EVOLUTION OF THE GALAXIES

by Eben van Zyl

When we look at the fine gradations between the various classes of galaxies in the three-dimensional classification of de Vaucouleurs, eg SA(s) and SAB(s) we may think that the various classes of galaxies may, through the millennia, change from one class to the other, eg Sa to Sb, to Sc or vice versa. In SAB(s) the central bar is only slightly visible and it is hardly distinguishable from the class SA(s). But the dynamics of these stellar systems show that there are great differences in mass between the various classes which preclude the evolution of one class into another. The actual physical merging of galaxies into each other, does take place as is shown in cases such as the "Rattail" galaxies, NGC 4038/39, also known as the "Antennae" because of their widely flung spiral arms. The galaxies NGC 5426/27 seem to be in contact prior to merging. In Centaurus A, a spiral galaxy is apparently moving through a huge elliptical galaxy. There is so much empty space between the stars, that two galaxies can move through each other without collisions between the individual stars. In Centaurus A the Hubble Space Telescope has revealed "blue" stars in the obscuring dust streak. These are "new" stars as is shown by their low and negative B-V colour indices. In comparison the stars of the elliptical galaxy are "old", being "red" because they have high B-V colour indices. The new stars in Centaurus A have been formed "recently" (a few tens of millions of years ago) by the compression of the gas and dust between the older stars.

We may think that a lenticular (lens-shaped) galaxy may yet develop spiral arms or that the arms of a spiral may wind up into the lenticular form. But there is no evidence for this. It would appear that each galaxy evolves along certain lines and that the most important parameter determining the evolution, is the amount of mass available for star formation and the amount of matter in the surrounding space. These masses determine amount of bending of the space-time, the curvature of space, as set out in Einstein's Theory of Relativity. Mass exercises gravity and the motions of masses of matter are constrained by the curved "world lines" of force between the masses of matter and centred on the gravitational centre.

The first matter which was formed in the cosmo-genesis (or big bang) when the temperature had dropped to 10⁹ degrees, was hydrogen, together with 20 to 28 % of helium and very little heavier nuclei of atoms. The microwave background radiation, which was present from the beginning, is very uniform, but the C O B E satellite has revealed that there are minute irregularities. It is these irregularities that caused the primeval matter to coagulate in clumps. Where these clumps moved in almost parallel lines they merged into more massive clumps. When two clumps approached each other from opposite directions, they spiraled around each other, or rather around their mutual centre of gravity, their barycentre and then revolved around each other in ellipses, thus forming a double star. The sizes of the ellipses in which they revolve, are inversely proportional to the masses of the clumps: the larger the mass, the smaller is the ellipse, but the shapes of the ellipses, their eccentricities, are identical. The line .joining two revolving bodies, always passes through their barycentre S.

For a star to form in the first instance, the matter must accrete by spiraling on to a centre of gravity. This spiral motion gives spin to the resultant star. Everything in the universe has spin. As more and more matter accretes, the pressure on the centre increases and this raises the temperature because the pressure exerted by the overlying layers, forces the separate particles closer together, thus increasing their rates of collision. Each collision liberates heat. Eventually the temperature rises so high that the accreting gas begins to glow, but it is not yet a star. Probably most of the matter in the universe was left in this form, which today we call "brown dwarfs".

The temperature has to rise to about 15 million degrees before the pressure succeeds in forcing the separate hydrogen nuclei, or protons into each other in what is called the proton-proton reaction. The proton is indicated by , the upper 1 indicating its mass and the lower 1 its positive electric charge. When the two protons are forced into each other, one proton loses its positive charge and this is shot out as a positron. The resultant particle is a deuteron . Besides the positron neutrinos, of no mass, but carrying away tremendous amounts of energy are shot out. Another proton is then forced into the deuteron to form a nucleus of helium of mass 3 and charge 2: and copious amounts of gamma rays U , are set free. Two of the helium nuclei are then forced together to form the stable isotope (four helium two) and two protons are liberated and the process continues. The gammarays are the source of energy which keeps a star going.

Einstein's equation E = mc² showed the equivalence of matter (m in grams) and energy E (in ergs), where c is the speed of light in centimetres per second, and this vast quantity is raised to the second power! From as little as 4 grams of solar matter, which is converted into helium, the amount of energy liberated is sufficient to boil away more than 9,5 million litres of ice-cold water and turn them into steam!

The gammarays produced in the centre of a star are repeatedly absorbed by neighbouring particles and just as readily re-emitted. This is the nature of plasma, a form of matter which is in thermal equilibrium.

With each absorption and re-emission of gammarays, their frequency is decreased. Beginning at 10²⁵ vibrations per second the gammarays, after about 100 million years, reach the surface of the star and by that time their frequency has been reduced to 10¹⁴, which is the frequency of visible light. The eyes of humans and other animals evolved and became adapted to this frequency, giving us the faculty of sight.

The first stars that formed must have been very massive because much matter was present in a very small volume. Stars were formed in swarms, as open clusters, and in groups of hundreds of thousands, the globular clusters, and in their thousands of millions, as galaxies.

The very massive stars which formed the centres of the first galaxies, consumed their hydrogen fuel at a prodigal rate. A star of 10 times the mass of the Sun consumes its hydrogen at 1000 times that of the Sun. Such massive stars could therefore not have existed for longer than a few ten million years, before they collapsed on their centres because of the sudden decrease of radiation which had balanced the crushing gravitational force of the overlying layers. This crunch on the centre caused the helium which had accumulated there, to undergo fusion into heavier atoms - the helium flash - forming atoms such as carbon, oxygen, neon, magnesium and silicon (of mass 28). When silicon was reached, things happened very swiftly. Each two silicon atoms were fused together to form an iron nucleus (mass 56). Up to the formation of iron energy is liberated with each step up the atomic ladder. Beyond iron energy has to be supplied from outside. Iron has the most densely packed nucleus of all atoms. When iron is formed, the collapse of the centre ceases and a rebound in the form of an indescribable explosion takes place - a supernova explosion - in which the star casts off most of its substance and blows itself to smithereens.

When a star goes supernova it leaves a highly compressed neutron star in its wake -- a star equal in mass to the Sun but having a diameter of 2 kilometres. A star of 10 times the Sun's mass, or more, leaves a core of less than 1 kilometre in size. This is a gravitational vortex (or black hole) where the gravity is so great that no radiation can escape because the escape velocity is greater than the velocity of light, and such a velocity cannot exist.

A galaxy having many massive stars in its nucleus, will experience intermittent and protracted supernova explosions. Seen from afar the nucleus will appear to be variable in its light intensity and variable with short periods, as short as months. These galaxies are known to us as quasars - they are 100 times brighter than a whole galaxy and variable and emit X-rays.

The quasars have distances of thousands of millions of light years, so that we see them by the light which left them thousands of millions of years ago when the universe was very young, not more than 10% of its present age. All galaxies will have gravitational vortices in their nuclei, some small, some very big. It is calculated that the centre of the Milky Way harbours a gravitational vortex of 300 million solar masses. The time of its variability when the supernovae lit up the nucleus is long since past. The matter which those supernovae hurled out, has gone to form the spiral arms.

The galaxy M82 in Ursa Major is a good example of a galaxy which is at present undergoing an explosion in its nucleus. This exploding phase shows that an old galaxy can also turn on the fireworks, or maybe M82 is still very young.

Thousands and millions of massive stars going supernova certainly can blow a galaxy apart, thus providing the raw material from which a second generation of stars can condense These stars find themselves caught in the gravitational field of the massive black hole in the nucleus. These stars then revolve around the nucleus, the furthest ones lagging behind as shown in the sketch. The stars at A will revolve faster than those at B, further from the nucleus N, and so on up to the stars furthest from the nucleus. The dotted curve shows the first rudiments of a spiral arm.

We also have barred spirals with varying degrees of development of spiral arms, as shown in the sketch. At the top we have a spiral in which the matter at the end of he central bar has just begun to trail behind. Secondly, the trailing matter becomes more prominent and so on until the spiral is almost completely wound up.

From this we can conclude that barred spirals eventually develop into ordinary spirals.

The Sun lies at a distance of about 30 000 light years from the centre of the Milky Way galaxy. Its rate of revolution relative to the centre is 250 km per second. To revolve once around the galaxy takes the Sun ( 30000 x 9,46 x 10¹² x 2p ) ¸ 250 seconds. This equals Years, and this comes to 226 million years for one revolution around the centre. In the 4,6 x 10⁹ years that the Sun has existed, it has made 20 revolutions. ( Lunar samples brought back by the Apollo astronauts (l969 - 1972) have shown a maximum age of 4,7 x 10⁹ years. The Sun must be at least as old as that ).

D N Schramm in "The age of the Elements", determined through his study of radioactive elements that there was a peak of supernova explosions at about 9 x 10⁹ years ago and a second peak at 5 x 10⁹ years ago. The Sun and its planets must have condensed from the material cast out by the second burst of supernovae 5 thousand million years ago.

In order to produce a burst of supernovae 9 x 10⁹ years ago, the first galaxies must have formed 4 to 5 thousand million years earlier, i.e. 13 to 14 thousand million years ago. The universe was then very young, so we are forced to conclude that galaxies formed very soon after the cosmo-genesis, or big bang. It is highly probable that the first galaxies that formed were ellipticals and that globular clusters formed at the same time as islands around the elliptical galaxies, since they both have old stars, stars poor in metals.

Barred spirals soon followed and they were followed by ordinary spirals.

Whereas elliptical galaxies and globular clusters largely contain yellowish stars, stars with high B-V colour indices, spirals also contain young stars that are called "blue" because they have small and even negative B-V colour indices. The nuclei of spiral galaxies also largely contain yellowish stars, showing that the nuclear bulges of spiral galaxies are older than the spiral arms.

All galaxies follow evolutionary paths that are determined by the amount of mass they contain. The mass of the central black hole also plays a dominant role in the course of a galaxy's evolution.

Mass also plays a dominant role in the evolution of a star. Stars of less than 2 1/2 times the Sun's mass will eventually cast off their overlying layers and end up as white dwarfs. Stars of 2 1/2 up to 8 times the Sun's mass will eventually go supernova and leave a neutron star behind. Stars of more 8 times the mass of the Sun will also go supernova and leave a gravitational vortex (black hole) behind. It seems likely that all galaxies have massive gravitational vortices in their centres.

An incontrovertible fact is that all the galaxies are moving away from each other with speeds proportional to their distances - the further they are the faster they recede. The universe is expanding because the fabric of the space-time continuum is expanding. This is called the inflationary model. The question arises whether the universe will expand forever or whether the expansion will eventually come to a stop, to be followed by contraction and an eventual big crunch, leading to a repeated big bang and expansion. If this is the case, the universe is cyclic, and consists of alternate expansion and contraction phases. The cycle from big hang through expansion, back to big bang, requires a period of 120 x10⁹ years - one hundred and twenty thousand million years!

Jan Eben van Zyl