INTRODUCTION

Just as one’s own birth into this world, the birth of the universe’s galaxies and stars is just as crucial to our existence. Life is solely dependent upon the stars and the galaxies in which they make their home. The sun is the central engine which fuels life on our planet. Without such amenities life as we know it would not exist.

It is easy to realize the importance of the processes of the formation of galaxies and stars, particularly our own, when looked at from this perspective. The processes and variables that were present in the creation of the galaxies and stars shed light on the formation of the universe in its present and future state. The heavens that look so familiar to us are actually an ever-changing and dynamic environment. Galaxies and stars form, evolve and change as surely as we do over our lifetime. This life span is on a far greater scale, however, than we can easily fathom. Whereas evolution and processes of creating galaxies and stars are an ongoing operation, which to the untrained eye is nearly imperceptible, science has enabled us to get a greater understanding of the processes.

Galaxy Formation

In a seemingly lifeless cloud of gas and small objects, a galaxy is about to be born. That seemingly empty, cold mass of gaseous clouds holds the essential ingredients of a future galaxy. This almost empty stagnant environment contains all the matter and energy to create a huge galaxy containing hundreds of thousands of powerful stars and planets. There are several forces at work in huge primordial gas clouds. Gravity and centrifugal force being the most importent.

With centrifugal force from the original Big Bang and moving at a half million miles per hour, this large area of gasses is spinning in unison. There are several theories on how this primordial gas comes together to form a galaxy. The three most respected theories were proposed in the 1960’s and 1970’s and are still being debated today. The first theory posed in the 1960’s assumes that galaxies were formed by contraction of one enormous cloud of gas. The second theory claims galaxies were formed by several smaller clouds of gas merging into one. The third theory states that galaxies were formed by numerous small gas clouds merging into one.

The centrifugal force of the spinning gas cloud along with the gravitational pull within the clouds work together to give a galaxy its shape. All galaxies began with an elliptical shape, which is mostly a result of the centrifugal force of the spinning gas cloud. Like pizza dough being spun into the air the gas cloud flattens to a thin disk shape. Only the small center of the galaxy retains a spherical shape do to its slow rate of rotation and density. This is an example of slow angular velocity at the center of the galaxy. Many Galaxies evolve into much more complex shapes. There are four types of galaxies. These four types or groupes are devided by their shape. This classification system was developed by the astronomer Hubble and is named the Hubble classification. The four types of galaxies are spiral, barred spiral, elliptical, and irregular galaxies.

A typical spiral galaxy contains about one hundred billion stars and is approximately one hundred thousand light years across.(Kaufman p.478) Spiral galaxies are characterized by a large central bulge and spiral arms. They are characterized as well as classified by the thickness of the central bulge. The spiral arms are made up of millions of relatively young stars in a constant orbit around the center of the galaxy. Our Milky Way galaxy is an example of such a spiral galaxy.

A barred spiral galaxy is charaterized by a destinct bar running through its nucleus. The arms of a barred spiral galaxy originate not from the nucleus but from bars running through the galaxy’s nucleus. Like the spiral galaxy, the barred spiral galaxy has a central bulge containing the majority of mass in the galaxy.(Kaufman p.479 and p.481)

The spiral arms in both a spiral galaxy and a barred spiral galaxy are formed by density waves that move threw the forming galaxy. This process is simmilar to spinning a lopsided beach ball into the air. The unbalanced ball wobles as it spins. If the ball was viscus the denser area would begin to pull away forming a spiral arm.

An elliptical galaxy is of-course elliptical in shape. Elliptical galaxies are almost void of interstelar gas and dust. Thislack of gas and dust inhibits star formation.

Irregular galaxies can come in many different shapes. They are put in this group because they don't fit into any of Hubble's other classifications. Irregular galaxies like the three other types are further broken down into more specific groups based on other characteristics.

Galaxies are not the largest units in the universe. Most galaxies are related to others in large galaxy clusters. A large galaxy cluster may contain thousands of galaxies. A small or poor galaxy cluster may contain only a few dozen galaxies. There are two types of galaxy clusters a regular cluster and an irregular cluster. A regular cluster has a spherical shape with most galaxies being consentrated in the center of the cluster. An irregular cluster has no destinct shape and has galaxies equally despersed throughout. Our galaxy, the Milky Way, is a member of a poor irregular galaxy.

A galaxies shape, size, and mass rarely change after its formation. One way formed galaxies do change is through galaxy collision. Since galaxies are spinning through the universe they sometimes run into each other. These collisions can throw stars out of galaxies or cause one galaxy to canabolize matter from another, aiding in its growth. These collisions must truely be catastrophic events.

Galaxy Formation Conclusion

Galaxies come in many shapes and sizes but they all form under the same processes. The two most importent forces are centrifugal force and gravity. These forces work together on interstellar gas and dust to form all galaxies. It is this same ingrediants and forces that come together to form the stars and planets that make up galaxies.

STELLAR FORMATION

The processes we have just described in galaxy formation occur simultaneously, as forces acting within the forming galaxies create stars. Most of the matter in a mature galaxy is contained in these stars.

A star is born through intertwining gravitational forces as cold, dense clouds of gas are being compressed. Two main ways in which a star can be born, are through the result of compression of the remnants of a super nova or from the pulling together of interstellar dust and clouds of a nebula.

As an aging star begins to deteriorate and die, one of the possibilities is that the star reaches a critical mass where as the outer layers are too heavy, and the core can no longer support its immense weight. When this happens the star collapses in on itself until it cannot collapse any further. The rapid implosion of matter comes to a halt when it cannot fall any further, and that energy is transformed into a massive shock wave that bounces back away from the core in all directions taking all of the matter with it in a monstrous explosion called a supernova. From the remnants of this violent death comes the rebirth of new stars resulting from the element rich explosion.(Kaufman p.411)

The most common “breeding ground” for stars is a nebula. These masses of interstellar dust and glowing gas contain essential elements for the composition of stars. Many nebulae have young stars forming at the same time. The intense radiation given off by the new stars fueling themselves causes the surrounding gases to glow which in turn gives the nebulae its ethereal luster. The solar nebulae is comprised of essential elements for the production of new stars. (Kaufman p.364)

Those most essential element is hydrogen. As the cloud of gas is pulled together, it becomes more dense and opaque. Once this stage is attained, the effect that gravity has on the mass begins to accelerate. This intense squeezing together of a cloud of dust and gas is what begins star formation.

As the gas cloud condenses it gets a vigorous rotation. This rotation causes some dust and gaseous particles to be slung away by centrifugal force. Some of these particles may eventually come together to form planets, or secondary stars.

Once this process is in motion the intense compression of the mainly hydrogen containing mass creates intense heat in the protostar’s core. If the cloud has enough mass, that is the cloud has the critical mass to generate enough heat in its core to begin the process of hydrogen fusion,(process of having enough mass and pressure to fuse two hydrogen atoms together). The process of hudrogen fusion is were four hydrogen atoms are squezed together till they combine to form one helium atom. The snow balling of this reaction results in the birth of a star.

However, if the protostar does not have enough mass and pressure to generate the necessary energy for hydrogen fusion in its core, then the mass of clouds and dust will be fated to contract into what scientist’s refer to as a brown dwarf. These failed stars are in essence hydrogen-enriched planet.(Kaufman p.231)

CONCLUDING REMARKS

The formation of galaxies and stars is one of the most fundemental and essential processes in our universe. It is both an ancient and continuous process. This process gives perpetual birth, life, death, and rebirth to the universe. It is amazing that this seemingly random and sometimes violent formation of heavenly bodies has worked so harmoniously to create the universe we know today.

The formation of the galaxies and stars play an integral part in our daily life. It is the sun that gives life to the Earth and a habitable environment for its inhabitants. It will be the sun and the continual processes of the universe that will eventually claim the Earth and all life on it, although the evolution of our galaxy will continue uninterrupted as it has over the past billions of years.

Glossary of terms

angular velocity- The velocity of an object due to the angle at which it lies, as compared to a larger source of gravity.

barred spiral galaxy- Galaxy with spiral arms originating at the ends of a bar of stars running through the galaxy's nucleus.

Big Bang- Theory of how our universe began.

brown dwarf- A body of very dense clouds and dust resembling a planet.

centrifugal force- A force which acts upon an object due to its rotation.

density waves- A dense area of clouds or dust that move threw a forming galaxy to form a spiral arm.

galaxy clusters- A number of galaxies related and moving in relation to each other.

large galaxy cluster- A galaxy cluster containing hundreds or thousands of galaxies.

small, poor galaxy cluster- A galaxy cluster containing only a few dozen galaxies.

regular cluster- A galaxy cluster which has a spherical shape with most galaxies condensed near the center.

irregular cluster- A galaxy cluster which has no particular shape with galaxies evenly destributed threwout.

interstellar dust and gas- Particles of matter which pervade the universe.

nebula- The breeding ground for new stars. Cluster of glowing matter pulled together by gravity.

primordial gas cloud- Great clouds of gas and dust which may serve to be breeding grounds for new stars.

protostar- The stage in a stars lifecycle where it has not yet reached the H-R diagram.

spiral arms- Long thin bands of stars patruding from s galaxies center.

spiral galaxy- A galaxy characterized by arched lanes of stars and glowing nebulae.

super nova- The violent death of a aging star. Can produce the stituation where new stars can form.

Sources

Kaufman, William J. III. Galaxies and Quasars. San Fransisco: W.H. Freeman and Company, 1979.

Kaufman, Willian J. III. Universe (fourth edition). New York: W.H. Freeman and Company, 1994.

Silk, Joseph. A Short History of the Universe. New York: Scientific America Library, 1994.

Trinh, Xuan Thuan. The Birth of the Universe: The Big Bang and After. New York: Harry N. Abrams, Inc., 1993.

World Wide Web

http://www.astro.washington.edu/strobel/evolution.notes/evolution.html

http://spacelink.msfc.nasa.gov

http://astro.washington.edu/strobel/galaxy.notes/galaxy_pic.html