The Sun, a star, pictured alongside Jupiter and EarthThe main difference between stars and planets is that stars have high temperatures compared to planets. Stars undergo nuclear reactions—they burn hydrogen in their cores, releasing massive amounts of energy. In order to be hot enough for these reactions to take place, stars need to be extremely big. They must have a mass of at least 75 times that of Jupiter, the largest planet in the Solar System. Because they radiate energy, stars are very bright objects. Planets do not generate their own energy through nuclear reactions. They reflect some of the radiation coming from their parent star.
Stars form when a cloud of dust and gas collapses under its own gravity. Planets form out of the material that collects in a disc spinning around the star during the early stages of its formation (when it is known as a “protostar”).
Stars are made of hydrogen and helium gases. Planets may be composed of a mixture of rock, metal, ice and water (like the Earth), or gas that collects around a solid core of rock, ice or metal (like Jupiter or Saturn).
It is possible to see five planets with the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. Mercury, which appears in the evening or morning but never in the middle of the night, is often hard to spot because it is always close to the Sun.
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