Violent universe
The cosmos is not the serene place it first appears… it is full of aggressive offenders
The cosmos is not the serene place it first appears – it is full of aggressive offenders
As ultraviolet radiation cascades down from space, Earth’s place as a life-friendly planet is under threat. The surface of our world is bathed in this high-energy radiation, and it penetrates our oceans to depths of 75 metres (246 feet). The damage is catastrophic. The intense glare fries ocean plankton, and those organisms which survive divert all their energy to repairing their DNA rather than photosynthesising. Oxygen levels then drop as carbon dioxide levels rise.
The knockout blow for many larger species comes as dwindling plankton numbers offer scant food resources, with the effect rocketing up the food chain.
Thankfully this fictional future is very unlikely, but what would have caused such devastation? The answer is a gamma-ray burst (GRB). They often emit as much energy in a few seconds as our entire galaxy does in a year. If such a volley of gamma rays were to strike our planet, it would rip molecules in our atmosphere apart, releasing an army of ultraviolet photons to devastate the world’s biosphere. Luckily these catastrophic events are rare, particularly in our own Milky Way. In fact, we’ve never observed one in our galaxy. Even if one were to go off, it would have to be aligned almost perfectly with Earth. However, studying them reveals just how violent our universe can be.
As the majority of GRBs observed by astronomers are in distant galaxies, working out exactly what causes them is notoriously tricky. However, most researchers agree that they are related to the death of stars. When a star at least eight-times bigger than the Sun begins to die, it alters the way it fuels itself. During the main chunk of its lifetime it was converting hydrogen into helium via nuclear fusion. However, it now starts to fuse helium into carbon and begins to bloat outwards. The core continues to fuse heavier and heavier elements until it creates a dense, iron core. The core then collapses under its own weight before rebounding and sending a shock wave through the star’s outer layers, causing it to explode as a Type II supernova – an event so energetic it can outshine all the stars in a galaxy and be seen even during daylight hours.
GRBs are associated with a particularly vicious form of supernova, known as a hypernova, which can be up to 50 times more energetic than the ordinary variety. They are thought to form when the iron core of a star at least 30 times more massive than the Sun collapses to form a black hole. Twin jets of energetic radiation surge away from the region close to the black hole, and the gamma rays are believed to be produced by collisions between the jets and the outer layers of the star.
At least that’s the favoured origin mechanism for around 70 per cent of observed GRBs – those that last longer than two seconds. These are known as long gamma-ray bursts. However, roughly 30 per cent of GRBs are much shorter, typically lasting just 0.2 seconds. Rather than the collapse of a single star, these short gamma-ray bursts are believed to originate from the collision of two neutron stars
– the remnants of dead stars smaller than those which form black holes. The size of a city, a neutron star is so dense that a teaspoon full of its material weighs more than a mountain range, and their magnetic fields can be a trillion-times stronger
than our Sun’s. Computer simulations show that when two neutron stars spiral inwards towards each other they merge to form a black hole. The magnetic fields merge, too, and the overall strength of magnetism is boosted by a thousand times. Eventually the new black hole’s magnetic field aligns into jets, which can power a short GRB.
Not content with being responsible for these mighty events, neutron stars are also thought to be behind another of the universe’s rogues’ gallery of violent offenders: magnetars. Souped-up versions of single neutron stars, they boast a significantly stronger magnetic field. It is estimated that ten per cent of neutron stars end up as this enhanced variety. With a rotational period of at least one second, they also spin more slowly than their traditional counterparts, which can rotate dozens of times in the same period. Their stronger magnetic field can yield intense flashes of gamma rays and X-rays, but not for long – the typical lifetime of a magnetar is just 10,000 years, a blink of an eye in astronomical terms. There are thought to be at least 30 million extinct magnetars in our Milky Way galaxy alone.
Sticking with the merging remnants of dead stars, there is another type of supernova which is sometimes the result of two white dwarfs colliding together. Less massive than neutron stars or black holes, white dwarfs are the leftovers from stars about the size of the Sun. If two Sun-like stars were orbiting around each other in a binary system, when they both die the resulting white dwarfs can collide with each other. However, there is a limit to how massive a single white dwarf can be – known
“a neutron star is so dense that a teaspoon of its material weighs more than a mountain range”