Alpha Centauri is the star system nearest to our Sun. About how long does light from Alpha Centauri take to travel to Earth, and what does this tell us about its distance?

Question

Which of the following is not a unit of distance?

Answer 1

You can also arrive at this answer by working with the actual distance in kilometre instead of starting from the light year definition.

Alpha Centauri is located at a physical distance of roughly \( 4 \times 10^{13} \) kilometre from the Sun.
Light covers about \( 3 \times 10^5 \) kilometre in every single second.
Dividing the distance by the speed gives the travel time in seconds: \( \dfrac{4 \times 10^{13}}{3 \times 10^5} \approx 1.33 \times 10^{8} \) seconds.
Converting seconds to years, since one year has about \( 3.15 \times 10^7 \) seconds, gives \( \dfrac{1.33 \times 10^{8}}{3.15 \times 10^{7}} \approx 4.2 \) years.
This matches the standard figure quoted for Alpha Centauri, confirming that its light takes about 4.2 years to reach us, which is also why it is described as being 4.2 light years away.

Answer 2

You can also arrive at this answer by working with the actual distance in kilometre instead of starting from the light year definition.

Alpha Centauri is located at a physical distance of roughly \( 4 \times 10^{13} \) kilometre from the Sun.
Light covers about \( 3 \times 10^5 \) kilometre in every single second.
Dividing the distance by the speed gives the travel time in seconds: \( \dfrac{4 \times 10^{13}}{3 \times 10^5} \approx 1.33 \times 10^{8} \) seconds.
Converting seconds to years, since one year has about \( 3.15 \times 10^7 \) seconds, gives \( \dfrac{1.33 \times 10^{8}}{3.15 \times 10^{7}} \approx 4.2 \) years.
This matches the standard figure quoted for Alpha Centauri, confirming that its light takes about 4.2 years to reach us, which is also why it is described as being 4.2 light years away.

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