The Sun, compared in size with Jupiter, the largest planet, and Earth The Sun is a typical star, a spinning ball of hot gas made up almost entirely of hydrogen (three-quarters of its mass) and helium. To us it is of crucial importance since no life could exist without it, but it is simply one of hundreds of billions of stars in the Milky Way Galaxy—itself one of billions of galaxies in the Universe. The Sun, although larger than most stars, is tiny when compared to supergiants; some astronomers classify the Sun as a “yellow dwarf”. The Sun represents more than 99% of all the matter in the Solar System, planets included. Almost exactly a perfect sphere, its diameter is more than 100 times that of Earth.
The Sun with a segment removed to reveal its internal layers. The arrows show the hot gas circulating in the convective zone.At the centre of the Sun is the core, a region of incredible pressure (200 billion times that on the Earth’s surface) and intense heat—about 15 million°C (25 million°F). This is the Sun’s nuclear furnace, where the energy that keeps it shining, is created. It produces massive amounts of energy by “burning” about four million tonnes of hydrogen every second. Hydrogen atoms fuse together to form helium. Energy from this reaction flows out from the core through the radiative zone to the convective zone. Here, in a continuous cycle, hot gas bubbles up to the surface before sinking down to be reheated again.
The highest prominences (arches) on the Sun's surface are often more than 600,000 kilometres (about 400,000 miles) high, twice the distance from the Earth to the Moon.
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