Astronomers from Ohio State University have calculated the odds that a supernova occurring within our galaxy will be visible from Earth sometime within the next 50 years. The odds are nearly 100 percent that such a supernova would be visible to telescopes, in the form of infrared radiation. However, the odds dip to 20 percent or less that the supernova would be visible to the naked eye in the night sky.
The calculations mean that astronomers have a very great chance of being able to detect a supernova fast enough to observe what happens at the very beginning of a star's destruction. Though supernova remnants have been observed, no one has yet been able to witness the beginning of a star’s demise.
Supernovae are stellar explosions that occur at the end of life for stars more massive than our Sun. The massive star explodes when it has used up all its hydrogen fuel. Its core collapses just before it explodes, ejecting most of its mass into space. Supernovae can be triggered in two ways: either by the sudden re-ignition of nuclear fusion in a compact star; or by the collapse of the core of a massive star.
Though scientists can observe these stars go supernova in other galaxies, they cannot 100 percent understand how it occurs. Through calculations and computer models, astronomers have worked out the physics of supernovae; their models do seem to watch what is seen in the sky. Technologies have now advanced to the point where astronomers can learn much more about supernovae by catching the next one in our galaxy and studying it from the beginning of its demise. Being able to measure the changes in infrared radiation from start to finish of a supernova will prove or disprove current theories.
Astronomers today have sensitive detectors for neutrinos, which are sub-atomic particles emitted from the core of a collapsing star, and gravitational waves, which are created by the vibrations of the star’s core. Detecting neutrinos and gravitational waves helps astronomers hunt supernovae occurring in the Milky Way. We may not be able to actually ‘see’ light from any supernova as our galaxy is filled with dust, which absorbs light and therefore might obscure a supernova from our view.
A supernova within our own galaxy would allow neutrino detectors and gravitational wave detectors to take measurements, as the machines’ sensitivity does not allow them to take measurements from other galaxies. Supernovae outside our galaxy are detected reasonably often, whereas those within our galaxy are detected about twice a century. Galactic dust dims the optical light from stars near the centre of our galaxy by a factor of nearly a trillion by the time the light reaches us infrared light is not as affected by this dust and so is only dimmed by a factor of 20.
Balancing all these factors allowed the astronomers to work out that they have a nearly 100 percent chance of witnessing a supernova within the Milky Way during the next 50 years. In an ideal scenario, neutrino detectors like Super-Kamiokande (Super-K) in Japan would sound the alert as soon as they detect neutrinos and would also indicate the location from which the particles were coming from. As supernovae expel neutrinos immediately after the explosion but don’t brighten in any light until minutes, hours or days later, infrared detectors could be honed onto the location pinpointed by the neutrino detectors and catch the supernova before it brightens. The same would happen with gravitational wave observatories such as LIGO and AIGO.
Not all neutrinos come from supernovae. Some come from nuclear reactors, and others come from Earth’s atmosphere or from the Sun. supernovae would cause short bursts of neutrinos to be detected within seconds of each other, but occasionally glitches in the electronics do the same thing. Co-author Mark Vagins, an American neutrino expert working at Super-K, along with others has built a scale model of a particular kind of neutrino detector in a new underground cave in Japan. Vagins and co-author John Beacom, professor of physics and astronomy and director of the Center for Cosmology and Astro-Particle Physics at Ohio State, have described the new detector, called EGADS for "Evaluating Gadolinium's Action on Detector Systems”.
EGADS is only 200 tons, and therefore much smaller than the Super-K, which is 50,000 tons. Both facilities are composed of a tank of ultra-pure water. The water in EGADS is spiked with a small amount of the element gadolinium, which has a great affinity for neutrinos. When a neutrino from a supernova within the Milky Way enters the tank, it can collide with the water and release energy and some neutrons. The gadolinium will absorb the neutrons and then re-emit its own energy. The resulting signal would resemble a heartbeat – with one detection signal followed by another a microsecond later. It is hoped tat EGADS’ signal will allow neutrino detector teams to identify supernova signals much more accurately. The Super-K scientists hope to add gadolinium to the tank as early as 2016.
For those of us who do not have access to telescopes which can detect infrared radiation, the odds of seeing a Milky Way supernova with our naked eyes are low, and depend on our latitude on Earth. Johannes Kepler spotted the last supernova in our galaxy in 1604, when he was in northern Italy. That supernova was 20,000 years away in the constellation Ophiuchus. The probability of a galactic supernova being visible with the naked eye from somewhere on Earth within the next 50 years is about 20-50 percent, and more than likely people in the southern hemisphere will be more likely than most to see it because they can see more of the galaxy in the night sky.
The results will appear in an upcoming issue of The Astrophysical Journal; the unpublished paper is available here. Scott Adams, the head author of the study, has summarised the findings in a video: