The central body of the Solar System, and by far the nearest star to us—the only one that we can examine in great detail. It is classified as a G2V star: a yellowish star with an effective temperature of 5772 K (spectral type G2) and a main-sequence dwarf (luminosity class V). Its apparent visual magnitude is −26.7, but its absolute magnitude is only +4.82. The Sun is largely hydrogen (71% by mass), with some helium (27%) and heavier elements (total 2%). Its age is estimated to be about 4.6 billion years. The energy produced by nuclear reactions at its core is transferred to the surface layer, the photosphere, by radiation through the inner two-thirds of its radius and thence by convection for the outer one-third. The transfer of energy from the core to the surface takes 10 million years. At its centre the temperature is calculated to be 15.6 million K and the density 148 000 kg/m3. The vast majority of its energy escapes to space from the photosphere. The photosphere is marked by small-scale features such as granulation and supergranulation, as well as larger-scale features related to solar activity such as sunspots and faculae, which are associated with regions of strong magnetic field. The Sun’s sidereal rotation period is about 25 days at the equator, and 27–28 days at latitude 40°; near the poles the period is 33.5 days. The adopted mean value, corresponding to latitude 17°, is 25.38 days.
Sun: Physical Data |
Diameter | Inclination of equator to ecliptic | Mean axial rotation period (sidereal) | Mean density |
1 392 530 km | 7°.25 | 25.38 d | 1.41 g/cm3 |
Mass | Luminosity | Volume (Earth = 1) | Escape velocity |
1.989 × 1030 kg | 3.83 × 1026 W | 1.3 × 106 | 617.3 km/s |
The photosphere is only a few hundred kilometres thick, and its temperature steadily decreases with height to about 4400 K at the temperature minimum. Above this is the chromosphere, where the temperature is between that of the temperature minimum region and about 20 000 K. There is a rapid rise of temperature with height through the transition region and into the corona, where the temperature is 2 million K or more.
Solar activity rises and falls in an approximately 11-year cycle that manifests itself in the varying number of sunspots and corresponding active regions. The magnetic polarities of pairs of sunspots, which are opposite in the leading and following parts, are reversed in each successive cycle, so that there is a 22-year magnetic cycle. A continuous stream of particles, the solar wind, flows out into interplanetary space at 300–750 km/s, with high-speed streams emanating from coronal holes.