30 - 8/30 Journey Through Space and Time First stars born...

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8/30 –Journey Through Space and Time First stars born 2-300 million years ago We couldn’t be orbiting a first-generation star o Would be made up of only those first elements – hydrogen, helium Other elements created in stars then flung out into the universe Light year (ly) – Distance light travels in one year –about 10 trillion km Star – A self-luminous, gravitationally bound ball of gas that shines or used to shine because of nuclear reactions in its core. The sun is a typical star. –doesn’t reflect light Planet: A body massive enough to be round due to gravity (>600 km diameter, but not massive enough for nuclear reactions ever to begin), orbiting a star, and far more massive than all other objects in its vicinity. –new definition, different from reader o Has to orbit a star—however there are some free-floating planets Planetary system – a collection of planets and smaller bodies orbiting a star Galaxy- Large (typicall 5,000 to 200,000 ly across) gravitationally bound system
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30 - 8/30 Journey Through Space and Time First stars born...

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