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17 October 2015

The Last Supernova in the Milky Way

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Astronomers around the world are eagerly awaiting the appearance of the first supernova in the Milky Way galaxy for over four hundred years. The last one was “Kepler’s star,” which, in 1604, was visible with the naked eye in the constellation Ophiuchus. Although named after Johannes Kepler (1571-1630), he did not discover it. However, it was the German astronomer, best known for his laws of planetary motion, who conducted the most comprehensive observation of the supernova for over a year after detecting it on October 17, 1604. Kepler thought he had detected a new star, which outshone all others at night and was even visible during the day for over three weeks.

X-ray, optical & infrared composite image of Kepler’s Supernova remnant. Credit: NASA/ESA

Today we know that what Kepler observed was not the birth of a star but rather its sudden death, in the form of a violent explosion. These explosions offer an explanation for why there are elements on Earth heavier than iron, which is impossible to produce during the normal life of a star. Furthermore, supernovae leave behind stellar remnants, whose study is very helpful for increasing our knowledge about the mechanisms that produce supernovae, and their investigation raises new questions.

One of these questions, for example, concerns dark energy. While studying supernovae, a group of scientists concluded that, contrary to what had been believed up until then, the expansion of the universe was accelerating (and for this discovery they won the Nobel Prize in Physics in 2011). The problem is that the mass of the universe cannot explain this acceleration. One had to assume that gravity acted in a different way, pushing masses away from one another, not drawing them together.

In this way a new energy entered the picture, a “dark” energy residing in empty space. And since energy is equivalent to mass, dark energy meant a new contribution to the total mass of the universe, though a different type of matter – dark matter. We now know that about 3% of the universe is composed of ordinary matter, 30% of dark matter and 67% of dark energy.

Although Kepler’s supernova was not a unique phenomenon (just 30 years earlier a very similar one had been observed in the constellation Cassiopeia), no other supernova that has occurred in our galaxy has been observed since. According to astronomers, there are a few candidate stars in the Milky Way that could become supernovae, such as Eta Carinae, located about 7,500 light-years from Earth. Perhaps it has already exploded. If it had done so in the Neolithic period, we might see in the near future, but if it went supernova today, it would take another 7,500 years before humanity could observe it.

More information on the science of supernovae in “The world after the Revolution: Physics in the Second Half of the Twentieth Century”

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