Alex Filippenko
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Why is the sky dark at night? In an infinitely old and large Universe the sky should be ablaze with light at all times. There are several possible answers to this paradox, each of which has profound cosmological implications. The relative youth of the Universe is now known to be the main explanation.
11) Understanding the Universe: An Introduction to Astronomy, 2nd Edition: Our Home-The Milky Way Galaxy
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Starting a series of lectures on galaxies, you begin with our home galaxy, the Milky Way. The band of light by that name is simply the Galaxy seen edge-on from our place within it. You also explore the nebulae in our Galaxy, many of which are regions of stellar birth.
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Brown dwarfs are low-mass objects whose dim glow is caused by slow gravitational contraction and the fusion of deuterium, a heavier but far less abundant isotope of hydrogen. Free-floating planets have even less mass than brown dwarfs and are not associated with any star.
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According to general relativity, the fate of the Universe is tied to its global geometry. If the Universe has positive curvature, like a sphere, it must eventually collapse in a "Big Crunch." If it is flat or has negative curvature, however, it will expand forever.
18) Understanding the Universe: An Introduction to Astronomy, 2nd Edition: Jupiter and Its Amazing Moons
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Beyond Mars lie the four gas giants: Jupiter, Saturn, Uranus, and Neptune. Jupiter is the Solar System's largest planet by far. Its moons include Io, which is volcanically active, and Europa, which may have an ocean of liquid water below its frozen surface.
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A voyage to another star would require exact information on distances and properties of the stars. This lecture shows how astronomers measure the distance to nearby stars and how they determine their surface temperatures, which are the basis for stellar classification.
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