The radius of the Sun, Rq, is 109 times that of Earth, but its distance from Earth is 215 Rq, so it subtends an angle of only ¥2° in the sky, roughly the same as that of the Moon. By
comparison, Proxima Centauri, the next closest star to Earth, is 250,000 times farther away, and its relative apparent brightness is reduced by the square of that ratio, or 62 billion times. The temperature of the Sun's surface is so high that no solid or liquid can exist there; the constituent materials are predominantly gaseous atoms, with a very small number of molecules. As a result, there is no fixed surface. The surface viewed from Earth, called the photosphere, is the layer from which most of the radiation reaches us. The radiation from below is absorbed and reradiated, and the emission from overlying layers drops sharply, by about a factor of six every 200 km (124 miles). The Sun is so far from Earth that this slightly fuzzy surface cannot be resolved, and so the limb (the visible edge) appears sharp.
The mass of the Sun, Mo, is 743 times the total mass of all the planets in the solar system and 330,000 times that of Earth. All the interesting planetary and interplanetary gravitational phenomena are negligible effects in comparison to the force exerted by the Sun. Under the force of gravity, the great mass of the Sun presses inward. To keep the star from collapsing, the central pressure outward must be great enough to support its weight. The density at the Sun's core is about 100 times that of water (roughly six times that at the centre of Earth), but the temperature is at least 15 million K, so the central pressure is at least 10,000 times greater than that at the centre of Earth, which is 3,500 kilobars. The nuclei of atoms are completely stripped of their electrons, and at this high temperature they collide to produce the nuclear reactions that are responsible for generating the energy vital to life on Earth.
While the temperature of the Sun drops from 15 million K (27 million °F) at the centre to 5,800 K (5,500°C, 10,000°F) at the photosphere, a surprising reversal occurs above that point. The temperature drops to a minimum of 4,000 K (3,700°C, 6,700°F), then begins to rise in the chromosphere, a layer about 7,000 km (4,300 miles) high at a temperature of 8,000 K (7,700°C, 13,900°F). Above the chromosphere is a dim, extended halo called the corona, which has a temperature of 1 million K (1.8 million °F) and reaches far past the planets. Beyond a distance of 5-Rq from the Sun, the corona flows outward at a speed (near Earth) of 400 km per second (km/sec; 250 miles/ sec); this flow of charged particles is called the solar wind.
The Sun is a very stable source of energy. Its radiative output, called the solar constant, is 137 ergs per square metre per second (ergs/metre2/sec), or 1.98 calories per square cm per minute (cal/cm2/min), at Earth. The solar constant is defined as the total radiation energy received from the Sun per unit of time per unit of area on a theoretical surface perpendicular to the Sun's rays and at Earth's mean distance from the Sun. It is most accurately measured from satellites where atmospheric effects are absent. The solar constant does increase, but only by 0.2 percent at the peak of each solar cycle.
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