THE DEATH OF STARS

After a star has cast off its upper layers of gas, the rest which collapses on to the core must not exceed the Chandrasekhar Limit which is 1,44 solar masses. If the remainder exceeds this limit it will go on collapsing to become a neutron star. Whereas the size to which the white dwarf collapses is about the same as the size of the Earth, the neutron star collapses to a size of about 20 kilometres. The density of a white dwarf is 88 kg per cubic centimetre (8000 times the density of lead) while that of a neutron star is the same as that of a large atomic nucleus, 2,7 x 10¹⁴ grams per cubic centimetre. It is all that is left when a star undergoes a supernova explosion - the rest gets blasted into space.

Stars that are more than 2,5 times the mass of the Sun cannot blast off enough material to be left with less than 1,44 solar masses and they therefore go supernova and collapse down to the neutron stage or even down to a black hole.

Stars of mass 5 times that of the Sun spend only a few hundred million years in the Main Sequence of the Hertzsprung-Russell diagram whereas sun-like stars spend 10 milliard years there. Stars of 10 solar masses spend only 30 million years in the main sequence. Sanduleak -69° 202 in the Large Magellanic Cloud had been well studied and was known to be a star of 8 - 10 solar mass, a B3 giant of absolute magnitude -6,5. It went supernova on 24 February 1987 when it was photographed in the act by Professor Ian Shelton of Toronto University while he was in Chile. The light curve of this supernova SN 1987A, was the first curve to show a drop in brightness due to the collapse before the bolometric magnitude showed an increase in brightness of magnitude 1,6 to a broad peak, typical of Type II supernovae.

The Hubble Space Telescope succeeded in photographing rings around the star which are due to light reflected from the layers of gas which had previously been blown off from the core of the star, previously being some hundreds of thousands of years ago!

Although the bubbles of gas blown off from a supernova, gradually dissipate into space and become invisible, the region in the immediate vicinity of the Large Magellanic Cloud shows some 30 such bubbles as remains of supernovae that must have taken place not so many millions of years ago. The brightest stars in the Magellanic Clouds have the same absolute magnitudes as the brightest stars in the Milky May Galaxy that are due to go supernova eventually.

Many white dwarfs have companion stars. Because the gravity of the white dwarf is so highly concentrated, the white dwarfs in binary systems strip gas off from the companion stars so that the masses of these white dwarfs increase. When their masses exceed the limit of 1,44 solar mass they undergo mighty supernova-explosions in which all the matter of the star is blown into space and no residue in the form of neutron stars is left. This type of supernova is the mightiest known and it is typified as Ia. These Ia-supernovae are being monitored at the very edge of the observable universe, 6 to 10 milliard lightyears distant.

The redshifts of these Ia supernovae show that their speeds of recession are greater than those calculated from the Hubble formula V = H_oD. The astronomers studyinq these Ia supernovae therefore conclude that the expansion of the universe is accelerating. This goes against the laws of dynamics which state that the rate of expansion of the universe must decrease and eventually come to zero. What the astronomers who propound the idea of an acceleration in the rate of expansion, lose sight of, is that the light from the Ia supernovae left their sites of origin 6 to 10 milliard years ago when the rate of expansion of the universe was greater than it is now, thus proving that the rate of expansion is continually decreasing. .

It has been calculated that the rate of expansion of the universe will come to zero after 60 milliard years. Then an ever-accelerating rate of increase of contraction will follow for another 50 milliard years, to be followed by another big bang. The universe is thus cyclic by nature, having an endless series of expansions and contractions. This dispels of the question: "What was there before the big bang?" Before the big bang there was a previously contracting universe which led to the singularity of the big bang and before that there was an expanding universe and so on ad infinitum...

Throughout the life of the universe, the matter spewed into space by supernovae is recondensed into succeeding new stars and planets. The most massive stars were the first to go supernova, to be followed later by less massive stars. D N Schramm in his treatise "The Ages of the Elements" found that there were two peaks of supernova explosions after the big bang, one 9 milliard years ago and another 5 milliard years ago. It was during this last peak of supernovae that the Sun and its planets were born. The Sun is thus a third generation star since the big bang. The death of stars therefore means the regeneration of new stars!

Jan Eben van Zyl