernovae every year–367 in 2005 and 551 in 2006

  • After exploding, a supernova can emit the same amount of energy over a period of a few months as the sun would put out over 10 billion years

  • In a galaxy the size of the Milky Way, supernova occur on average once every 50 years