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The life of stars between 11 and about 50 times the mass of the Sun

The Crab Nebula
The Crab nebula. The expanding debris from a supernova recorded by the Chinese, that exploded on 4 July 1054, just before the Norman Conquest of England.
(© European Southern Observatory)
These stars live comparatively faster and more dramatic lives than lower mass stars. Like all stars, they start by converting hydrogen into helium but they are at least 100,000 times brighter than the Sun and have surface temperatures of 30,000 K or more. A star 25 times the mass of the Sun gets through its life about 1000 times faster. Because there is more mass present, as each nuclear fuel is exhausted in the core the force of gravity is able to increase the central pressure and temperature and fuse the end product of the previous nuclear reactions to produce elements of ever increasing atomic weight.

If we were to peer into the core of such a star as it nears the end of its life, we would see a structure rather like that of an onion. In the outside layers we would find hydrogen being fused into helium. In successively deeper layers we would see helium being fused into carbon and oxygen, carbon being fused into neon and magnesium and so on up the periodic table of elements. At the very centre of the star where the temperature is about 8,000 million K, a core of iron is being created through the fusion of silicon. In a star twenty times the mass of the Sun this process only lasts for about a week. One reason for the increasingly rapid rate of evolution as heavier fuels are fused is that, at the higher temperatures required, increasingly more energy is lost in neutrinos. These fly straight out of the star instead of taking millions of years to diffuse to the surface as do the photons.

A property of the atomic nucleus known as its 'Binding energy' means that elements with atomic weights less than that of iron can be fused to create heavier elements, with the simultaneous release of energy, while elements heavier than iron release energy when split into lighter elements. This means that once the core of a star is made of iron it is no longer possible to produce more energy merely by compressing it to start a new fusion reaction. The force of gravity is indifferent to this problem and, in the absence of an energy source, will compress the core and raise its temperature to about 10,000 million degrees. At this temperature the photons split the iron nuclei into protons and neutrons. Thereby undoing millions of years of nuclear synthesis in a fraction of a second. The protons combine with the free electrons to form more neutrons, releasing a huge flux of neutrinos and in about one tenth of a second an iron core that was about 12,000 km in diameter collapses to form a neutron star about 20 km in diameter. This leaves the outer layers of the star unsupported, which now collapse and bounce on the dense, virtually incompressible neutron core. As a result, a large amount of Gravitational Potential Energy is released, in an enormous explosion called a supernova of 'Type II'. The name Type II distinguishes them from supernovae of Type Ia, which have a completely different origin. Type Ib supernovae on the other hand, are caused by a similar mechanism to Type II but take place in more massive stars.

X - rays from the crab pulsar
Part of the Large Magellanic Cloud, at a distance of 165,000 light years, before and after the explosion of the supernova SN1987A in February 1987.
(© David Malin, Anglo Australian Observatory)
During the course of this huge explosion many heavy elements are created, as the flood of neutrons collides with the nuclei of elements made in the outer layers of the core before the explosion. Silicon and sulphur atoms combine to form radioactive nickel, which then decays into radioactive cobalt and subsequently into iron. Evidence for this was seen in 1987, in the fading light curve of a bright supernova called SN1987A, which for a while was entirely powered by radioactive decay energy, beautifully confirming the theory of what happens during such an explosion.

X - rays from the crab pulsar, The neutron star
The neutron star at the heart of the Crab nebula is also a pulsar that rotates 30 times per second.
(© NASA/Chandra Space Observatory)
For a few weeks after the explosion the supernova can outshine an entire galaxy of 100,000 million stars. This is in spite of the fact that only 0.01% of the energy is in the form of light and most of the remainder is in the form of neutrinos. Neutrinos are particles that travel at almost the speed of light and only very weakly interact with matter, to the extent that it would take a light year thickness of lead to stop them all. In spite of this, during the explosion of SN1987A, about 10 neutrinos were detected from it, after they had passed right through the Earth.

Shortly after the explosion, if the star is large enough, it is possible that the neutron core collapses and becomes a black hole. Otherwise the neutron star, which may be spinning 30 times per second, can be detected as a pulsar. But only if the beam of electrons coming from its surface is orientated towards the Earth. Then we can detect a radio or light pulse, each time the beam sweeps past us, like the flash of light from a lighthouse.

Meanwhile the debris from the explosion streams out into space at a speed of about 5,000 kilometers per second, mixing with the existing interstellar gas. After 20,000 years it will have slowed down and formed a roughly spherical cloud of gas about 100 light years in diameter. Eventually this debris will completely mix with other dust and gas clouds and one day, become part of another star.

The Vela Supernova remnant
Part of the Veil nebula. This is the remains of a supernova that exploded 5,000 years ago and is still expanding at 160 kilometers per second.
(© David Malin, Anglo Australian Observatory/Royal Observatory Edinburgh)
Supernovae are the source of many of the heavy elements such as iron, cobalt, nickel, titanium, silver and gold that we find on Earth. The Earth contains material from many supernovae that occurred before our solar system was born.

There is also evidence from the isotopes seen in meteorites that the expanding debris from a nearby supernova may have triggered the collapse of the gas cloud out of which our Sun and planets were made.

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