# There are 86400 seconds per day and 365 days per year

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Unformatted text preview: our Galaxy, at a rate of one per second… How long would it take you? A.a few weeks B.a few months C.a few years D.a few thousand years Answer D 19 Now, lets think about the vast number of stars in our galaxy… Answer D A simple calculation shows that 100 billion seconds is nearly 3,200 years. There are 86400 seconds per day and 365 days per year. So, that is 32 million seconds per year. So, 100 billion seconds/32 million sec/yr = 3,200 years. Remember that the 3,200 years is just to COUNT the stars, not even to give them names or to study them to determine if they have planets. How many galaxies are there? Wikimedia Commons The Andromeda Galaxy 20 We’ve considered the immense size of our Galaxy, but it is just one of many in the Universe. This image shows the nearest large galaxy to our own is the Andromeda Galaxy, 2 million light years away. Just think, the light we see from this galaxy today started its journey before the ﬁrst humans walked the Earth. So, let’s think about how many galaxies there are out there. 21 This image is the Hubble Ultra Deep Field - the deepest image of the universe ever taken in visible light. 800 exposures from the Hubble Space Telescope were combined to make this image. The exposures added to 1 million seconds or 11.3 days of viewing time combined. This single image contains an estimated 10,000 galaxies. Nearly every object you see here is a galaxy (the handful of stars in the image can be identiﬁed by their cross-like appearance due to light reﬂecting of the struts of the telescope). The most distant galaxies in this image up to 13 billion light years away from Earth! The image covers an area equal to roughly one thirteen-millionth of the total area of the sky - no bigger than a grain of sand held out at an arm’s length. If we extend that over the entire sky, we can infer that the observable universe contains about 100 billion galaxies! How many stars are there in the...
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## This note was uploaded on 10/23/2013 for the course ASTR 100 taught by Professor Dunne during the Fall '12 term at University of Illinois, Urbana Champaign.

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