The angular momentum problem that defeated Kant and Laplace—why the planets have most of the solar system's angular momentum while the Sun has most of the mass—can now be approached in a cosmic context. All stars having masses that range from slightly above the mass of the Sun to the smallest known masses rotate more slowly than an extrapolation based on the rotation rate of stars of higher mass would predict. Accordingly, these sunlike stars show the same deficit in angular momentum as the Sun itself.
The answer to how this loss could have occurred seems to lie in the solar wind. The Sun and other stars of comparable mass have outer atmospheres that are slowly but steadily expanding into space. Stars of higher mass do not exhibit such stellar winds. The loss of angular momentum associated with this loss of mass to space is sufficient to reduce the rate of the Sun's rotation. Thus, the planets preserve the angular momentum that was in the original solar nebula, but the Sun has gradually slowed down in the 4.6 billion years since it formed.
At the centre of the solar system is the Sun, the star around which Earth and the other components revolve. It is the dominant body of the system, constituting more than 99 percent of its entire mass. The Sun is the source of an enormous amount of energy, a portion of which provides Earth with the light and heat necessary to support life.
The Sun is classified as a G2 V star, with G2 standing for the second hottest stars of the yellow G class and the V representing a main sequence, or dwarf, star, the typical star for this temperature class. (G stars are so called because of the prominence of a band of atomic and molecular spectral lines that the German physicist Joseph von Fraunhofer designated G.) The Sun exists in the outer part of the Milky Way Galaxy and was formed from material that had been processed inside a supernova. The Sun is not, as is often said, a small star. Although it falls midway between the biggest and smallest stars of its type, there are so many dwarf stars that the Sun falls in the top 5 percent of stars in the neighbourhood that immediately surrounds it.
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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