Astronomy & Astrology - Sun (astronomy)

Sun (astronomy) - I. Introduction & Index

Sun (astronomy), closest star to Earth. The Sun is a huge mass of hot, glowing gas. The strong gravitational pull of the Sun holds Earth and the other planets in the solar system in orbit. The Sun's light and heat influence all of the objects in the solar system and allow life to exist on Earth.

The Sun is an average star—its size, age, and temperature fall in about the middle of the ranges of these properties for all stars. Astronomers believe that the Sun is about 4.6 billion years old and will keep shining for about another 7 billion years.

For humans, the Sun is beautiful and useful, but also powerful and dangerous. As Earth turns, the Sun rises over the eastern horizon in the morning, passes across the sky during the day, and sets in the west in the evening. This movement of the Sun across the sky marks the passage of time during the day. The Sun's movement can produce spectacular sunrises and sunsets under the right atmospheric conditions. At night, reflected sunlight makes the Moon and planets bright in the night sky.

The Sun provides Earth with vast amounts of energy every day. The oceans and seas store this energy and help keep the temperature of Earth at a level that allows a wide variety of life to exist. Plants use the Sun's energy to make food, and plants provide food for other organisms. The Sun's energy also creates wind in Earth's atmosphere. This wind can be harnessed and used to produce power.

While it lights our day and provides energy for life, sunlight can also be harmful to people. Human skin is sensitive to ultraviolet light emitted from the Sun. Earth's atmosphere blocks much of the harmful light, but sunlight is still strong enough to burn skin under some conditions. Sunburn is one of the most important risk factors in the development of skin cancers, which can be fatal. Sunlight is also very harmful to human eyes. A person should never look directly at the Sun, even with sunglasses or during an eclipse. The Sun influences Earth with more than just light. Particles flowing from the Sun can disrupt Earth's magnetic field, and these disruptions can interfere with electronic communications.

Index

II. Physical Characteristics	A. Importance to Earth
II. Physical Characteristics	B. Role in the Solar System
III. The Sun as a Star	A. The Sun's Place in the Milky Way
	B. Comparisons with Other Stars
	C. Composition of the Sun
	D. The Sun's Remote Past and Distant Future
IV. The Sun's Energy	A. Scale of the Sun's Energy
	B. Nuclear Fusion in the Core
	C. The Proton-Proton Chain
	D. Solar Neutrinos
V. Inside the Sun	A. Radiation and Convection
	B. The Oscillating Sun
	C. Helioseismology
	D. Rotation
VI. The Sun's Atmosphere	A. The Photosphere


	B. The Chromosphere
	C. The Corona

	D. The Sun's Winds
VII. History of Studying the Sun	A. Beginning of Scientific Study
	B. Spectroscopy
	C. Studying the Sun's Photosphere and Sunspots
	D. Study of the Sun's Energy
	E. Discovering the Structure of the Sun
	F. Solar Research from Space

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Contributed By:

Randolph H. Levine, M.A., Ph.D. Scientific Computing Marketing Manager, Digital Equipment Corporation. Author of The Sun as Seen from Space. Contributor to Solar Physics and Astrophysics Journal.

Kenneth R. Lang, B.S., B.A., M.S., Ph.D. Professor of Astronomy, Tufts University. Visiting Senior Scientist, NASA Headquarters. Author of Sun, Earth and Sky and The New Sun.