astro092211 - Astronomy Sept. 22nd, 2011 The Solar System:...

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Astronomy Sept. 22 nd , 2011 The Solar System: 8 planets and 1 star Nearby stars (alpha Centayuri, Sirius) Loose star clusters , “open clusters”: several thousand stars spread over a region ~30 light years across Stars close together in the sky Born together, grow old and die together Globular star clusters : 20,000 – several million stars spread over 60 – 150 light years Galaxies : Vast “islands of stars”, 10 12 - 10 6 stars, 100,000 light years across Galaxy groups : like our “Local Group”, a small number of large galaxies and many small galaxies Galaxy clusters : hundreds of large galaxies. Scientists can be stubborn and silly about names. .. For example, these objects are “planetary” nebulae, even though they have nothing to do with planets. Working with angles Angular size is directly proportional to physical size and inversely proportional to distance. Angle (in radians) = linear diameter
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astro092211 - Astronomy Sept. 22nd, 2011 The Solar System:...

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