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Suppose you go outside and throw an apple straight up into the air. The Earth's gravity slows the upward speed of the apple until it finally stops--and then starts falling back down. But suppose you could throw the apple so fast that Earth's gravity could never quite make it stop and fall back down. That speed is called escape velocity -- for Earth, the escape velocity is 25,000 miles per hour. On planets or stars with more gravity, the escape velocity would be even higher. Some planets and stars have the same mass but different sizes. In that case, the smallest one will have the greatest gravity at its surface. Stars can change size (and gravity) very dramatically. A star's gravity works not only on objects outside the star but also among the particles that make up the star. These particles attract one another and the star would collapse if it weren't for the nuclear energy pushing outward. When the nuclear fuel runs out, there is no more energy to keep the star from collapsing into a very small and dense sphere. The ultimate size of this sphere depends on the original mass of the star. Oddly, the largest stars result in the smallest spheres. If a star has a mass less then 1.4 times the sun's, gravity among its particles will not be able to overcome the tendency of the electrons, protons, and neutrons to remain separate. The forces will balance when the star has become a white dwarf the size of a small planet. If a star has between 1.4 and 3.6 solar masses, the gravitational force is so great that electrons combine with protons to form neutrons. The resulting neutron star will be only a few miles in radius. For stars with more than 3.6 solar masses, even the forces that hold elementary particles apart cannot overcome gravity and the entire star shrinks until its radius is essentially zero. In the region very near this pinpoint of matter, gravity becomes almost infinite. Even four or five miles from this singularity, gravity is so great that the escape velocity equals the speed of light. Einstein's general theory of relativity showed that light, though it does not react to gravity in the same way as ordinary matter, is nevertheless affected by strong gravitational fields. In fact, light itself cannot escape from inside this region. The imaginary surface at this radius is known as the "event horizon." From outside, the event horizon would appear perfectly dark (since no light can escape) and hence would appear to be a black hole in space. How do astronomers observe black holes? When charged particles accelerate, they emit electromagnetic radiation (like visible light or X rays). Astronomers look for regions in the sky with radiation consistent with charged particles accelerating toward a black hole. Researchers attempt to rule out other possible causes of this radiation to prove that black holes really do exist.