If we see a supernova of a star that is 25 light years away from us, we know it exploded 25 years ago.
true
false

True

Explanation:

- Primarily, a light year is a unit of distance. So 1 light year is equivalent to 9.46 x 10^12 km that a light travels in one Julian year i.e 365.25 day.

- Hence, so for an object 25 light years away the time it takes to reach us:

                                    t = (25*9.46*10^15 m) / (3*10^8 m/s)

                                    t = 25 years.

- Hence, it takes 25 years for the light to travel through vacuum to travel 25 light years.

answer; equivalent to a one(1) horsepower motor;

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