For centuries, humankind has looked at the stars, and for just as many years humankind has tried to explain the existence of those very same stars. Were they holes in an enormous canvas that covered the earth? Were they fire-flies that could only be seen when the Apollo had parked his chariot for the night? There seemed to be as many explanations for the stars as there were stars themselves. Then one day an individual named Galileo Galilei made an astounding discovery: the stars were replicas of our own sun, only so far away that they seemed as large as pin pricks to the naked eye. This in turn gave rise to many more questions. What keeps the stars burning? Have they always been glowing, or are they born like humans, and thus will they die? The answers to all these questions can be summed up in two words; stellar fusion. Therefore one can begin to understand the stars by understanding what fusion is, how it affects the life of a star, and what happens to a star when fusion can no longer occur. The first question one must ask is, "What is fusion?" One simple way of explaining it is taking two balls of clay and mashing them into one, creating a new, larger particle from the two. Now replace those balls of clay with sub-atomic particles, and when they meld, release an enormous amount of energy. This is fusion. There is currently three known variations of fusion: the proton-proton reaction (Figure 1.1), the carbon cycle (Figure 1.2), and the triple-alpha process (Figure 1.3). In the proton-proton reaction, a proton (the positively charged nucleus of a hydrogen atom) is forced so close to another proton (within a tenth of a trillionth of an inch) that a short range nuclear force known as the strong force takes over and forces the two protons to bond together (1). One proton then decays into a neutron (a particle with the same mass as a proton, but with no charge), a positron (a positively charged particle with almost no mass), and a neutrino (a particle with almost no mass, and no charge). The neutrino and positron then radiate off, releasing heat energy. The remaining particle is known as a deuteron, or the nucleus of the hydrogen isotope deuterium. This deuteron is then fused with another proton, creating a helium isotope (2). Then two helium isotopes fuse, creating a helium nucleus and releasing two protons, which facilitate the chain reaction (3). This final split is so violent that one-half of the total fusion energy is carried away by the two free protons. The second fusion variation, the carbon cycle, starts with a carbon nucleus being fused with a lone proton (1). This creates a nitrogen isotope. One proton then decays into it's primaries -- a neutron, positron and neutrino. The positron and neutrino separate from the nuclei as another proton fuses with the cluster. This creates a nitrogen nucleus which is then fused with yet another proton, forming an oxygen isotope (2). One proton then decays again as still another proton is forced into the nucleus (3). This final fusion splits into a nitrogen and a carbon nucleus; the nitrogen carries away the majority of the fusion heat, while the carbon goes back into the cycle. The triple-alpha process, the last known variety, is perhaps one of the simplest fusion reactions to understand. In this process, two helium nuclei fuse together to form a beryllium nucleus (four protons and four neutrons) (1). Almost immediately after this, another helium nucleus is forced into the cluster, creating a carbon nucleus of six protons and six neutrons (2). In this reaction, all of the heat given off is short-wavelength gamma
rays, one of the most penetrating forms of radiation. Each variety of fusion occurs depending on the size and age of the star. This will affect core temperature, causing the corresponding variety of stellar fusion. Now that fusion has been explained, one can learn how it occurs in the different star types. All stellar bodies start off as protostars, or concentrations of combusting gases found within large clouds of dust and various gases. These protostars, under their own gravity, collapse inward until it's core has been heated and compressed enough to begin proton-proton fusion reactions. After that starts, a star's mass will determine how long and through what kind of reactions it will go through. Generally, there are three classes of stars which can form: dwarfs, sunclass stars, and giants. Dwarfs begin as protostars of low size and mass (most protostars fall under this category). These stars, which have on average less than one-third the mass of our sun, go through very basic existances. One variety is the red dwarf, which has at least one-third the mass of the sun. Because of it's low mass, the red dwarf is predicted to last thousands of billions of years. The gravitational pressure of the star will cause the proton-proton reaction to occur in it's core, but after all the hydrogen has been fused into helium, the star lacks the pressure to begin the triple-alpha process. It is predicted that it will then contract into and inert, compressed ball of gas known as a black dwarf. Another variety of dwarf is the brown dwarf, which is so light (less than one-tenth the mass of the sun) that it lacks the pressure to even begin the proton-proton reaction, and becomes a black dwarf within just a few hundred million years, it's nuclear fuels unexpended. Sun-class stars are massive enough to move past the hurdle that the dwarves encounter and continue on the fusion chain. With a mass of two to five times that of the sun, the core of these stars rise to several million degrees Kelvin, bringing the surface temperature to approximately 6,000 degrees. After ten billion years, the inert helium in the core has compressed and the released heat ignites a hydrogen shell around the core. The energy given off by the combustion causes the stars size to double. The star continues to grow into a super-giant, raising the core temperature so high that in what's known as a helium flash, the helium core fuses into carbon. The series of these reactions causes varying shells of helium, hydrogen, and fusing hydrogen until the lack of pressure to fuse carbon ends the fusion in the core, it's gaseous surroundings dissipating, leaving a highly compressed and hot ball of carbon known as a white dwarf. Giants, the largest of all stars, have the shortest and most complex lives of any of the stars. These bright blue monstrosities begin from protostars which are hundreds of times the size of our sun. Within only a hundred million years, the proton-proton reaction at the core ends. The star is now six times the sun's size, and almost four times as hot. Once the core has changed to helium, the heat from it's compression causes the star to double in size. The star now makes it's final journey into oblivion. Most stars end their lives by lacking pressure to continue fusion and calmly fade into inert masses. This is not the case with giant class stars. After a mere 9 or 10 million years, all of the hydrogen atoms in the core have fused into helium (Figure 2.1). This causes a temporary pause to the fusion in the core, allowing gravity to compress it. This compression raises the core temperature to 170 million degrees Kelvin (from 40 million degrees during the proton-proton reaction phase). This energy is transferred to the hydrogen envelope surrounding the core,
expanding it to a thousand times the diameter of our sun. After this, most of the events of importance that occur happen in the core. With one million years to go, the collapse of the star raises the core temperature enough to halt the collapse and fuse it's core into carbon and oxygen while fusing the outer shell into helium (Figure 2.2). It remains this way for almost a million years. With a thousand years to go, most of the helium in the core is gone. This again pauses fusion, and collapse continues. The periods of collapse and fusion get increasingly shorter as time goes on. Once the collapse raises the temperature to 700 million degrees Kelvin, the carbon/oxygen core begins to fuse into neon and magnesium, creating layers around the core that continue to fuse hydrogen into helium, and helium into carbon (Figure 2.3). With a mere seven years to go, the core temperature of 1.5 billion degrees, the neon atoms in the core begin to fuse into more oxygen and magnesium, giving the star an onion-like appearance, each layer being denser toward the center (Figure 2.4). With one year to go, the core temperature reaches two billion degrees, fusing the oxygen core into sulfur and silicon (Figure 2.5). Only a few days to go, and the core temperature soars to three billion degrees, fusing the core into tightly compressed iron, which has a mass of almost 1.44 solar masses (the mass of our sun is one solar mass) (Figure 2.6). Since iron cannot fuse into anything further, the core continues to collapse under it's own gravity. With a tenth of a second to go, the iron core is collapsing at approximately 45,000 miles a second, packing the earth-sized core into a sphere only ten miles across. The iron atoms become so compressed that the nuclei melt together, creating enough heat to fill the core with neutrinos. The core has now reached maximum crunch, meaning it can no longer contract (Figure 2.7). The repulsive force in the core becomes so strong that it overpowers the gravitational force, and the core recoils and projects matter in a shock wave that bursts through all the outside layers. Almost one hundred percent of the energy is released as neutrinos, the first outwardly noticeable sign of the death of the star. The shock wave dissipates all of the surrounding layers, leaving a small dense sphere composed of neutrons which is known as a neutron star. This final explosion can be seen for thousands of years. Most remain neutrino stars , but if the core had more than three solar masses, it's gravity continues to collapse it, condensing the star into a singularity, or point of infinite mass and density. The gravity of this singularity is so great that even light cannot escape. This is what is known as a black hole. Through examining the above circumstances, one can now understand what solar fusion is, and how a star is directly connected to it. And yet one must take the information with a grain of salt. Scientists have only determined these facts from the information they now have. Everyday new things are discovered that may discredit all we believe to be fact. One can only hope that one day we as a people can learn enough to prove once and for all the exact nature of the universe.
Stellar Fusion ~ The Cosmic Ballet ~
Dylan Richards Chemistry 30 Mr. Hartley October 20, 1996
Bibliography Time - Life Editors, Voyage Through the Universe - Stars. Time - Life Books Inc., 1990