(1998).ISO/media/astdict/s/graphics/sevol.jpg) Stellar evolution. A schematic evolutionary track on the Hertzsprung-Russell diagram for a star of one solar mass. |
Stars form in clusters in the clouds of gas and dust of the interstellar medium. The material of the protostar condenses and collapses. The core heats up through the release of gravitational energy until the temperature is high enough for the nuclear fusion of hydrogen into helium to take place. The time taken for this process depends strongly on the mass of the protostar. A star of 10 solar masses takes only 300,000 years, compared with 30 million years for a star the same mass as the Sun.
Hydrogen burning in the core continues until the fuel supplies are exhausted. During this phase, the star is on the main sequence of the Hertzsprung-Russell diagram. Again, the timescale is reduced dramatically with increasing mass. The Sun has a main-sequence lifetime of 10 billion years (of which about half have passed), as compared with only 500 million years for a star three times as massive.
When hydrogen burning in the core stops because the fuel is used up, a fundamental change in the star's structure takes place as adjustments are made to compensate for the loss of the energy source. The inert core contracts rapidly. In the process, gravitational energy is released, which heats the surrounding layers of hydrogen to the point where hydrogen burning recommences, but in a shell around the core. The result of the new outpouring of energy is to push the outer layers of the star further and further outwards. As this gas expands, it cools, and the star becomes a red giant. The combined effect of the increase in size and decrease in temperature is to maintain a more or less constant luminosity.
Meanwhile, the helium core continues to contract until a temperature of a hundred million degrees is reached, high enough for the fusion of helium into carbon and oxygen to begin. Helium burning starts.
Eventually, all the helium in the core is consumed. What happens subsequently depends on the mass of the star. In the more massive stars, contraction of the core after each fuel has been exhausted raises the temperature sufficiently to ignite a new, heavier fuel. Ultimately, a situation can be reached in which the central core has been converted to iron, while around the core, in a series of shells, silicon, oxygen, carbon, helium and hydrogen are being burnt simultaneously. Once a star has developed an iron core of about one solar mass no new reactions are possible. At this stage, the core contracts until it implodes catastrophically, setting off a supernova explosion. The naked core that remains becomes a neutron star.
In lower-mass stars, such as the Sun, the central temperature never gets high enough to progress beyond the burning of hydrogen and helium in concentric shells. Instabilities develop that result in the outer layers of the star being separated from the core to form an expanding shell of gas, called a planetary nebula, that gradually disperses into space. In fact, significant mass is probably lost from most stars through stellar "winds", particularly during the later phases of evolution.
The remaining core cools and shrinks, becoming more and more compressed until it is about the size of the Earth. The matter becomes degenerate and a white dwarf is formed. There is no internal source of energy and the white dwarfs continue to radiate and cool.
The evolutionary progress of a star is often demonstrated by plotting an evolutionary track on a Hertzsprung-Russell diagram. The illustration shows such a track for the Sun. Hertzsprung-Russell diagrams for star clusters illustrate the differential effect of mass on the rate of stellar evolution and can be used to determine the ages of clusters.
The outline of evolution given here is for single stars. Membership of a binary or multiple system may profoundly influence the course of a star's evolution if mass transfer takes place.