Josh Smith, of Cashmere High School, asks :-

Can scientists predict when a star will collapse?

Frank Andrews, an astronomer at Wellington's Carter Observatory, responded.

No. When a star collapses violently the result is an enormous explosion. This is called a Super Nova. Only large stars, which have used up their available fuel, can produce a Super Nova.

If we look at the interior structure of a star we find that the nuclear thermal reactions which are its source of energy take place only in the central core where temperatures and pressures are extremely high. The core and surrounding outer layers of the star are supported by the energy produced in the core. Thus a normal, stable star must maintain an Energy Balance and a Pressure Balance.

The energy balance means that the amount of energy produced in the star's core must equal the amount radiated from the surface. The pressure balance requires that the pressure generated by the outward flow of energy must equal the force of gravity, which is trying to collapse the star. In this way stars like our Sun can maintain an almost constant output of energy for billions of years Should the energy flow from the core be reduced the energy balance will alter and this now effects the pressure balance. The star will contract, which under normal circumstances restores the equilibrium. As the star contracts, the pressure and temperature rises causing the rate of energy production to increase. This enables the balance to be re-established. Similar readjustments will take place if energy production increases.

When stars become old it becomes impossible for them to maintain their energy and pressure balances. As a result such stars become unstable and exhibit a number of symptoms characteristic of old age. For example: all stars except the smallest, will swell up and become red giants. They are also likely to fluctuate in brightness, pulsate rhythmically and shed matter into space as a stellar wind.

Aging stars showing the above symptoms are possible candidates for collapse. How the star collapses and what it collapses into, depend on its core mass. We call the mass of our Sun one solar mass.

Stars like the Sun are too small to become a super nova. They finish up as white dwarfs.

If the star has a core mass of more than 1.4 solar masses but less than about 6 solar masses it will become a super nova. In the core of such stars reactions will start which are "endothermic". This means that they absorb heat instead of giving it out. The result is that the energy and pressure balances are completely destroyed and catastrophic collapse of the core to a neutron star about 15 km in diameter results. The outer layers of the star are blasted into space as a super nova remnant. The resulting neutron star is rotating very rapidly, perhaps 30 times a second, and if the magnetic pole of the neutron star sweeps the Earth astronomers will observe a pulsar. Can you think of the reason why neutron stars are spinning so rapidly?

For stars with a core mass greater than about 6 solar masses the onset of the endothermic reactions also trigger catastrophic core collapse and a super nova results. In this case, however, the core of the star collapses to form a black hole.

The most massive stars known are perhaps more than 100 times heavier than the Sun. These are called "Hyper Giants". They are the stars, which are certain to become super novae and leave black holes. Such stars can be more than three million times brighter than the Sun and are extremely unstable. They evolve extremely rapidly. Eta Carina is perhaps the best known example of such a star.

Although we are able to observe stars that are excellent candidates for catastrophic core collapse we still cannot predict the exact time at which this will occur. All we can say is that a particular star is likely to become a super nova in the next few tens or hundreds of thousands of years.

Eta Carina is probably the closest example of a star that is going to become a super nova in the "astronomically near future"........But don't hold your breath. It could easily take another 100,000 years!!!!