Children of the stars

By Charlie Hetherington

SciTech Editor

The Supernovae are big. Very, very big. They are so unimaginably enormous, physicists employ a rough guide to their size: however colossal you can imagine, the reality is bigger. To illustrate the principle, take the following question: which is brighter? A nuclear bomb on your forehead or the Sun (if it was large enough) going supernova seen from Earth? The answer is the supernova, by a billion times.

Supernovae are rather toasty

With this rough rule of thumb established, it’s safe to conclude supernovae are rather toasty. Indeed, anything within a 30 light year ‘kill-zone’ would be blitzed by highly ionising radiation. Any planet, such as Earth, would have its atmosphere stripped by travelling atomic nuclei, known as cosmic rays. This layer normally protects against ultraviolet light; without it, all but the most basic forms of life would be killed.

Luckily, there are no stars large enough to go supernova close to Earth. Nevertheless, ones further afield, in a cluster called the Local Bubble, may still have had steered the evolution of life on Earth. By measuring levels of a radioactive iron isotope from the exploding stars within ancient seabed, researchers at the University of Kansas determined that a major period of supernovae activity occurred within the past 10m years, showering the Earth in an abnormal amount of cosmic rays.

This period corresponds to a major shift in the Earth’s biosphere. Small amounts of charcoal in oceanic sediment indicate wildfires became much more prevalent throughout the globe. As grasses are more resistant than trees to fire, vast swathes of forest were converted to savannah. The cause of the sudden increase in wildfires was unknown, until Dr Melott and Dr Thomas proposed an intriguing explanation.

When cosmic rays slam into the Earth they interact with its atmosphere, breaking apart molecules and letting loose electrons. These electrons go on to free others in, creating cascades that culminate in a lightning bolt.

However colossal you can imagine, the reality is bigger

Recent measurements made in Armenia saw this cascading effect, confirming the theory. The researchers then calculated the effect the extra cosmic rays from supernovae had on lightning generation and found it increased nearly 50 times – the culprit unmasked.

What makes this discovery particularly interesting is the impact the shift from forest to grassland had on hominids, or human-like primates.

By forcing us out of our tarzan, tree-swinging ways and onto just two feet, our hands were freed to get up to all sorts of trouble.

Photograph by NASA

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