Sun

Sun

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The Sun is located in the "suburbs" of the Milky Way galaxy, around 30,000 light-years from the center and within one of its spiral arms. It revolves around the galaxy's center at an average speed of 155 miles per second, taking 225 million years to complete each circuit.

Although the Sun is just one star among an estimated 400 billion stars in the Milky Way galaxy, it is the closest star to Earth. More importantly, it is the only star that provides Earth with enough light, heat, and energy to sustain life. Also, the strong gravitational pull of the Sun holds the Earth in orbit within its solar system.

Many people forget that the Sun is a star because it looks so big and different when compared to other stars and because the Sun appears in the sky during the day, whereas other stars only appear at night. Because the Sun is so close to the Earth, its luminosity (brightness) overwhelms the brightness of other stars, drowning out their daytime light.*

*Because of its relative nearness, the Sun appears about 10 billion times brighter than the next brightest star.

Size and Distance

Archimedes (287 b.c.e.–212 b.c.e.) placed the Sun at the center of the solar system. Observations of Galileo (1564–1642) supported this heliocentric theory, nullifying the ancient belief that the Earth was the center of the solar system.

Before the Sun's distance was known, Aristarchus (310 b.c.e.–230 b.c.e.) knew that the Moon shines by reflected sunlight. Aristarchus decided that if he measured the angle between the Moon and the Sun when the Moon is half-illuminated, he could then compute the ratio of their distances from the Earth. Aristarchus estimated that this angle was 87, resulting in the ratio of their distances at sin 3°.

Before the invention of trigonometry, Aristarchus used a similar method to calculate the inequality , reasoning that the Sun was 18 to 20 times farther away from the Earth than the Moon. As calculations were refined, the angle between Moon and Sun was shown to be 89°50′. Astronomers learned that the Sun is actually 400 times farther away from the Earth than the Moon.

Scientists now know that the Earth-Sun distance is 93 million miles. This distance was discovered when radar signals were bounced off Venus's surface to determine the Earth-to-Venus distance. At a speed of 500 mph, a journey from the Earth to the Sun would take 21 years.

Ancient civilizations thought that the Sun and Moon were the same size. Yet the Sun's diameter is really 864,338 miles across, which is more than 400 times the Moon's diameter and about 109 times the Earth's diameter. The Sun has a volume 1.3 million times the Earth's volume. Thus, a million Earths could be packed within the Sun. Although enormous compared to Earth, the Sun is an average-sized star.

German mathematician Johannes Kepler (1531–1630) devised his laws of planetary motion while studying the motion of Mars around the Sun. The Sun's mass can be calculated from his third law with the equation T ² where T is the period of Earth's revolution (3.15 ×10⁷ seconds), r is the radius of Earth's revolution (1.5×10¹¹m), and G is the planetary constant (6.67 ×10¹¹ Newton's m²/kg). To find M _s (Sun's mass) insert these values into the equation and solve

M_s = (π)(1.5 × 10¹¹m)³/(6.67 × 10⁻¹¹N m²/kg)(3.15 × 10⁷s )²
       = 2.0 × 10³⁰ kilograms.

Therefore, the Sun's mass is about 300,000 times the Earth's mass. Yet with respect to other stars, the Sun's mass is just average.

Time and Temperature

The Sun's rotation is similar to the Earth's rotation (one rotation every day), but because the Sun is gaseous not all parts rotate at the same speed. Galileo first noticed that the Sun rotates when he observed sunspots moving across the disk. He found that a particular spot took 27 days to make a complete circuit. Later observations found that the Sun at its equator rotates in slightly over 24.5 days. At locations two-thirds of the distance above and below the equator, the rotation take nearly 31 days.

In order to support its large mass, the Sun's interior must possess extremely large pressures and temperatures. The force of gravity at the core's surface is about 250 million times as great as Earth's surface gravity. No solids or liquids exist under these conditions, so the Sun's body primarily consists of the gases hydrogen (73 percent) and helium (25 percent).

Within the Sun's core, nuclear fusion reactions release huge amounts of energy. About 5 billion kilograms of hydrogen convert to helium, releasing energy each second. The core temperature is about 15 million kelvin, with a density of 160 grams per cubic centimeter. Based on mathematical calculations, the solar core is approximately the size of Jupiter, or approximately 75,000 to 100,000 miles in diameter. The amount of hydrogen within the Sun's core should sustain fusion for another 7 billion years.

For now, the Sun is bathing the Earth with just the right amount of heat. High-energy gamma rays (the particles created by fusion reactions) travel outward from the core and ultimately through the outer layers of the photosphere , chromosphere , and corona , losing most of their energy in the process. Along the way to the Sun's surface, the temperature of the gamma rays has dropped from 15 million to 6,000 kelvin. Yet even at these temperatures, the Sun is in the middle range of stellar surface temperatures.

Measuring the Sun

Information available about the Sun has increased with revolutionary scientific discoveries. Early telescopic observations allowed scientific study to begin, showing that the Sun is a dynamic, changing body. Later developments within spectroscopy, and the discovery of elementary particles and nuclear fusion, allowed scientists to further understand its composition and the processes that fuel it.*

*Astronomers now believe that the Sun is about 4.6 billion years old and will shine for another 7 billion years.

Recent developments of artificial satellites and other spacecraft now allow scientists to continuously study the Sun. Among the advances that have significantly influenced solar physics are the spectroheliograph, which measures the spectrum of individual solar features; the coronagraph, which permits study of the solar corona without an eclipse; and the magnetograph, which measures magnetic-field strength over the solar surface. Space instruments have revolutionized solar study and continue to add to increased, but still incomplete, knowledge about the Sun.

see also Archimedes; Galileo Galilei; Solar System Geometry, History of; Solar System Geometry, Modern Understandings of.

William Arthur Atkins with

Philip Edward Koth

Bibliography

Nicolson, Iain. The Sun. New York: Rand McNally and Company, 1982.

Noyes, Robert W. The Sun: Our Star. Cambridge, MA: Harvard University Press, 1982.

Washburn, Mark. In the Light of the Sun. New York: Harcourt Brace Jovanovich, 1981.

Internet Resources

"The Sun." NASA's Goddard Space Flight Center. <http://www-istp.gsfc.nasa.gov/Education/Isun.html>.

"Today from Space: Sun and Solar System." Science at NASA. <http://www.science.nasa.gov/newhome/pad/sun_today.htm#anchor1452757>.