The renewed threat of an arms race has elicited a revision to the dreaded Doomsday Clock, a harbinger to nuclear and climate change disaster. The clock now stands at two-and-a-half minutes to midnight, the closest to midnight since 1953, when thermonuclear devices of immense catastrophic capabilities were being tested. And yet, the world’s entire nuclear arsenal and its potential destructive power pales in comparison to some of the more energetic events that occur rather frequently in the cosmos.

Supernovae, in general terms, are caused by the explosions of stars. The first kind of supernovae (named Type II) refer to the explosive final throes of a giant star as it runs out of fuel at the end of its lifetime. Small and medium-sized stars, such as the sun, continue burning fuel for billions of years and slowly swell up to red giants, before gradually losing most of their mass to become white dwarfs.

The same can’t be said for giant-sized stars, which go through their entire fuel supply in a few million years. This might seem paradoxical, since giant stars have several times more fuel than smaller ones, but it’s precisely their big size that causes them to burn fuel more rapidly.

Like all stars, giant stars start off by fusing hydrogen to helium in their core. This is followed by fusion of helium to carbon, and then further fusion of carbon to oxygen, neon and heavier elements. When a giant star’s core starts producing iron, its days are numbered.

Unlike all other elements that came before it, fusion of iron requires more energy than it produces. This results in a sudden loss of energy generation at the star’s core, and the balance between the star’s gravitational pressure and the core’s energy production is toppled. In a fraction of a second, the star collapses, crushing its core’s components to a very small region of space. A massive explosion ensues, followed by further collapse of the star’s material.

A second type of supernovae (Type Ia) occur when white dwarfs, the remnants of small or medium-sized stars, accrete material from a nearby star or gas cloud. On reaching a certain size, referred to as the Chandrasekhar limit, a white dwarf loses its stability and completely annihilates itself in a massive explosion.

Supernovae observations have been documented as early as 10 centuries ago. In 1054AD, Chinese astronomers witnessed a supernova (probably Type II) which many astronomers nowadays believe was the cause of the famous Crab nebula, a supernova remnant in the Taurus constellation. Just a few years before, in 1006AD, the brightest supernova (probably Type Ia) ever witnessed in recorded history was observed, reaching a brightness about 16 times that of Venus. Both supernovae occurred in our galaxy, the Milky Way, and both were visible to the naked eye. Recently, thanks to better telescopes, supernovae in very distant galaxies have also been observed.

The question that comes to mind is – will we see a Milky Way supernova in our lifetime? A few candidates do, in fact, exist. The bright red giant star Betelgeuse, easily visible with the naked eye in the Orion constellation, is at the final stages of its lifetime. Betelgeuse might explode any time between now and a million years, and its explosion would see the star become as bright as the full moon for a few months. No need to fret, however, as Betelgeuse’s supernova will have no adverse effects on Earth whatsoever, since it is too far away!

Josef Borg is currently a PhD student within the Institute of Space Sciences and Astronomy, University of Malta, and also Vice-President of the Astronomical Society of Malta.

Did you know?

• The Sun is approximately halfway through its lifetime. At about 4.6 billion years old, our sun has used about half of the hydrogen it has available for fusion in its core. It will continue burning hydrogen for about 5 billion more years, after which it would expand to a red giant.

• You can see a few thousand stars with the naked eye. From most light-polluted locations, only a few dozen bright stars can be seen. However, the naked eye would be able to make out around 6,000 stars from a very dark location, all within our own galaxy.

• A red dwarf will be the last star in the universe. Since smaller stars have a longer lifetime, the last star in the universe will be a red dwarf. In about a trillion years, the last red dwarf will gradually run out of fuel and fade away, leaving a cold, dark universe.

For more trivia see: www.um.edu.mt/think

Sound bites

• The Cassini spacecraft has entered the grand finale of its 20-year mission. The spacecraft has been in the Saturnian system since 2004, following its launch from earth in 1997. After deploying the Huygens probe in 2005, which made history by landing on Saturn’s largest moon, Titan, Cassini has been orbiting Saturn and its moons, collecting valuable information and sending incredible images of the gas giant. Cassini is expected to make a series of 22 dives between Saturn and its famous rings, until it takes a final plunge into Saturn’s atmosphere this September.

• Astronomers have discovered an ‘iceball’ earth orbiting a star 13,000 light years away. The discovery was made thanks to gravitational lensing effects. As a star passes in front of a more distant star as seen from earth, a temporary increase in brightness of the distant star can be noted. Because of the planet orbiting the star closer to us, a secondary brightness increase can be observed in the readings.