The term solar constant implies a belief in a constant luminosity output for the Sun, but this may not be completely correct. This is found by measurement of the solar constant, the energy received per square meter (1,360 watts/m 2) by a surface perpendicular to the direction of the Sun at a distance of 1 astronomical unit and multiplying by the surface area of a sphere of radius 1 AU. The total energy emission of the sun, or luminosity, is 4 × 10 26 watts. This differential rotation reveals that the Sun is not solid, but is gaseous or liquid. Sunspots aren't seen at higher latitudes, but use of the Doppler effect for light observed at latitude 75° reveals a longer period of 33 days. For example, the equatorial rotational period is 25.38 days, but at latitude 35°, the period is 27 days. Observations of the sun reveal that different parts of the Sun rotate at different speeds. The rotation of the sun is made evident by the sunspots that cross the solar disk in about two weeks, then disappear, and then reappear at the opposite limb (or curved edge) two weeks later. The solar mass is equivalent to 330,000 earth masses, or 2 × 10 30 kg, for a mean or average density (mass/volume) of 1.4 g/cm 3. As a result, we cannot observe this interior region from the outside. Below the photosphere, the physical conditions of the material of the solar interior prevent light from escaping. Above the photosphere the solar atmosphere is transparent, allowing light to escape. What we see when we look at the sun, however, is not a solid, luminous surface, but a spherical layer, called the photosphere, from which the bulk of the solar light comes (see Figure ). The solar diameter equals 109 Earth diameters, or 1,390,000 kilometers. Evidence from other stars has also revealed their life histories, allowing us a better understanding of the part and future of our particular star. In turn, the study of stars shows us that our Sun is merely an average star, neither exceptionally bright nor exceptionally faint. But to astronomers, the Sun is the only star that can be studied in great detail thus, studying the sun is vital to the understanding of stars as a whole. ![]() The energy that we receive from the Sun dictates the environment on Earth that is so important to humanity's existence. SETI-The Search for Extraterrestrial Intelligence.Internal Structure Standard Solar Model.Interior Structure: Core, Mantle, Crust.Minor Objects: Asteroids, Comets, and More.Origin and Evolution of the Solar System.
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