Question

1 Light year is equivalent to how much metre?
(A) $9.5 \times {10^{15}}$
(B) $950 \times {10^{15}}$
(C) $9.5 \times {10^{19}}$
(D) $905 \times {10^{15}}$

Answer 1

Hint: As the name might suggest, a light-year is not a measurement of time, but a measurement of distance. It is equal to 9.7 trillion kilometres, which needs to be expressed in metres.

Complete Step by Step Solution: A light-year is the distance a beam of light travels in a single Earth year, or 6 trillion miles (9.7 trillion kilometers).
9.7 trillion kilometres when expressed in numbers is ${10^{12}}km$ where kilometres is written as $km$.
$9.7 \times {10^{12}}km = 9.7 \times {10^{12}} \times {10^3}m$ since $1km = {10^3}m$.
Thus, by the law of indices, we write,
${\text{1 light year}} = 9.7 \times {10^{15}}m$.
On the scale of the universe, measuring distances in miles or kilometers doesn't cut it. In the same way that we may measure the distance to the grocery store in the time it takes to drive there ("The grocery store is a 15-minute drive away"), astronomers measure the distances of stars in the time it takes for light to travel to us. For example, the nearest star to our sun, Proxima Centauri, is $4.2$ light-years away.

Hence the correct answer is option A.

Note: Measuring in miles or kilometers at an astronomical scale would be extremely cumbersome and impractical. Starting in our cosmic neighbourhood, the closest star-forming region to us, the Orion Nebula, is a short 7,861,000,000,000,000 miles away, or more simply, 1,300 light-years away. The center of our galaxy is about 27,000 light-years away. The nearest spiral galaxy to ours, the Andromeda galaxy, is $2.5$ million light-years away. Some of the most distant galaxies we can see are billions of light-years from us.
Measuring in light-years also allows astronomers to determine how far back in time they are viewing. Because light takes time to travel to our eyes, everything we view in the night sky has already happened. In other words, when you observe something 1 light-year away, you see it as it appeared exactly one year ago. We see the Andromeda galaxy as it appeared $2.5$ million years ago. The most distant object we can see, the cosmic microwave background, is also our oldest view of the universe, occurring just after the Big Bang some $13.8$ billion years ago.