When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. Stars develop when gravity compacts clouds of gas. Star The basic building block from which galaxies are made. Solar system The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets. Also called the habitable zone or life zone, the Goldilocks region is an area of space in which a planet is just the right distance from its home star so that its surface is neither too hot nor too cold. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Based on that definition, IAU ruled that Pluto did not qualify. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. American Heritage Dictionary of the English Language, Fifth Edition. Planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. After the story of Goldilocks and the Three Bears, in which the young girl Goldilocks enters the bears' house during their absence, tastes their bowls of porridge, and finds one too hot, another too cold, but the last one just right. All animals and many microorganisms need oxygen to fuel their metabolism. the area around a star (such as the sun) in which there is sufficient energy present to sustain life. Oxygen A gas that makes up about 21 percent of the atmosphere. Liquid A material that flows freely but keeps a constant volume, like water or oil. But if it’s just right - in that so-called Goldilocks zone - water could pool as a liquid and support life. It also can’t be too far (or the extreme cold would freeze any water). This distance would be not too close to its sun (otherwise the extreme heat would evaporate liquids). Goldilocks zone A term that astronomers use for a region out from a star where conditions there might allow a planet to support a kind of life. Boil To heat a liquid to the temperature at which it turns to vapor.
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