What slows down the contraction of a star-forming cloud when it makes a protostar?

If you mix chlorine lithium in nickel together which two elements would be the most reactive

Light from the sun reaches the earth in 8.3 minutes. the velocity is 3.00 x 10^8 m/s. how far is earth from the sun? i know how to get to d = (3 x 10^8 m/s)(498 sec). i just don't know how to get from there to the answer being 1.5 x 10^11 m.

In positive numbers less than 1, the zeros between the decimal point and a non-zero number are blank significant