Astronomy is the science of that which happens beyond our Earth. Images like this one of stellar jet activity in the Carina Nebula, taken by the Hubble Space Telescope, help us understand the universe. Click ahead for more astronomical theory images.
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Human civilizations have been studying extraterrestrial phenomenon for far longer than we've had a Hubble Space Telescope. This Toltec astronomical observatory in Yucatan, Mexico, is around 1,000 years old.
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Despite the work of early astronomers, for millennia, most people understood very little about the stars and the skies. Next, we'll examine the debate about the center of the universe.
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Circa 135 A.D., Ptolemy (Claudius Ptolemaeus), an Alexandrian astronomer, mathematician and geographer, described a system of astronomy based on the theory that the sun, planets and stars revolve around the Earth -- the Ptolemaic System.
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Ptolemy (c. A.D. 100-170) used the movements of the sun, moon and planets to deduce that celestial bodies revolved around our Earth. His geocentric model went mostly unquestioned until the Renaissance.
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Without these three -- (L to R) Isaac Newton, Johannes Kepler and Galileo on the Astronomers Monument at the Griffith Observatory in Los Angeles -- where would science be?
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Shown here is a close-up of a painting of Nicolaus Copernicus, the first astronomer to suggest that the sun is the center of the universe. See a statue of him next.
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Copernicus (1473-1543) proposed an alternative model to Ptolemy's: the heliocentric model, in which the Earth and planets revolve around the sun. Click ahead for more Renaissance astronomers.
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Tycho Brahe (1546-1601) and Johannes Kepler (1571-1630) were Renaissance astronomers. Brahe attempted to marry the heliocentric model and the geocentric model with a combined plan. Kepler authored the Laws of Planetary Motion.
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Who could have guessed back in the Renaissance that sleek space telescope designs would one day result from the work of the earliest astronomers?
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Galileo Galilei (1564-1642) is often considered the chief astronomer of his time. He built some of the first powerful telescopes, promoted the heliocentric solar system in a cultural environment that was hostile to that idea, and made many extraordinary observations about the heavens.
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One of Galileo's first telescopes. He put the device to more serious purposes than were originally intended for it, and in the process saw whole new worlds.
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The reflecting telescope invented by Sir Isaac Newton in 1671. By using a curved mirror to reflect and focus the light, the length of the telescope was dramatically reduced.
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The Crab Nebula shows the effects of a distant supernova, or an exploded star. Some experts believe T Pyxidis -- only 3,260 light years from us -- could one day collapse into a type Ia supernova. If it does, the gamma radiation would end life on Earth.
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Albert Einstein's theories of special and general relativity helped us develop our understanding of gravity, motion and the structure of space-time. The atoms in the Eagle Nebula don't experience time equally but relatively, depending on their velocity.
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The big bang theory was nicknamed by astronomer Fred Hoyle. According to the theory, the universe began in an incredibly dense, hot state, then went through rapid expansion and cooling, giving us the galaxies we know today.
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While many people believe that the big bang theory refers to an explosion, it actually refers to the expansion of the universe.
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The Planck Satellite collects data that helps scientists refine theories like the big bang.
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Peter Higgs, the man for whom the Higgs boson particle was named, tours the Large Hadron Collider at CERN in Switzerland.
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Put into orbit in 1990, the Hubble Space Telescope allowed us to see farther than ever before and helped us understand the expansion of the universe.
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Astronomers can now observe the existence of black holes -- collapsed stars so dense that even light cannot escape them. Black holes began as theories, but now we believe we have visual evidence of entities like Sagittarius A at the center of our galaxy.
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This image shows evidence of the supposed black hole at the center of the Milky Way by its effects on surrounding matter. A doomed star can be seen trapped in a fierce orbit around the entity. Click ahead for more theories about space and our universe.
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The Big Crunch theory paints a cyclical picture of universe creation.
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Wormholes are theoretical punctures in spacetime. If we imagine the universe is the three-dimensional surface of a four-dimensional taco shell, a wormhole may allow direct passage through the middle of the taco, from one side of the shell to the other.
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Joseph M. Newman's film This Island Earth (1955) gave us an iconic alien life form. Astronomy is rife with theories -- both expert and amateur, both sober and sensational -- about extraterrestrial life. What do experts say about possible alien intelligence?
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Some scientists, such as astrophysicist Stephen Hawking, believe the sheer size of the universe makes alien life probable. However, it's possible that such life is not intelligent, or that we would never be able to cross the distances to encounter it.
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Ultra-high-energy cosmic rays -- lone particles launched through space with amazing energy -- are a phenomenon of mostly unknown origin. Do they originate from giant black holes at the centers of galaxies, or from gamma ray bursts like the one above?
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This photograph, taken in 2004 by the Hubble Space Telescope, is the farthest back we have ever seen into the distant, early universe. The galaxies seen here are the oldest known, from around the time of the big bang. As the universe continues to expand, what will happen to us?
If our planet becomes uninhabitable, could we simply relocate? First we'd have to find a planet with a composition similar enough to Earth's. Then we'd have to figure out how to get there. Check out our images of Earth-like planets located light years away in the Kepler telescope pictures!
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