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Astronomy is the scientific study of the objects visible in the night sky by means of telescopes and associated instruments that analyze the radiation received from these objects.
The greenhouse effect is the physical mechanism by which the atmosphere helps to maintain Earth's surface temperature within a range comfortable for organisms and ecological processes.
An element is defined by the number of protons in the nucleus of its atoms, but its chemical reactivity is determined by the number of electrons in its outer shell—a property fundamental to the organization of the periodic table of the elements.
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