Stellar Evolution

Stellar Evolution An essay by David.R.Gilson When we look up at the night sky, it is hard to imagine that those tiny points of light are actually fiercely burning balls of gas, just like our sun. As with life on earth, there is a great variety in the state of each individual star. Each was born at a different time, and each is at a different point in it's life. In the void of space there are many free atomic particles (almost all hydrogen). Over the great distances that separate these particles the only force that acts on them is the gravitational forces that radiate from all the other bodies of matter in the universe. When there are enough of these in a certain volume of space, the gravitational force that they all feel is strong enough to start to pull them all towards their respective centre of mass. At this point in a star's evolution, the amount of material that is participating in the gravitational collapse is the controlling factor in how the star will develop and, eventually, meet it's fate. As the cloud continues to collapse, it develops a low opacity due to the increasing density. The loss in gravitational potential energy (relative to the centre of mass) causes all of the particles to radiate infra-red radiation. This gives a rise to the cloud's luminosity and hence gives it a dull red glow. This is known as the "Helmholtz - Kelvin Contraction". As the cloud collapses further still, the temperature rises as the particles interact more and more with each other trying to push apart because of the repulsive electrostatic forces. The opacity of the cloud increases significantly, the result of this being that the luminosity falls. The great increase in temperature triggers nuclear reactions in the core of the proto-star, where the density is at it's greatest. This is known as the "Hayashi Stage". Nuclear reactions within the core are now firmly established. The remaining dust cloud surrounding the star is dispersed by solar wind or radiation pressure. The body is now a fully fledged star. This is called the T-Tauri stage. The fuel that a star 'burns' is atomic hydrogen. Hydrogen atoms collide with other hydrogen atoms so energetically that they undergo nuclear fusion. In such a nuclear reaction initially, two hydrogen atoms fuse to form Deuterium, an isotope of hydrogen. 1H+1H --> 2H + Anti-Electron + Neutrino This continues as shown to form Helium (4He). 1H + 2H --> 3He + Gamma ray 3He + 3He --> 4He + 2(1H) To give some idea of the scale of energy produced, (going by the best estimates) the sun consumes 607.6 million tons of hydrogen per second, and produces 603.2 million tons of helium per second and looses 4.4 million ton per second as radiated energy (described by E=mc2 ). As already mentioned, the mass of the star determines what route the star will take through it's life. The life history of a star can be charted on what is known as a "Hertzprung Russell Diagram" (HRD). This is a plot of spectral class (which indicates the surface temperature and apparent colour of a star) against the absolute magnitude of the star. (the absolute magnitude is defined as the apparent brightness of star if it were ten parsecs away. A parsec is defined as the distance to an object that subtends a parallax angle of one arc-second, this is approximately 3.09x1013 Km.). If all of the known stars are plotted on an HRD, a correlation of stars called the "Main Sequence" exists. This is where all of the 'middle aged' stars lay. On a HRD, a newly born star enters the main sequence at the bottom and travels upwards with time. When the star enters it's next stage of evolution it leaves the main sequence, the point at which it leaves (and consequently, where it ends up) depends on it's mass. The stages of stellar evolution that are known to us are now described in the following text. If a star is around 0.1 Solar Masses (i.e. one tenth of the mass of the sun), it forms what is known as a

"Red Dwarf". This is a small red coloured star, it is the least dense type of star. It is because of the low density that the that the nuclear reactions are less vigorous and hence only give a dull red appearance. Red Dwarfs however, are the longest lived stars because they use up their nuclear fuel at a lower rate than other stars. When Red Dwarfs die they simply disperse all their matter into space (a dignified affair). Stars between 0.1 and 1.4 solar masses (the class of star that the sun belongs to) , will burn their fuel for about 10,000 million years. Using up hydrogen and then their helium. As the fusion products approach heavier elements such as carbon and iron, the nuclear reactions produce less energy and become less frequent (when the fusion products are completely iron no more reactions occur. Fusion does not produce any energy with elements heavier than iron). At this stage the nuclear reactions spread from beyond the core to use up the remaining hydrogen. This causes the star to swell enormously in size. Such a star is called a "Red Giant". The size of a red giant can be several time the radius of the earth's orbit around the sun. It should be noted that the sun is expected to befall this fate and that Mercury, Venus and possibly the earth will be encompassed by the expansion of our sun. When a red giant has reached it's maximum size, the outer layers of gas drift off into space. Such a layer of gas is called a "Planetary Nebula" (the name was given by William Herschel in 1785, who said that they looked like small disks of planets). At the centre of the resulting planetary nebula is an intensely bright star, that is about the size of the earth, which is called a "White Dwarf". The planetary nebula forms only about 10% of the mass of the previous red giant, so the white dwarf contains about 90% of the mass of the star in a volume of that of the earth. White dwarf's are very dim and very dense. The internal structure was postulated by S. Chandrasekahr, "Electron degenerate matter - all electrons in ground state." A white dwarf star has no internal heat source, it just radiates what heat it has into space. As time goes on, the white dwarf becomes dimmer and eventually becomes a black dwarf (definitely not to be confused with a black hole) and fades into oblivion. Massive stars, larger than 3 solar masses, are destined to have a much more exotic life. These massive stars burn more brightly (and mostly are blue to white in colour), than other lighter stars, before they leave the main sequence. As they get older the nuclear reactions spread beyond the core, as in their lighter counterparts. Also, as in common with lighter stars, these massive stars undergo an expansion. When they expand they are called "Super Giants". As the super giant expands the ensuing nuclear reactions spiral out of control and culminate in an implosion and then a brilliant explosion called a Supernova. Such an explosion can release so much energy that it can outshine entire galaxies for a limited period of time. The remnants of a supernova form what is known as a Nebula, a ghostly cloud of stellar gas. Nebulae form a nursery for new stars to develop - Stellar recycling. At the centre of the remains of the dead star is what is known as a Neutron Star. This is a star that is composed purely of neutrons. This is so because the extreme pressure caused by the initial implosion triggers nuclear reactions where protons and electrons combine to form neutrons. This is the reaction. Electron + Proton ----> Neutron + Neutrino The neutron star emits X-Rays from it's polar regions. If the neutron star acquires any rotational energy, it behaves like a light house. On earth we observe these X-Rays as short pulses while the X-Ray emission sweeps past. If this happens, the star is called a Pulsar. If the number of neutrons in the neutron star (or pulsar) is greater than ~1057 the nuclear forces can no longer resist the gravitational forces. If this happens an uncontrollable collapse occurs. The collapse tends towards a mathematical point. The gravitational forces from this body are so strong, that up to a certain point even light cannot travel fast enough to escape the gravity well. It is this property that gave these objects their name - Black Holes. The ultimate fate for any star. It is these unobservable bodies that are speculated to provide the required gravitational forces that hold together galaxies, the habitat of stars, that is, unfortunately, beyond the scope of this essay. Thank you for reading. David.R.Gilson

References Lecture notes by Dr.Steigman for Astronomy course - Dept. app. physics, Hull University (1997, semester 2) Collins Pocket Guide, "Star and Planets", 2nd ed. - Ian Ridpath, Wil Tirion. Harper Collins publishers, 1996 (ISBN 0-00-219979)