Michael A. Harris
Birth of a Star
The birth of a star begins with a huge mass of dust and gas known as a giant molecular cloud, or nebula. Giant molecular clouds are so massive that as much as a million stars can form from them.1 In our own galaxy alone there are thousands of such giant molecular clouds. Despite how massive these clouds are though, they are not very dense because they are spread out over vast distancesup to 200 light years in diameter. However, relative to the interstellar medium, these giant molecular clouds are considered very dense and can be more than a thousand times denser. To give perspective, Earth's atmosphere is about a trillion time more dense that a giant molecular cloud.18
These massive clouds of dust and gas are not uniform in density. As a result, in the areas that have more density, gravity begins to pull together more and more particles and gases. In the most massive of Giant molecular clouds, gravity can independently pull together the massive amount of material. However, in smaller clouds or fragments of giant molecular clouds where there is only ten to hundreds of solar masses worth of material it is not uncommon that a shock wave from a nearby exploding star or the passage of these clouds through an area of higher gravity be required for condensation of the material to occur.17
As these areas become more massive, gravity becomes stronger and pulls even more material together. Gravity eventually causes areas within the giant molecular cloud to collapse. As collapse occurs, gravitational energy is released and the gas becomes very hot. As the cloud collapses internal pressure from all the gas molecules begins to push outward. It is this battle between the inward pull of gravity and the outward push of internal pressure that characterizes the stars fight for life and eventual forfeit to death.2 As this battle is occurring, the material is not yet a star. This point when the material is contracting and heating up that it is called a protostar.
Protostars are not yet stars because they have not reached internal temperatures high enough to begin nuclear fusion. As the cloud of gas and dust contract because gravitational forces are more powerful than the internal pressure forces, the temperature of the cloud increases. It is not until the protostar reaches a temperature of 10,000,000 Kelvin that its hydrogen gas fuses to form helium.3 It is at this point that a true star has formed. As the star fuses its hydrogen into helium, the internal pressure increases and begins to balance the inward force of gravity and the star becomes stable. It is at this point that the star is said to be a main sequence star.4 Before continuing with a discussion of the various stages of stellar evolution, it is important to discuss the Hertzsprung-Russell diagram.
The Hertzsprung-Russell diagram
In two independent studies early in the twentieth century Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell focused their attention on two characteristics of a starits luminosity and its temperature. Hertzsprung and Russell plotted the luminosities and temperatures of stars on a graph in which the luminosity increases upwards on the "Y" axis and the temperature decreases toward the left on the "X" axis. Both astronomers came to the same discovery that a star's luminosity is related to its temperature. In most cases, the relationship follows that as the stars luminosity increases its temperature also increases.
For most stars this relationship between temperature and luminosity holds true. These stars, when plotted on the H-R diagram, create a line of stars that go from the bottom left (cool and not very luminous) to the upper right (hot and very luminous). Those stars that follow this relationship and are thus plotted in this general line on the H-R diagram are known as main sequence stars. However, not all stars follow this pattern or relationship that the more luminous the star the hotter it is. Looking at the H-R diagram, one notices that some stars fall outside the main sequence belt. For example there is a group of stars which when plotted fall above and to the right of the main sequence. For these stars, their luminosity is high but their temperature is low. These stars are known as red giants and supergiants. In addition, there is an area on the H-R diagram in which stars lie below and to the left of the main sequence. These stars have low luminosities but very high temperatures. Such stars are known as white dwarfs.
Why is this important? Well stellar evolution can be described by how a star's luminosity and temperature change over time.5 These changes are a result of the competition between the inward force of gravitational contraction and the outward force of pressure. As a star fuses its hydrogen into helium, both its temperature and luminosity change. Therefore, scientists can use the Hertzsprung-Russell diagram and a stars position on it to describe its current evolutionary stage. All stars begin as main sequence stars and are first plotted allow the main sequence line since their luminosity is in relationship with their temperature. Stars will spend about 90 percent of their life in the main sequence stage, which explains why about 90 percent of the known stars are plotted in this area.19 It is convenient, therefore, to discuss the evolution of stars with respect to where they lie on the H-R diagram.
When a star becomes a main sequence star, nuclear fusion has begun and it has become a stable star. A main sequence star's position on the H-R diagram is dependent on the star's initial mass. A Star with high mass will have a higher luminosity and higher temperature. A low mass star therefore will be plotted with a lower luminosity and temperature. The initial mass also determines how long a star remains as a main sequence star. High mass stars fuse their hydrogen much faster than low mass star.6 Therefore, a high mass star will become unstable quickly and move onto the next stage in a stars life cycle whereas low mass stars will burn the fuel more slowly and remain as a main sequence star for a longer period.
Main Sequence Stars
A main sequence star fuses its hydrogen into helium and spends most of its life in this stage. The amount of time spent as a main sequence star is dependent on how fast the star fuses its hydrogen. The speed at which a star fuses its hydrogen is, in turn, dependent on how much mass the star has. More massive stars have a stronger gravitational attraction force and therefore higher temperature are required to provide the pressure to counteract gravity. The higher temperatures allows more fusion, which leads to a shorter time as a main sequence star.7 The opposite is true for low mass stars. The most massive of stars only spend a few million years as main sequence stars while low mass stars may remain for 10's of billion years. Our sun will remain as a main sequence star for another five billion years. It is currently about five billion years oldhalf way through its life cycle. Up until now, all stars follow the same steps in the life cycle to become a main sequence star. However, now a star's mass will determine whether it will become a white dwarf, neutron star, or a black hole.
Becoming a Red Giant
The next stage in the life of a star occurs when all the hydrogen in the core is fused into helium. Since energy and heat are no longer being produce by the fusion of hydrogen, the core cools and begins to contract. Helium, which now makes up the core requires much higher temperatures to fuse and therefore does not begin to fuse until gravitational force cause the core to collapse and heat up to even higher temperatures. Before that happens though, the outer hydrogen shell that now exists around the helium core increases in temperature due to the contracting helium core that is heating up and producing enough heat to allow the hydrogen shell to fuse its hydrogen that is nearest to the core. The heat produced from this fusion moves outward and causes more hydrogen farther out in the hydrogen shell to fuse. This causes the outer shell to expand, which in turn causes its temperature to decrease. The expanding shell causes the luminosity of the star to increase. Thus the stars position on the H-R diagram changes as it temperature decreases and its luminosity increases. The now larger, cooler outer shell takes on a reddish color and the star is now called red giant.8
The next step in the evolution of a red giant is when the helium core that has been contracting gains enough energy to begin fusion. When the core heats up to 100 million K the helium begins to fuse. The heat generated from this process causes the surface temperature of the star to increasethus changing its position on the H-R diagram again. During this process, the stars plot moves to the left on the H-R diagram.
As the helium is used up in the core, the process repeats itself with oxygen and carbon fusion in the core after contraction has risen the temperature again. The outer shell that is now helium that fused from hydrogen is heated, expands, cools and turns red again. The hydrogen and helium shell begins to drift off into space and creates a huge shell around the carbon/oxygen core. This shell is known as a Planetary Nebula.9
Final Stage for Intermediate Mass Stars (5-10 times the mass of the Sun)
As the hydrogen and helium shell drifts off into space, the core continues to contract but there is not enough heat generated to cause the carbon/oxygen core to fuse into other elements. Therefore, the core simply contracts until it become very dense and the gravitational forces balance the internal pressure forces generated when the electrons are squeezed together by gravity. At this point it becomes a white dwarf.10
White Dwarf
As stated above, a white dwarf stars forms when the gravitational force balances the internal pressure force in the core. This occurs when the electrons are squeezed together so tightly that they can no longer be squeezed together and therefore the core cannot contract any further. If further contraction were to occur it would break a fundamental rule that govern how electrons behave, which is that no two electrons can occupy the same space at the same time doing the same thing.22 When the core becomes so dense that further contractions is prohibited, that gas is said to be degenerate. White dwarfs are stars with degenerate electron cores.
Since white dwarf stars can no longer contract, energy is no longer being produced in the core. Therefore, eventually the star will dissipate all of its heat energy and will stop glowing brightly. The star will become dimmer and dimmer until it no longer shines at all. It will be a cold stellar object.
A white dwarf star is so dense that one with a mass the same as the Sun will have a radius the same as Earth. It should be noted that the larger the mass of the star the smaller its final radius will be.23 The material in a white dwarf is so dense that a teaspoonful would weigh more than a garbage truck here on Earth.24
An Indian-American astrophysicist, S. Chandrasekhar, made the discovery that a white dwarf shrinks as the mass in the star increases. Chandrasekhar calculated that a white dwarf with a mass of about 1.4 solar masses (Msun) or larger would have a radius of zero.25 That means that not even the pressure from the degenerate electrons would stop the core from contracting further if the star's mass is higher than 1.4 Msun. Therefore, the maximum mass that a star can have and still become a white dwarf in known as the Chandrasekhar limit. Stars that have masses beyond this limit will further evolve and end their life in a different manner.
Final Stages of High Mass Stars (10 to 150 times the mass of the Sun)
In the case of a star that is 10 to 15 times the mass of the Sun, the core continue to collapse and is able to reach temperatures that allow the carbon/oxygen core to fuse into new, heavier elements. The cycle of core fusion, expansion, contraction and fusion again continues in high mass stars until the core in made of iron. Silicon, which fuses to iron, is the last fusion process that releases energy and allows the star to stay stable and balanced. As a result, the star's core continues to collapse without fusion taking place. The result is a core made of degenerate neutrons. Such a star is known as a neutron star. It has been found that if a high mass collapsing star's core is three time the mass of the Sun or less it will stabilize as a ball of degenerate neutrons. The result is called a neutron star.11
Neutron Star
The neutron star begins where the white dwarf left off. Again, if the mass of the star is more than 1.4 solar masses than the core will continue to contract. As the core become very dense and gravity squeezes the degenerate electron together more and more, some of the electrons are squeezed into the atomic nuclei. As the electrons are squeezed into the atomic nuclei they combine with the protons to form neutrons. As more and more electrons combine with protons to form neutrons, the pressure pushing out against the gravitational force become less and further contraction of the core ensues. Further contraction forces more of the protons in the atomic nuclei to combine with the electrons to the point that the atomic nuclei are saturated with neutrons and can no longer hold on to them.
What results as the neutrons are squeezed out and forced together is a degenerate gas of neutron much like the degenerate gas of electron that formed the white dwarf. Like the electrons the neutron resist being squeezed together and will reach a stable state in which no more contraction of the core will occur. Calculations have shown that a star with 3 solar masses will reach a stable state and remain a degenerate core a neutrons.25
If the mass of the star exceeds the above mentioned limit, its core of neutrons becomes unstable and continues to collapse and becomes and very dense object from which light cannot escape. This object is known as a black hole.12
The Sun's Evolution and the Fate of Life on Earth
To better understand how Earth will be affected by the evolution of the sun it is important to first describe the relationship between the sun, Earth, and life. For this Unit I will briefly describe, or outline, this relationship. Ideally, the subject of Life's relationship with the natural resources on Earth and those relationships with Earth's natural cycles and process would be covered in earlier units. Here we really want to focus on how the Sun affects Earth's natural resources such as water, gases, and soils. Life on Earth owes its existence to liquid water and the proper amounts of certain gases such as carbon dioxide, nitrogen, and oxygen to name a few.
In the past, the Earth had much more carbon dioxide. This is important because carbon dioxide allows solar radiation to pass through it but does not allow radiant heat from the Earth's surface to pass through. Instead it absorbs this heat and reradiated most of it back to the Earth surface. This process has come to be known as the greenhouse effect. This is an important process for life, for without carbon dioxide (and a few other greenhouse gases like methane and water vapor) the temperature of Earth's atmosphere at sea level would be 15 degrees Celsius (60 degrees Fahrenheit) colder.21 That is not to say that life is better off with increasing levels of carbon dioxide. If there is too much carbon dioxide then the Earth's temperature will rise too high and life will be threatened. It should be noted though that the Earth started with more carbon dioxide in its atmosphere than it currently has now. However, the sun's energy output was about 30 percent less and therefore in balance enough to create an environment that was not too cold or too hot for life to take hold. Overtime as the sun's energy output has increased the amount of carbon dioxide in the atmosphere has decreased well. That is, except for the past 50 to 80 years as the industrial revolution took hold. Increased use of fossil fuels and the cutting of forest have caused the Earth's carbon dioxide levels to increase. With the increase in carbon dioxide and other greenhouse gases, it is uncertain what will happen as the sun produces more energy and bathes Earth in ever increasing amounts of radiation. What is certain though is that no matter what our atmosphere is made of, eventually, the sun's will evolve through stages of life and eventually the energy it produces will be too great and will disrupt the delicate relationship between the sun's energy, Earth's natural resources and process, and all living organisms.
With a mass of 1.989 X 1030 kg, our sun is classified as a low mass star.13 And like all stars it began as a huge giant cloud of dust and gas. Gravity pulled these materials closer and closer together until the intense heat at the center cause the hydrogen to fuse into helium thus forming a protostar. Like all other stars on the main sequence, our sun has continuously increased the amount of heat energy being given off. That is to say that in the past our sun did not bath Earth in as much solar energy as it does today. In fact, it currently gives off 30 percent more heat energy than it did at its birth.20
Currently the sun is a main sequence star. It has been burning it hydrogen core for about 5 billion years and is expected to continue as a main sequence star and fully burn up the hydrogen in its core in about another 5 billion years.14 Relatively soon after all the hydrogen in the core is burned the sun will become a red giant as the outer hydrogen shell expands. This outer shell will expand to a size greater than that of the orbit of Mars, which is about 430 times larger than its current size.15
As the outer shell expands, the sun's luminosity will increase to about 20,000 times its current value. However, the surface temperature will lower to about 3,500 K while the core temperature reaches 100 million K.16 Earth will be bathed in more solar radiation and energy as the outer shell expands and nears Earth, thus casing the surface temperature of Earth to raise. It will continue to heat Earth until it is too dry and to hot for life to exist. Eventually, the burning hydrogen shell will envelope the Earth as well as Mercury, Venus, and Mars.
