When a star’s helium is exhausted it expands to a red giant. Candidates like the sun get off the bus at this point because they fail to reach enough temperature to burn the next element. This is the case for bodies with a solar mass of 1.44 or less. They become planetary nebulae and white dwarfs.

Incidentally, we should not confuse this limit with the condition of the supernova white dwarf. Here we are talking about a star on the point of leaving the main sequence. The white dwarf in the binary system has already completed its journey out of the main sequence and its increase in mass is a new feature in its path to destruction.

Stars far heavier than the sun, however, begin burning carbon, and then the heavier elements climbing up the periodic table towards iron. At each stage, the star expands, and contracts as heavier and heavier elements are created, and each time the process is accelerated. A star might burn hydrogen on the main sequence for billions of years. But once the process gets to the heavier elements the events occur over much shorter periods. For example, a star will burn at the silicon stage for only a few days.

A star with a solar mass of 15 will take 10 million years to process hydrogen fusion and one million years for the next stage, helium. Carbon takes just 300 years, oxygen 200 days and silicon two days. This is because more and more energy is consumed to produce the heavier elements as far as iron. Fusing iron, however, absorbs more energy than is released, shutting off its power supply. So whatever the mass of a star, once iron is formed, there is never enough energy to drive the fusion process further, outward pressure disappears, and inward gravity wins the battle.

Iron forms in the last day of life and it takes one second for the star to collapse. This happens at one third the speed of light, the collapse crushing the star down to a diameter of 25 kilometres. Even the atoms start to crush each other, and the rebound produces the greatest explosion known in the cosmos: a supernova. It is only through this extraordinary rebound that the nuclei of iron are forced to combine to form even heavier metals. It is a rare event, which is why such metals are in short supply.

Incidentally, it is thought that a double star supernova exploded in this way more than 5 billion years ago and produced pretty well all our iron. For example, the mass of Earth well demonstrates the amount and distribution of the elements. In round numbers its mass comprises iron (32%), oxygen (30%), silicon (15%) and magnesium (14%)—just four elements accounting for 91% of the earth’s mass. Sulphur, nickel, calcium and aluminium account for a further 7.5%, leaving just over 1% for all the rest, some 90 plus elements.

Nova

A nova was first termed because its bright flash was thought to be the birth of a new star. Instead, it is similar to a supernova, the death of a star. So what is the difference between a binary exploding in a supernova and a binary exploding as a nova? Both involve a massive star orbiting a common centre of gravity with a white dwarf. And in both cases material transfers to the dwarf increasing its mass. The essential difference is that a nova occurs as an explosion on the surface of the white dwarf; whereas a supernova occurs when the donated material crushes the white dwarf so much that it explodes from the centre.

In the case of a supernova binary the material transfer to the white dwarf continues to increase, triggering a fusion of carbon and oxygen, until it collapses and explodes destroying itself.

In the case of a nova outer material from the bigger star falls down onto the white dwarf finally triggering a thermonuclear explosion on its surface. The result is an increase in brightness by many magnitudes, and an increase in energy output by a factor of a million or more. The nova outburst occurs in visible light. The explosion continues until the fuel is exhausted.

Gamma rays

A supernova is certainly the most violent death of a star, and the most dangerous event in the universe. If it happened just 12 light years away it would scorch the Earth.

When a supernova occurs we see only 1/10 000th of its energy as light. By far the most outburst is in the form of gamma rays. These photons are the most powerful in the universe with the shortest wavelength of 10^-11 or less.  In this way a supernova releases trillions of times our Sun’s energy. And though this catastrophic event leaves behind a corpse, it is also responsible for the birth of everything we see around us.  A supernova is an agent of change, good and bad.

WR104 could soon go supernova. This is a Wolf-Rayet star in the Constellation of Sagittarius some 8 400 light years away. It is impossible to predict the time of this event, nor can anyone predict whether WR104 will produce a gamma-ray burst. To occur, the star has to be spinning fast enough to make an axis for the gamma-ray beam to emerge. But we do not know whether WR104 is spinning fast enough. The fact that this is a binary system could mean that the spin may be high enough. Because in such a scenario the two stars yank each other around and spin each other up.

[See also Gamma Ray Burst in the discussion on Black Holes]

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.

Last updated: Saturday, 20 March 2021