Lesson 1 Astronomy: The sun
The sun is the closest star to Earth and is the center of the solar system. The Sun is a medium-sized star in an outer arm of the Milky Way Galaxy. As seen from Earth, the sun is by far the brightest object in the sky; the sun therefore determines the usual distinction between day and night. The sun is responsible for the vast majority of heat in the Earth's atmosphere and is the main source of energy for life on Earth.
The sun is made up of very hot plasma. It contains more than 99% of the solar system's matter, mainly hydrogen and helium. The inner part of the sun is so hot and dense that nuclear fusion occurs: about 600 million tons of hydrogen are converted into helium every second. The energy released is largely emitted in the form of radiation, including visible light. The surface of the sun shows a varying number of sunspots, which are caused by local magnetic fields that inhibit convection.
The Sun is classified as a dwarf star (yellow dwarf). The Sun evolved about 4.6 billion years ago and is now halfway along the main sequence. In a few billion years, the sun will enter its next phase of development. The hydrogen within the solar core will run out, causing the core to collapse under its own weight. The Sun will then increase in heat and size (red giant), engulfing Venus and Mercury and making Earth uninhabitable. At the end of its life cycle, the sun will shed its outer layers (planetary nebula phase) and transform into a white dwarf.
The sun has great cultural and mythological value due to its enormous influence on humans. The sun was and is considered a deity in some cultures: sun gods appear in many different mythologies. The synodic rotation of the Earth and its orbit around the sun form the basis of solar calendars, including the Gregorian calendar, which is widely used worldwide
The Sun is a medium-sized type G main sequence star (spectral class G2V). It has a mass of about 1.989 × 1030 kg (1989 quadrillion tons), equal to 332,946 times the mass of Earth. The sun contains 99.86% of the mass of the entire solar system. This mass consists mainly of hydrogen, in the outer layers about 91 mole percent or 70 percent by mass. The other common element is helium, about 9 mole percent or 28 percent by mass. In the center of the sun, where hydrogen is converted into helium through nuclear fusion, the hydrogen content is probably lower (35 percent by mass) and the helium content is higher (63 percent by mass).
Type G stars are among the 15% brightest stars in the Milky Way: most stars are much smaller (red dwarfs)
