Our lives are intimately linked to the stars, but in ways much more down to earth than the romantic views of them. As we all know, our sun is a star and the thermonuclear reactions that are continuously taking place inside it are what provide and sustain life on our planet. What do we get from the sun? We get carbon, oxygen, calcium and iron, courtesy of stars that disappeared billions of years ago (Naeye, 1998). Star formation is a study in contradictions because the formation of a star begins with atoms and molecules floating freely through space that are brought together through gravity to form masses that become stars.
Stars go through three major stages of development in their transformation from infancy to adult stars: a collection of dust and gases, protostar, full-blown star. Pictures of these various stages are mind-boggling in their beauty and bring one to an immense sense of awe at the machinations of the universe. Scientists believe that stars begin as a collection of interstellar dust and gases (Frank, 1996). This mass of dust and gases forms a cloud that begins shrinking and rotating until it eventually develops into what is called a protostar.
Once the protostar reaches sufficient ass, it then begins the process of converting hydrogen to helium through a series of nuclear reactions, or nuclear fusion until it becomes a full-blown star (Astronomy, 1995). Those protostars that are too small to complete the nuclear fusion die out to become what are known as brown dwarfs (refer to photo at right). Thanks to an image from the Mt. Palomar observatory, astronomers have obtained the first image of a brown dwarf, named Gliese 229B (or GL229B).
It is a small companion to the red star, Gliese 229, which is approximately 19 light-years from Earth in the constellation Lepus. GL229B is too hot and massive to be classified as a planet, yet at the same time it is too small and cool to be able to shine like a typical star in fact, it is actually at least 100,000 times dimmer than our own sun and is the faintest object ever to be discovered orbiting another star. As a star forms, it is this fusion-powered heat and radiation emanating from the core of the star which keeps the star whole (Watery Nurseries, 1997).
If it werent for this, the star would actually collapse under the stress of its own weight. However there is a balancing act that takes place within the star between radiation nd gravity (which provides fuel for the star) that prevents this and makes it possible for star to have a life span of billions of years. The big question, though, is how does this whole process get started and what actually makes it possible for these masses meld together to form a star, instead of just exploding back into cosmic particles?
What actually happens is that the clouds of gas and dust are actually drawn into compaction through self-created gravitational collapse. As the picture at left (from the Hubble Telescope) shows, these clouds go through continuous implosion to become solid masses. Scientifically speaking, it is logical to assume that this implosion should actually generate so much heat that the gas and dust expand, rather than come together and yet this is not the case. The reason why, scientists believe, is due to water molecules that are formed during this process.
It is the addition of these charged molecules, called hydronium, that they believe provide the ingredient necessary to prevent further expansion of the gasses and dust, thereby allowing the continuance of implosion until the star finally forms a solid mass. Hydronium is made up of three hydrogen atoms and one oxygen ion. In theory, it has the ability to transform into water (H2O) plus one independent hydrogen atom, as long as it is able to capture a free- floating electron from somewhere.
It takes hundreds of millions of years for the particles of dust and gas to come together into these gigantic clouds that can span hundreds of light-years in size. The clouds are dominated by their two prime elements of hydrogen and helium while particles of dust make up about one percent of a clouds mass. In addition, there are other molecules present that contribute to the molecular structure of the cloud, such as ammonia and other arbon-based elements. Each cloud contains enough elements to create approximately ten thousand new stars.
It takes many millennia for a collapsing gas cloud to fragment into thousands of dense, rotating clumps of gas that will eventually become newborn stars. The cores of these gaseous clumps are continuously compacting more and more as their rotation becomes faster and faster and, over time, the cores become elongated. Some of these elongated cores are hypothesized to eventually become binary and multiple star systems by virtue of the fact that the cloud is stretched out so much.
Over time, stars naturally change. Once the star enters its maturity, a stage where nuclear reactions begin to stabilize, it will spend the majority of its existence there. As they age and enter the late evolution stage, they often swell and become red giants which can evolve into novas, planetary nebulas, or supernovas. By the end of its life, a star will change into a white dwarf, black dwarf, or neutron star depending upon the composition of its original stellar mass.
Thanks to NASA’s Hubble Space Telescope we have gained new insight into how stars might have formed many billions of years ago in he early universe. This picture from the Hubble shows a pair of star clusters, which might be linked through stellar evolution processes. There are actually a pair of star clusters in this picture which are located approximately 166,000 light- years from the Large Magellanic Cloud (LMC) in the southern constellation Doradus. According to astronomers, the clusters, for being so distinctly separate, are unusually close together.
In the past, observations such as this were restricted to clusters within our own Milky Way galaxy. Because of the fact that the stars in he Large Magellaniv Cloud do not have many heavy elements in their composition, they are considered to be much more primordial than other newly forming stars and, therefore, more like scientists speculate stars were like in the early universe. There is an ongoing debate among astronomers as to the importance of disks in the formation process.
Many astronomers believe that most of the matter that makes up the star actually starts off inside a disk which spirals inward until it coheres into a star. There have actually been observations of massive disks as they orbit infant stars and it is these observations which have ed scientists to believe that disk accretion is very important to the process of star formation. The key to understanding star formation is the correlation between young stars and clouds of gas and dust.
Usually the youngest group of stars have large clouds of gas illuminated by the hottest and brightest of the new stars. The old theory of gravity predicts that the combined gravitational attraction of the atoms in a cloud of gas will squeeze the cloud, pulling every atom toward the center. Then, we might expect that every cloud would eventually collapse and ecome a star; however, the heat in the cloud resists collapse. Most clouds do not appear to be gravitationally unstable, but such a cloud colliding with a shock wave can be compressed disrupted into fragments.
Theoretical calculations show that some of these fragments can become dense enough to collapse and form stars. Astronomers have found a number of giant molecular clouds where stars are forming in a repeating cycle. Both high-mass and low -mass stars form in such a cloud, but when the massive stars form, their intense radiation or eventual supernova explosion push back the surrounding gas and compressive period. This compression in turn can trigger the formation of more stars, some of which will be massive.
Thus a few massive stars can drive a continuing cycle a star formation in a giant molecular cloud. While low-mass stars do form in such clouds along with massive stars, low-mass stars also form in smaller clouds of gas and dust. Because lower mass stars have lower luminosities and do not develop quickly into supernova explosions, low-mass stars alone can not drive a continuing cycle a star formation. Collapsing clouds of gases do not form a single object; because of nstabilities, it fragments producing an association of ten to a thousand stars.
The association drifts apart within a few million years. The sun probably formed in such a cluster about five billion years ago. Stars are supported by the outward flow Of energy generated by nuclear fusion in their interiors. The energy generated Keeps each layer of the star hot enough so that the gas pressure can support the weight of the layers above. Each layer in the star must be in hydrostatic equilibrium; that is, the inward weight is balanced by outward pressure. Stars are elegant in their simplicity.
