Sun
Period of Rotation: 25.4 days at equator, up to 30 days at
poles.
Mass: 333,000 x that of Earth.
Diameter: 870,000 miles (1.4 million kilometers).
Temperature: 27 million °F (15 million °C) in core; 11,000°F (6,100°C) at surface.

Among stars, it’s an average size, an average temperature. Looking at the galaxy that contains it and more than one hundred billion other stars, you’d never take notice of our sun, revolving around the center of the Milky Way, about halfway out on the inside edge of one of the galaxy’s spiral arms.

In our planet’s sky, of course, there’s no missing it. The sun is the largest object in the solar system, and all life on Earth depends upon its nurturing energy.

When we look at the sun (through safety filters, of course), we see its photosphere, the outer, visible layer that has a temperature of 11,000°F (6,100°C). Sometimes sunspots potmark the surface; sometimes there are none. These areas look darker because they are cooler than the surrounding gases—although they still sizzle at temperatures that range from 6,700°-9,400°F (3,700°-5,200°C).

Hot as the solar surface is, it’s an oasis compared to the superheated goings-on at its heart. Deep inside the sun’s core, the high rate of energy generation and the intense pressure—some 333,000 times (that of) Earth—heats the gas to 27 million°F (15 million°C). The process that fuels the sun is the ceaseless fusion of hydrogen into helium. This, in turn releases energy in the form of gamma rays that bounce from one atom to another until, after a million years or so, the energy finally reaches the sun’s surface in the form of visible light and heat. Every second, the sun releases as much energy as 90 billion 1-megaton hydrogen bombs.

Our average star is about 40 percent along in its expected 12 billion-year lifespan. About six billion years from now, it will have used up its hydrogen fuel and begin fusing its helium nuclei. The sun will swell into a red giant, so huge that it may encompass the Earth’s orbit. After another billion years the red giant will eject its outer layers, leaving only a small white dwarf star that will cool and fade away.

A SPOTTED FACE Created by strong concentrations of the sun’s magnetic field, sun spots come and go in regular cycles on the sun’s photosphere. Every 11 years sunspot activity reaches a maximum, a period known to scientists as solar max. Measuring barely half as hot as the surrounding solar surface, sun spots look dark through a telescope only because they are relatively cooler than the rest of the photosphere. Sunspots can be enormous—as wide as 62,000 miles (100,000 kilometers). They appear small only because the sun is so huge. Some think there may be a link between sunspot activity and weather conditions on Earth. A period of sunspot inactivity in the late 17th century, called the Maunder Minimum, coincided with an abnormally cold period in northern Europe now known as the Little Ice Age.

A SOLAR CROWN The outer part of the sun’s atmosphere, called the corona, is the region where arc-shaped and jet-like solar prominences appear. These immense clouds of gas can travel at speeds up to 15,000 miles (24,000 kilometers) per hour, to erupt 80,000 miles (128,000 kilometers) above the sun’s surface. It is only during a total solar eclipse that the corona, a thin veil of charged particles floating slowly away from the sun, is visible to the eye from Earth. Its shape changes constantly, and thus no two solar eclipses appear the same. SOLAR WIND Scientists have known for decades that the sun bathes all around it in a wind of charged particles. Most of them stream out through so-called holes in the sun’s corona, or outermost layer, where magnetic fields open out into space, allowing the gas to blow away. The solar wind is not a steady stream, but varies greatly in its intensity. Some of this variation seems to be due to “puffing” of vast clouds, like puffs of smoke through a chimney, controlled by dynamic patterns in the sun’s huge magnetic field. Under normal circumstances, the solar wind flows past Earth at about 300 miles (450 kilometers) per second. Solar disturbances carried in the solar wind can cause power line surges and radio interference on Earth. The solar wind can also trigger the aurora.

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