US computer company, manufacturer of the Macintosh range of computers.
The star at the center of the Solar System. Its diameter is 1,392,000 km/865,000 mi; its temperature at the surface is about 5,800 K (5,530şC/9,980şF), and at the center 15,000,000 K (15,000,000şC/27,000,000şF). It is composed of about 70% hydrogen and 30% helium, with other elements making up less than 1%. The Sun's energy is generated by nuclear fusion reactions that turn hydrogen into helium at its center. The gas core is far denser than mercury or lead on Earth. The Sun is about 4.7 billion years old, with a predicted lifetime of 10 billion years.
US astronomers reported finding water on the Sun 1995. The water, in the form of superheated steam, was located in two sunspots where the temperature was only 3,300 K (as opposed to 5,800 K elsewhere on the surface).
At the end of its life, it will expand to become a red giant the size of Mars’s orbit, then shrink to become a white dwarf. The Sun spins on its axis every 25 days near its equator, but more slowly toward its poles. Its rotation can be followed by watching the passage of dark sunspots across its disc. Sometimes bright eruptions called flares occur near sunspots. Above the Sun’s photosphere lies a layer of thinner gas called the chromosphere, visible only by means of special instruments or at eclipses. Tongues of gas called prominences extend from the chromosphere into the corona, a halo of hot, tenuous gas surrounding the Sun. Gas boiling from the corona streams outward through the Solar System, forming the solar wind. Activity on the Sun, including sunspots, flares, and prominences, waxes and wanes during the solar cycle, which peaks every 11 years or so, and seems to be connected with the solar magnetic field. The unmanned space probe Pioneer 9 achieved solar orbit in 1968, and reported data on solar radiat