Thanks to a fortuitous observation with NASA’s Swift satellite, astronomers have caught a star in the act of exploding for the very first time, a major breakthrough in unravelling longstanding mysteries about how such explosions really work.
Stars much more massive than our Sun end their lives in supernova explosions as they run out of fuel for the thermonuclear reactions that power them. With no more energy being released at the star's core, the core collapses, sometimes into a superdense neutron star or a black hole. Usually supernova explosions are observed once the firework display is well under way, days or weeks after the supernova is born, but this latest supernova, SN2008D, is a rare catch; not only is it closer to Earth than any type of supernova ever observed in the act of exploding, but its gamma-ray free radiation gives the clearest picture ever of a star at the instant its explosive death transforms it into a supernova.
For four decades theorists had predicted that a bright burst of X-rays should be produced as the shock wave from the supernova blasts out through dense material surrounding the star, but capturing this precious moment would require knowing in advance where to point a telescope. On January 9, luck intervened and the impossible become reality. During a scheduled observation of the galaxy NGC 2770, a bright burst of X-rays was spotted in one of the galaxy's spiral arms. Alicia Soderberg of Princeton University, who was working on the data, immediately scrambled a 38-person international scientific team to scrutinise the new object using both orbiting and ground-based telescopes, providing key information that showed the object evolving in a pattern similar to other supernovae.
"The data confirmed that what we were seeing was indeed a supernova and not some new type of object,” says Sonderberg. “That initial X-ray burst thus is the earliest observation ever of an exploding star.”
Scientists had planned on studying supernova 2007uy in galaxy NGC2770, which was already several weeks old when seen in this visual, ultraviolet image (top left) and X-ray image (bottom left), when they got more than they bargained for with new supernova 2008d (right hand images). Image: NASA Swift team.
Vital new details from this supernova will help scientists settle longstanding controversies about the mechanisms of supernova explosions, and will allow cradle-to-grave analyses of the entire explosion process.
"We think that every core-collapse supernova will show an X-ray burst like this one,” says Soderberg. “If so, with the right instruments, we should be able to discover and study several hundred of them every year. Astronomical instruments planned for the future should then allow us to finally unravel the mystery of how these explosions occur.”
An independent group of scientists, lead by Dr. Maryam Modjaz, Miller Postdoctoral Fellow UC Berkeley, has determined that the stellar explosion was probably from a star originally more than 30 times the mass of the Sun, but with a radius less than or equal to that of the Sun, that must have shed its outer envelope of hydrogen but retained its next-inner layer of helium, placing it into the category of Wolf-Rayert stars that lose their mass rapidly by means of a very strong stellar wind. Further data suggest that the core of the doomed star is not round, but aspherical, which is important for understanding the geometry and the theories of stellar explosions.
“This newly born supernova is going to be the Rosetta stone of supernova studies for years to come,” says Soderberg.