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The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as light and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometres (864,000 miles), or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System.
Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. It is thought that almost all stars form by this process.
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Graz ( GRAHTS, German: [ɡʁaːts] (listen); Slovene: Gradec) is the capital city of Styria and second-largest city in Austria after Vienna. As of 1 January 2019, it had a population of 328,276 (292,269 of whom had principal-residence status). In 2015, the population of the Graz larger urban zone (LUZ) stood at 633,168, based on principal-residence status. Graz has a long tradition as a seat of higher education. It has four colleges and four universities with more than 60,000 students. Its historic centre (Altstadt) is one of the best-preserved city centres in Central Europe.
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