Approximately how long did the Sun spend as a collapsing protostar before fusion began?

• A. 1 million years
• B. 10 thousand years
• C. 50 million years
• D. 100 thousand years

Answer: (D)

During the early life of a Sun‑like star, gravity pulls in surrounding gas and the object contracts, forming a protostar. The time spent in this protostellar phase before hydrogen fusion can sustain the energy output is the pre‑main‑sequence period. For a star like the Sun, it takes tens of millions of years for the core to heat up enough to ignite sustained hydrogen fusion and settle onto the main sequence. In the solar case, estimates place this contraction phase at roughly 30 to 50 million years (often cited around 40–50 million years). The initial collapse to a protostar happens much sooner, but the longer period of contraction and heating before fusion begins dominates the timescale here. So the Sun spent on the order of tens of millions of years as a collapsing protostar before fusion began, not just hundreds of thousands of years.