Stellar time bombs

The increasing mass of a star contributes to its explosion leading to release of dangerous nuclear radiation, which has the capacity to ruin the protective ozone layer, writes C Sivaram.

Every second, at least one massive star in the universe is exploding, after collapsing catastrophically under its own strong gravitational field.

These titanic stellar explosions, called supernovae, constitute one of astronomy’s grandest spectacles in the cosmos.

In a few weeks, they emit as much energy as the Sun would radiate in its entire lifetime of ten billion years!

One can imagine how huge this is, by considering, that every second, the Sun radiates as much energy, as mankind would consume in several million years.

So the amount of energy the Sun will produce in ten billion years is massive. And a supernova releases this amount of energy in a few weeks.

Thus, for several weeks, a supernova could outshine all the other stars in a galaxy put together.

Stellar explosions

However, since there are close to a hundred billion galaxies in the universe, even though every second a supernova explosion may be occurring somewhere, statistically speaking, on an average, a supernova is expected to occur in a particular galaxy, once or twice every century.

In our galaxy, for instance, two spectacular supernovae occurred – one in 1572 (witnessed by Tycho Brahe) and one in 1604.

Ever since, there has been a possible supernova viewed by astronomer John Flamsteed in the 18th century, now seen as the bright nebula, Cassiopeia A, believed to be the remnant of that explosion.

Earlier, the Chinese witnessed the 105u AD supernova (now hosting the Crab pulsar). In 1006 AD, there was a very bright supernova in our galaxy, which became as luminous as a quarter moon.

In February, 1987, a massive star exploded in our neighbouring satellite galaxy, the Large Magellanic cloud (LMC).

On January 21 this year, there was a supernova eruption in the galaxy M82, just 11 million light years away.

There was a report of a supernova that exploded in 2010, which appeared 30 times brighter than normal.

This has now been attributed to the light from the distant supernova being magnified by the gravitation lens effect caused by a massive galaxy lying between the source and our galaxy.

There are two main types of supernovae.

The collapse of the massive star in the LMC in 1987 caused what is now dubbed as Type II supernova.

The 1054 AD Chinese supernova was a Type II supernova.

The supernovae seen in our galaxy in 1006, 1572 and 1604, as well as the one witnessed this year in the galaxy M82, belong to Type Ia.

This kind of supernova occurs when stellar material (for example, a companion star) piles onto a white dwarf, pushing it over the Chandrasekhar limit.

Most stars, including the Sun, end up as white dwarfs after they exhaust nuclear fuel in their cores.

In these lower mass stars, the core is predominantly carbon and oxygen (the stars are not massive enough to trigger thermonuclear reactions that produce heavier elements).

With no further nuclear reactions taking place, the core collapses and forms a dense stable remnant called a white dwarf, which would cool over several billion years.

However, if a white dwarf becomes more massive than about one-and-a-half times the solar mass, its gravity would make it collapse, heating it to a billion degrees.

At these temperatures, the entire star would detonate as a gigantic nuclear bomb and would be blown to smithereens. The nuclear reactions would end up producing mainly Iron.

Ticking time bomb

A white dwarf in a binary system could cause matter to accumulate on its surface continuously, and if pushed over the limiting mass, would explode as described above.

So, in this sense, it is a gravitational time bomb. Is there a time bomb like that in our neighbourhood?

Apparently, there is a binary star system called IK Pegasi, which consists of a white dwarf (close to the Chandrasekhar limit) with a companion star, which is separated from it by one-third the distance between the Earth and the Sun.

So, on a short-time scale, astronomically speaking, the white dwarf could explode. This system is only about 150 light years away.

The gamma rays from an explosion at this distance could end up destroying much of our ozone layer with consequent damage to the biosphere.

The nearest candidate for a Type II supernova is the giant star Betelgeuse, hardly 400 light years away.

This star is now observed to be shrinking rapidly and could explode in a few 1000 years (or less), again, generating harmful radiation, which could deplete the ionosphere and produce more nitrous oxides in the atmosphere.

A supernova blast at a point 100 light years away could destroy perhaps one third of the planet’s ozone layer. IK Pegasi is dangerously close to this.

It has been estimated that in the past two billion years, there could have been a few supernovae within 20 light years of the earth and they could have destroyed much of the ozone and caused much biological damage.

The abundance of the Iron-60 layer found in ocean sediments is proof of the existence of a supernova about two million years ago.

Far more dangerous than supernovae, are gamma ray bursts, which release in ten seconds, the energy equal to what the Sun emits in its ten billion years of lifetime.

Several such events occur everyday, but are mostly very far away. Two neutron stars merging could lead to a gamma ray burst, which even at a distance of 2000 light years can be lethal enough to destroy all life on Earth.

Indeed, the short period binary pulsar discovered a few years ago, is just that distance from us, and could merge in less than 1,00,000 years.

Another deadly gravitational time bomb in our vicinity.

There may be even undetected neutron star binaries nearby. At least, one of the earlier mass extinctions on earth (perhaps the one in the Devonian period) is attributed to either a supernova or a gamma ray burst.

Life in other worlds, present in locations or neighbourhoods like the galactic centre or closer to denser stellar habitats could well have been obliterated by these gigantic celestial time bombs.