Question

At what point in time do we say that the protosun became the Sun?

A) when thermonuclear fusion reactions began at its center
B) when it became hot enough to emit light and heat
C) when the temperature began to rise at its center
D) when planetary formation was complete

Answer 1

The protosun became the Sun when thermonuclear fusion reactions began at its center, specifically when the core temperature reached over 10 million K allowing hydrogen to fuse into helium.

Step-by-step explanation:

The protosun is considered to have become the Sun, our star, at the point when thermonuclear fusion reactions began at its center. This is when the core temperature exceeded 10 to 12 million K, enabling hydrogen to be fused into helium. The transformation from a protostar to a star is marked by the initiation of these nuclear processes, which are responsible for the Sun's energy output and light.

Despite other phases such as the protostar heating up or emitting light, or the completion of planetary formation, the defining moment of a star's birth is the onset of stable nuclear fusion at its core. The process of nuclear fusion within the Sun involves the conversion of hydrogen nuclei into helium and is primarily concentrated at the boundary of the helium core where the temperature is the highest.