Final answer:
After helium fusion begins in a low-mass star, its luminosity initially decreases due to the helium flash. Eventually though, as the star expands into a red giant, its overall luminosity increases.
Step-by-step explanation:
When helium fusion begins in a low-mass star, several changes occur. The core temperature rises until helium fuses into carbon in a process known as the triple-alpha process. After this helium flash, the star's luminosity and size initially decrease. However, eventually, as helium is used up, the star expands into a red giant again leading to an overall increase in luminosity. Therefore, the correct answer would be: 1) Its luminosity goes up.
After the helium flash, the release of energy from helium fusion changes the star's interior structure and leads to a temporary decrease in luminosity. As the star moves away from the main sequence band on the Hertzsprung-Russell diagram, its point shifts to indicate a new luminosity and temperature. As fusion progresses, there is an instability which often results in the outer layers being shed, forming a planetary nebula while exposing the hot core.