As you gaze up into the night sky, you may ask yourself how were all those stars created? Many astronomers have asked themselves that same question for centuries, but now we have an answer to most of the puzzle. As our knowledge of stars and the universe expands, the puzzle of how stars were created becomes closer to completion.
The Life of Stars
Every star that has ever shined, and that ever will, started its life inside a vast interstellar cloud of dust and gas called a nebula. Within the nebula, individual molecules of gas begin to shrink. As the density of these molecules become denser, they grow hotter. We know this because the fundamental laws of physics tells us that a collection of atoms will raise its temperature when it is contracted into a smaller volume. The contraction was slow at first, only heating the gas in the cloud to temperatures of a few hundred degrees, but as the clouds shrank in size, the temperature of the compressed gases slowly increased up to thousands of degrees. When a nebula becomes this small and hot the atoms move so rapidly and close together that they collide. When these atoms collide, the electrons are knocked loose of the nuclei. This occurs until the gases only contain free roaming electrons. Eventually after several million years of contraction, the temperature at the center of the clump of gas rose up to tens of millions of degrees. At this point, the hydrogen nuclei begin to fuse at the center, forming a star from what had been a contracting protostar.
What distinguishes the difference between two stars is its mass. The mass is the amount of material that the protostar was able to collect when it contracted to form a star. The amount of mass in a star also determines its self-gravitation. This gravitational force is pulling on every piece of the star. The larger the star, the higher the self-gravitation is. When a star has very high self-gravitation, it causes particles within the star to move more rapidly in random directions.
The temperature of a group of particles measures the average energy of motion per particle. When the temperatures are high, particles dance randomly. But when the temperature is lower, this means that the particles velocities will be more modest. The higher the self-gravitation within the star, the higher the temperature.
In order for nuclear fusion to occur within a star, the nuclei of the star must first overcome its mutual repulsion. Although the gravity of a star pulls the nuclei together, the repulsion occurs due to the fact that nuclei carry a positive charge. In nature, electric charges of the same sign will repel each other. When this occurs, every bit of speed will help. This is why the larger stars have higher temperatures. They have more mass than smaller ones, helping the particles in the nuclei to speed up, causing nuclear fusion to occur more rapidly.
Temperature is also a major key in nuclear fusion. At low temperatures of about fifteen million degrees Fahrenheit the process can barely occur. The nuclei are not moving rapidly enough to fuse together. Whereas at higher temperatures, nuclear fusion occurs more easily. Every million degrees makes a big difference during fusion.
Our sun is a typical star. Its temperature at the center is about twenty seven million degrees Fahrenheit. This temperature ranks just above the average of stars in our galaxy. Our suns mass also ranks somewhat above the average. Stars that have a greater mass than our sun have much greater rates of nuclear fusion, thus producing more energy of motion from energy of mass during every second of their lives. These are the stars that we can see brightly in the night sky.
Types of Stars
Some stars are classified by plotting the stars temperature by its luminosity. This chart is called the H-R diagram. There are seven different spectral types: O, B, A, F, G, K, and M. These different spectral types are due to changing amounts of hydrogen within the star. Basically, this chart shows the relation of the different types of stars to each other.
One type of star is called the red giant. Red giant stars are the largest stars in the universe known to man at this point in space exploration. They are about four hundred times the size of our sun. As a star ages, its core shrinks down to a much smaller size than before. Because of the stars self-gravitation, it also grows denser. The core contracts because of the small supply of protons. To produce the same amount of nuclear fuel, the star heats its remaining protons more than usual, so the rate of nuclear fusion is greater than before. This causes the star to generate a greater supply of energy then it even did with its greater supply of protons. In actuality, the star is like a motorcyclist racing to the next gas station so he won t run out of fuel. Thus causing the remaining fuel to be burnt at a much greater rate.
This increase of nuclear fusion has two significant effects on the star. First of all, the stars luminosity increases. Secondly, as energy from the core is released to the surface, the flow of extra energy loosens and expands the outer layers of a star. As the layer expands, the particles are spread out, and begin to cool down. When it has expanded as much as it is going to, the surface is only a couple thousand degrees. The stars surface now glows mostly in red light, rather than yellow or blue.
Some scientists believe that there are massive stars that are up to one thousand times the size of our sun. These stars, which have never been seen, would be more massive than any other star known to man. Scientists are still searching the sky s for evidence of the existence of these stars.
Another type of star that really is not quite a star is called a brown dwarf. This anomaly has never before been seen, but it is thought that a brown star is too small to have nuclear fusion occur, but too big to be considered a planet. These dwarfs have many of the same characteristics of normal stars. Jupiter is the largest known planet, and it does radiate some energy, but it is about fifty times to small to be considered a brown dwarf.
About fifty percent of the stars that can be viewed from Earth are binary stars or multiple systems. Binary stars are two stars rotating around a central gravitational pull between them. Sometimes these stars are similar to each other, but many times they are totally different. Because of these differences, the stars seem to change in brightness. One of the two stars may pass in front of the other causing the light to seem brighter or less bright. This is called an eclipsing binary. Some points in the sky are made up of several stars, called multiple systems. Castor, which is in the Gemini constellation, is an example of this, because it is made up of six stars. Three binary stars, all orbiting around each other at a central point.
The Death of Stars
After billions of years, the enormous heat that held gas particles in the sun dies down, because the supply of protons to the star has also slowed down. Gravity pulls these particles tightly together, and the star begins to collapse. It shrinks down to a fraction of its original size. The result of the star thereafter, depends on the amount of matter contained by the star.
One possibility that might happen to the star, is that it might collapse so much that it becomes what is called a white dwarf star. These white dwarfs are usually about the size of one percent of our suns size. These stars have a very hot and bright surface, but they do not let out very much light due to their small size. The atoms in these white dwarfs are so dense, that a piece of it the size of a marble would weigh more than one ton on Earth. The next step in the evolution for these stars is to cool down into what is know as a black dwarf, or a dead star.
Another possibility that might happen, is when a star with more matter than six times our sun collapses into what is called a neutron star. Because larger stars have a greater gravitational pull than smaller ones, the atoms making up the star pack very close together. When a star becomes a neutron star, it is because the electrons and protons in atoms are so close together that they are fused together to form neutrons. After the star becomes as dense as it is going to be, its diameter will be about ten miles. A marble sized piece of this star on earth would weigh about one hundred thousand tons.
Scientists believe that when a very large star dies, the resulting star may become more dense than even a neutron star. This is called a black hole. The gravitational force in these stars not only crush atoms into neutrons, but crush neutrons into an even denser material. A marble sized piece of this matter would weigh billions of tons here on Earth. The matter in these stars are so great that it pulls everything into it including: all matter, heat, light, X rays, radio waves, and any other form of energy. Because of this, no black hole can be seen or detected by scientists using any type of equipment.
Some scientists believe that large black holes may lie in the center of some galaxies. This might be the reason why some galaxies are active and why some are not. Scientists think that all galaxies once had black holes, and that is how a galaxy was formed. The nearest galaxy that is thought to have a giant black hole is galaxy M87. This galaxy is very active, and is the reason scientists believe in their theory of the formation of galaxies due to black holes. After this discovery, it is believed that black holes are not a rare anomaly in the galaxy, because there may be millions or even billions in our universe.
Sometimes when a larger star is going to die, nuclear reactions occur within it faster than the star can emit it. This is called a nova. When this happens, the star rapidly grows and becomes millions of times brighter than it once was. Once it reaches a certain point of growth, an explosion occurs. In a nova, the star is not destroyed, because only an outer layer of its gas is blown into space. Because of this, it is believed that stars may become novas several times.
After a star goes nova, the star can shrink down into a pulsar. Pulsars are believed to be neutron stars. Unlike normal radio waves received from stars, pulsars waves come in bursts or pulses. Scientists believe this is due to rotation. Because pulsars are so dense, they can rotate very rapidly. On pulsars there may be a hot spot which may give out more light and radio energy than the rest of it. Thereby causing it to send pulses of light and radio waves into space.
Many questions about stars have been already answered, but there is still so much that we don t know. Such as what are black holes? This question, and many others like it may be answered in the near future, as space exploration continues, and our understanding of the universe expands.