Four stages in the evolutionary cycle of a low-mass star (similar in mass to the Sun) are described in the table below:
| Stage | Description |
|---|---|
| P | Helium fusion begins in the core, causing the star to contract and settle onto the horizontal branch. |
| Q | A giant molecular cloud collapses under gravity, heating up to form a protostar. |
| R | All nuclear fusion ceases, and the outer envelope is ejected to form a planetary nebula, leaving a white dwarf remnant. |
| S | Core hydrogen is depleted, and hydrogen shell fusion causes the outer layers of the star to expand and cool into a red giant. |
What is the correct chronological order of these stages?
Q→P→S→RQ \rightarrow P \rightarrow S \rightarrow RQ→P→S→R
Q→S→P→RQ \rightarrow S \rightarrow P \rightarrow RQ→S→P→R
S→Q→P→RS \rightarrow Q \rightarrow P \rightarrow RS→Q→P→R
Q→S→R→PQ \rightarrow S \rightarrow R \rightarrow PQ→S→R→P