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star fieldWhy Do Stars Shine?
by Marjorie Dorfman

Why do stars shine and where does the light come from? These are good questions and the answers shall be addressed, albeit not brilliantly, below. Read on if you dare to shine.

All stars (even our own Sun) are hot balls of glowing plasma held together by their own gravity which is extremely intense. The interiors of stars heat up because of the gravitational friction caused by their constant inward crushing. The process that causes stars to shine is called nuclear fusion because smaller nuclei are fused into bigger ones. The energy released in this process is enormous and can also be harnessed for use as weapons. Stars begin as huge clouds of hydrogen gas. As the clouds contract, they heat up. When it gets really hot, individual hydrogen atoms collide and with the release of energy, combine into helium.

birth of starStars are born (not in Hollywood in this case) by a balance between stellar gas and gravity. The energy released in the matrix of the star travels to the surface where it is then radiated into space in the form of light, heat, ultraviolet light and radio waves. Fusion energy is eventually depleted and sometimes this triggers the fusion of helium into carbon, or carbon into even heavier elements. Finally, all the elements that can provide energy are exhausted, and the star beings its final collapse.

The fusion of energy converts into gamma rays, which then become trapped inside the star. These rays are constantly pushing outward to free themselves against the gravitational contraction of the star. Due to this process, stars remain one size and do not continue contracting. Gamma rays are restless; they jump around inside the star. After they are absorbed by one atom, they are emitted over and over again, many times per second. To offer some perspective on this process, a single photon can take 100,000 years to get from the core of the star to its surface.

If a star is rather large to begin with, the centralized temperature will maintain the fusion and the heat will gradually seep to its outer regions, causing those surfaces to rise to high temperatures as well. The way in which a star shines is dependant on its outer surface. Our sun, for example, has an outer temperature of 5000 degrees but other stars are much hotter, some as much as 50,000 degrees. Hot stars produce a much bluer light while red dwarf stars produce mostly red and infrared light because they are considerably cooler than their counterparts.

bodies in spaceStars shine because they have huge internal fusion reactors that release tremendous amounts of energy. Although a simple explanation, its revelation has been a long time coming. In 1939, Hans Bethe from Cornell University made "star history" by publishing for the first time a model of the basic reactions that power stars. He predicted the dependency between the energy our Sun receives from two separate nuclear fusion reactions and the temperature of the respective star.

Bethe received the Nobel Prize for physics but his theory proved problematic because it contradicted the predictions of biologists and geologists. He did succeed in demonstrating the amount of energy produced by nuclear fusion reactions, but his theory maintained that the Sun would remain hot for only 30 million years, which contradicts scientific predictions that reveal the earth is much older than that. This left a gap in his theory because it clearly indicated that a supplemental source of energy is the only explanation for the Sun’s longevity.

The missing piece to this complicated puzzle was found in Einstein’s Theory of Relativity and that famous equation, E=mc2. Obscurely translated, this means that as hydrogen gas is fused into helium, that fusion, which is of two different types, is converted into pure energy, which is subsequently released in great quantities. The first type is proton-proton fusion where the fusion of two protons creates a neuron which then captures a third proton. This produces a helium-3 nucleus that doesn’t know when to stop and goes on to fuse further and create a helium-4 nucleus. The second reaction refers to C-N-O and is a fusion, which utilizes oxygen and nitrogen to fuse protons into helium nuclei. In these cases, small traces of carbon are the catalysts.

Only one nuclear reaction can take place at a time, depending on the temperature of the star’s core. In the 1930s scientists misjudged the Sun’s core to be about 20 million degrees Celsius, when in fact it is only 14 million degrees. This mistake led Bethe to conclude that the Sun is dominated by C-N-O nuclear fusion reactions.

stars in the skyThe 20th century found mankind constantly thirsting for more knowledge about the universe in which we live. Einstein published his theory of relativity, resolving a long debate in the field of physics, and we have come along way since then. Since the 1930s, we have known that our Sun generates energy through proton-proton fusion reactions, and that C-N-O reactions are mostly taking place in massive hot stars. Bethe is a pioneer who paved the way, as even his mistakes were highly significant in the field of physics.

Consider the stars as wondrous things, for they help to keep us humble in the same manner as the ocean does whenever we dare to stand beside it.

Did you know . . . ?

© 2010 Marjorie Dorfman
Michigan State University
Newton BBS

We found this great book:

The Birth of Stars and Planets

by John Bally
and Bo Reipurth

Birth of Stars

Besides having an excellent and clear description of what our current theories say about how stars are born, this book has a spectacular set of photographs. Aimed at the casual, non-technical reader, the book still manages to convey the tremendous magnitude, majesty, and mysteries of the universe. Its main subject is the birth of stars, and mostly within our own galaxy, the Milky Way.

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