The nearest star beyond the sun is Alpha Centauri, 4.2 x 1016 meters away. If we received a radio message from the star today, how long ago would it have been sent?
The distance that light travels in a vacuum in one year, approximately 9.46 trillion (9.46 × 1012) kilometers and there are
1000 meters per kilometer so that's 9.46 x 10^15 meters per light year
so
4.2 x 10^16
---------- = 4.44 years
9.46 x 10^15
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http://www.answers.com/topic/alpha-centauri
Actually, that distance is not quite correct. It is 4.2 light years away, or 3.8*10^16 meters. Divide the distance by the speed of light to get the time. The correct answer is 4.2 years, but you will get a lower number if you use the value you quoted.
Recent data indicate your figure for the distance is OK. I got 4.1*10^16 m from more recent sources. Go with Lance's answer.
