Once hydrogen burning is well established, a star settles to a remarkably consistent relationship between temperature, luminosity, diameter and mass known as the main sequence. It will then very gradually increase its luminosity while decreasing its temperature, but the change is extremely slow. Our sun has been on the main sequence for about four and a half billion years, and will remain on the main sequence for another 5 billion years. During that first four and a half billion years, its luminosity increased by about 40% – enough that the early radiative equilibrium temperatures of its planets were about 9% below present values. Observationally, the temperature of earth has remained in a range that allowed the presence of liquid water throughout this period, so the change in solar output was offset by changes in the Earth’s atmosphere or surface reflectivity.
More generally, stars spend most of their lifetimes, and by far the most stable part, on the main sequence. A planet stable enough to support life, and especially one on which life has evolved, is most likely orbiting around a main sequence star.
How long a star remains on the main sequence again depends on its mass. Very massive stars are prodigal of their energy, and exhaust their hydrogen fuel quickly. A star five times the mass of the sun has ten thousand times the luminosity, and will exhaust its fuel in fifteen million years – a mere eye-blink in geological and evolutionary terms. A star half the mass of the sun would have a main-sequence lifetime of two hundred billion years – considerably more than the age of the universe. It would also, however, have a luminosity a mere 3% of the sun’s. A planet would have to be very close – and have a very short year – to get the amount of energy from that star that we do from the sun.
The end of life on the main sequence is catastrophic for any earth-like planets. The star becomes both brighter and cooler – which means a very considerable increase in its diameter. The sun, for instance, will expand to include the orbits of Mercury and Venus, and approach that of Earth. The core temperature will rise until it reaches roughly a hundred million degrees Kelvin, at which point two other fusion reactions become possible – the combination of three helium nuclei to make carbon, and the collision of an additional helium nucleus with the carbon to make oxygen. The resulting ed giants are elderly stars, either in the initial expansion phase or in the core helium burning stage The transition to helium burning is again disastrous for any planets in the vicinity, and may cause part of the star’s mass to be blown off into space. The star may go through a phase of periodic changes in luminosity. In general a red giant is far less hospitable to planets than a main-sequence star – but it does have a very important role in that it produces heavier elements than helium – elements that did not exist shortly after the big bang.
Eventually, the star runs out of helium in its core. Again it becomes unstable, often ejecting up to half its mass and thus adding two newly produced elements – oxygen and carbon – to the interstellar medium from which the next generation of stars will be born. For a star with a mass less than four times that of the sun, there is no other possible fuel, and the star shrinks to a white dwarf – a dead star.
For larger stars, the core temperatures may rise high enough (600 million degrees Kelvin) that fusion reactions involving carbon atoms are possible, producing neon and magnesium as well as additional oxygen. If the star has more than nine solar masses, the temperature may rise even higher, to a billion degrees Kelvin, at which point neon burning begins, producing still more magnesium and oxygen at the expense of neon, while at 1.5 million degrees oxygen burning begins. Oxygen burning produces silicon, sulfur and phosphorous, as well as more magnesium. Finally, at around 3 billion degrees Kelvin, silicon fusion begins, producing iron. Iron is the most stable element with regard to nuclear reactions, and no further thermonuclear reactions are possible once all the other elements are burned to iron “ash”.
The star goes through these stages with increasing speed. If it spent 7 million years on the main sequence, it will then spend 700 thousand years burning helium, 600 years burning carbon, a year burning neon, 6 months fusing oxygen, and a mere day to use up its silicon.
Without further fuel to keep up its temperature, the star is doomed. It will either become a supernova or collapse into a black hole. In the former case it returns a cornucopia of new elements to the interstellar medium for the production of the next generation of stars and planets, as well as sending out a shock wave that may well trigger the birth of new stars.
The elements returned to the interstellar medium are not only the ones produced by the primary reactions given above. Some of the reactions produce neutrons, and these neutrons can interact with the elements produced earlier to produce new isotopes and elements. In addition, the heat and pressure of the supernova explosion itself produces additional elements, some of which are intensely radioactive and no longer exist in the local part of our universe. But every element in our universe, except hydrogen and helium, is produced by the thermonuclear processes in stars.
Next week–planetary atmospheres.