Obvious and not-so-obvious differences and similarities distinguish planets and stars. Stars sometimes appear to twinkle in the night sky while planets shine steadily. Stars maintain a constant position over time, while planets appear in different areas at different times. Both planets and stars are formed by the accumulation of debris in space, however, and although some planets are formed mostly from rock, other planets and stars are gaseous.
A star is a sphere of gas that holds its shape because of gravity. Interestingly, stars exist in a constant state of hydrostatic support, which is a sort of tug-o-war between the gravity that's pulling to collapse it and the pressure that's generated within from hot gases pushing outward, according to the website for NASA's Goddard Space Flight Center. Stars are formed when clouds of gas and dust accumulate into a large enough amount that gravitational pull forms it into a mass. When the mass becomes large enough, it begins to heat up from the core.
Scientists tend to disagree on what exactly defines a planet. The most recent explanation was offered in August of 2005 by the International Astronomical Union, which determined that to be considered a planet an object must orbit a star, be large enough to have settled into a round shape and must have cleared the area around its orbit. This most recent definition successfully explained Pluto right out of the picture, a move frowned upon by many astronomers.
Stars are much farther away from Earth than planets are. This explains why stars appear to twinkle while planets do not. Turbulence in the Earth's atmosphere interferes with the light from a star. Planets are close enough and large enough that the interference isn't visible. Stars are composed mainly of gas, while planets form in the wake of stars, once most of the gas has all ready been used up. In the case of terrestrial planets, the debris left behind is mostly rock, which eventually forms together with gravity to become a new planet.
Several planets --Jupiter and Saturn -- are not terrestrial like Earth, meaning they're composed mostly of gases instead of rock. These planets, sometimes called "gas giants" are similar to stars. In fact, the only thing keeping them from becoming stars is their mass. If Jupiter was about 80 times more massive, it could easily become a star. Planets and stars are also alike in that they might both orbit other stars. A system of stars orbiting another star is called a multi-star system, according to a 2006 article in "Scientific American."
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