The Sun
The Sun is a yellow dwarf star (G2V), about 4.6 billion years old, and the dominant gravitational force in the Solar System. It has a diameter of roughly 1.4 million kilometers and contains around 99.8% of the Solar System’s mass. Nuclear fusion in its core converts hydrogen into helium, producing energy that warms the planets. Above the core lie the radiative and convective zones, followed by the visible photosphere (~5,500 °C), the chromosphere, and the much hotter corona (~2 million °C).
Source: science.nasa.gov
APODs including "The Sun"
Sunspot in the Old Solar Cycle
06/02/2008
A new cycle has begun on our Sun. Over the past year, the Sun's magnetic field has reset and now a new 11 year period is beginning. Pictured above in a specific color of light emitted by hydrogen is sunspot 10982, one of the last sunspots of the old solar cycle. The two dark lines visible just above and to either side of the bright sunspot are cool filaments held aloft by the Sun's magnetic field. Hot and cold regions are shown as regions of relative light and dark, respectively. A solar cycle is caused by the changing magnetic field, and varies from solar maximum, when sunspot, coronal mass ejection, and flare phenomena are most frequent, to solar minimum, when such activity is relatively infrequent. Solar minimums occurred in 1996 and 2007, while the last solar maximum occurred in 2001. Editor's note: This APOD originally ran under the title "A Sunspot in the New Solar Cycle." But even though the new solar cycle has begun, the old one has not ended yet and may run concurrently with the new cycle for up to a year or so. This sunspot is now thought to belong to the old cycle, not the new one, based on its measured magnetic polarity and location.