An investigation of our universe becomes the story of the stars, for aside from dust and gas, space contains: stars. Even such exotic objects as pulsars, neutron stars, and black holes are only the remains of stars. Almost all of the information assembled through the various branches of astronomical observation: visual, infrared, ultraviolet, x-ray and gamma-ray may best be examined in terms of the following question: What stage in the life history of a star do they describe?
Therefore, a grasp of the basic stages in the life history of a star provides an essential framework for astrophysical inquiry. It is difficult, perhaps impossible, to consider the various stages of life (and the sequence of these stages) in a star's history without being moved at the resemblance to our own life story. Here it seems is our own life story acted out in a macrocosmic drama before our eyes. It is now considered fact that the birthplaces of stars are the vast nebular clouds of dust and gas distributed throughout space (see nebulae, interstellar dust, T Tauri stars).
In these relatively cool and dark clouds, protostars form through a process of gravitational condensation or contraction. It is imagined that perhaps some outside force in the form of gravitational energy from a passing stellar object perhaps causes a dust cloud to begin the contraction process. These huge clouds are known to be non-homogeneous. They contain spots where the gas and dust is somewhat denser than in the surrounding regions of the cloud. Perhaps stimulated in some way, these denser areas attract still more material toward themselves until a huge amount of matter (many times the size of our solar system) is formed.
See Figure A, where several protostars are forming in a vast cloud complex. The contraction process becomes acute, for nothing within the protostar can hold up the crushing weight of gas and dust that continues to accumulate. A crisis is reached. Through a friction-like process, the ever increasing pressure and density inside the protostar causes the temperature to rise in the protostar's center or core until 10 million degrees initiates a thermonuclear reaction. This reaction releases enormous radiant energy that, pushing out from within, stops the contraction process and: a star is born!
From this point forward, the life story of a particular star is dependent upon the size or mass of the original protester. The collapse of the proto-stellar material takes a relatively short amount of time in the star's life and, once the thermonuclear ignition takes place, the star's surface temperature rises rapidly and the star settles down to about ten billion years worth of being a star, in the common sense of the word. It is important, at this point, to examine the struggle going on within the stellar interior.
Once born, the star must live and die. The death of stars is inevitable and the life process is often conceived as one of thwarting or putting off this inevitable death, and thus prolonging life. The most fascinating aspect of the star's life is the intense struggle between the forces of gravity and contraction on one hand (the so-called outer forces) and the internal forces of radiation pressure pushing outward. As long as there is radiation coming from within, the forces of gravitational contraction are balanced, and stellar life as we observe it continues. In fact, the entire life of the star has to be conceived in terms of a continuous conversion process. These two archetypal forces create the stellar shell, which is well below the actual surface of the star itself. The thickness of this shell as well as its position near to or far from the stellar core suffers continual change and adjustment throughout the life of the star. The incredible weight of the many layers of gas first initiated continues to maintain and contain the radiant process -- a cosmic crucible. This pressure, and the inevitable collapse that must occur, is forestalled and put off by an incredible series of adjustments and changes going on within the core of the star. First of all, hydrogen burning (initiated at the birth of the star) continues for around ten billion years and this constitutes a healthy chunk of the stellar lifetime.
Our Sun is about half way through this stage at present and we can expect the Sun to continue as it is today for another five billion years or so. The exhaustion of hydrogen signals the onset of drastic changes in the life of the star and the next stage of that life. The radiant pressure of burning hydrogen has been all that has held back the initial contraction of the protester and when this is gone the stars core continues to contract. It has no material strong enough to stop such contraction and the core again shrinks, causing increased pressure, density, and temperature.
When the temperature at the center of the star reaches 100 million degrees, the nuclei of helium atoms (by-products of the hydrogen burning stage) are violently fused together to form carbon. The ignition of this helium burning at the stellar core again produces a furious outpouring of radiant energy and this energy release inside the star's core (as the star contracts) pushes the surface of the star far out into space in all directions. The sudden expansion creates an enormous star with a diameter of a quarter of a billion miles and a low surface temperature of between 3,000-4,000 degrees K -- a red Giant. In about five billion years, the core of our Sun will collapse while its surface expands, and this expansion will swallow the Earth. Our planet will vanish in a puff of smoke. Red stars like Antares and Arcturus are examples of this stage and kind of star. This Helium burning stage of the red giant continues for several hundred million years before exhaustion. With the Helium gone, the contraction process again resumes, and still greater temperatures, densities, and pressures result. At this point, the size or mass of the star begins to dictate the final course of the life. For very massive stars, the ignition of such thermonuclear reactions as carbon burning, oxygen burning, and silicon burning may take place, creating all of the heavier elements. These later stages in stellar evolution produce stars that are very unstable. These stars can vary and pulsate in size and luminosity leading in cases to a total stellar detonation -- a supernova.
Like ourselves, a star may end its life In one of several ways. When all the possible nuclear fuels have been exhausted (all conversions or adjustments made), the inexorable force of gravity (the grave) asserts itself and the remaining stellar material is confined in a sphere called a white dwarf. As the star continues to contract (having no internal-radiation pressure left), the pressures and densities reach such strength that the very atoms are torn to pieces and the result is a sea of electrons in which are scattered atomic nuclei. This mass of electrons is squeezed until there is no possible room for electron change and the very force of this impossibility withstands further contraction. The resulting white dwarf begins the long process of cooling off out in space. Becoming a white dwarf is only possible for stars with a mass of less than 14 solar masses. If the dying star has a mass that is greater than this limit, the electron pressure cannot withstand the gravitational pressure and the contraction continues. This critical limit of 14 solar masses is termed the Chandrasekhar Limit.
To avoid this further contraction, it is believed that many stars unload or blow off enough excess mass to get within the Chandrasekhar limit. The nova process is an example of an attempt of this kind. In recent years, it has become clear that not all stars are successful in discarding their excess weight and for them a very different state results than is found in the white dwarf. We have seen that the electron pressure is not strong enough to halt the contraction process and the star gets smaller and tighter. The pressure and densities increase until the electrons are squeezed into the nuclei of the atoms out of which the star is made. At this point, the negatively charged electrons combine with the positively charged protons to produce abundant neutrons. The resulting neutron force is strong enough to once again halt the contraction process and we have another type of stellar corpse: a neutron star. For a more detailed account of the neutron star, see Pulsars.
We have one further kind of 'dead' star. There is a limit to the size of star that can become a neutron star. Beyond a limit in mass of 21 solar masses, the degenerate neutron pressure can not withstand the forces of gravity. If the dying star is not able to eject enough matter through a nova or supernova explosion, and the remaining stellar core contains more than three solar masses, it cannot become a white dwarf or a neutron star. In this case there are no forces strong enough to hold up the star, and the stellar core continues to shrink infinitely! The gravitational field surrounding the star gets so strong that space-time begins to warp. When the star has collapsed to only a few miles in diameter (yet still has the same mass!), space-time folds in upon itself and the star vanishes from the physical universe. What remains is termed a black hole. (see section on Black Holes)
It should be clear at this point that all of the many kinds of stars and objects in space could be ordered in terms of the evolutionary stage they represent in the life of the stars. Just as each of us face what has been called the "personal equation" in our lives, so each star's life is made possible by the opposing internal and external forces. In the end, it appears that the forces of gravity dominate the internal process of adjustment and conversion that is taking place, just as in our own lives the failure of our personal bodies is a fact. And yet fresh stars are forming and being born, even now. The process of star life is somehow larger than the physical ends to the personal life of a star or a man, and our larger life is a whole, continuum, and continuing process that we are just beginning to appreciate. Some of the ideas that are emerging in regard to the black hole phenomenon are most profound, and perhaps are the closest indicators we have of how the eternal process of our life, in fact, functions.
Copyright (c) 1997-99 Michael Erlewine