The galaxy in which we live, called the ‘Milky Way’, is around 100,000 light years in diameter. Remember that a light year is a unit of distance, not time – it is the distance light travels in a year. So 100,000 light years is about 9×10^17 km – which is 9 with 17 zeroes after it kilometres. Huge.
The earth is about 13,000 km in diameter – so you could fit around 6×10^13 earths side by side from one end of the Milky Way to the other. It’s a bit of a back of the envelope kind of calculation, but the real answer will be on that kind of scale – which is truly massive…
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barney commented on :
Look here:
http://www.astrotx.com/Planet%20Sizes%203.jpg
Then here:
http://bztv.typepad.com/photos/uncategorized/image004.jpg