A light year is a measurement of distance that is used in the space. It is the distance that a light traveled for 1 year on Earth or 365 days on Earth at a speed of 3 x 10^8 meters per second. To convert the units from light year to the SI unit meters, we simply make use of the definition of the unit. We do as follows:
18.9 light years ( 365 days / 1 light year ) ( 24 h / 1 day ) ( 3600 s / 1h ) ( 3 x 10^8 m/s) = 1.79 x 10^17 meters
Therefore, 18.9 light years is equal to 1.79 x 10^17 meters.