Final answer:
On the main sequence, as a star's luminosity increases, its temperature also increases. After the nuclear fuel is exhausted, stars evolve into giants with higher luminosity and lower surface temperature, moving upward and to the right on the H-R diagram.
Step-by-step explanation:
As luminosity increases on the main sequence, the temperature of the stars also increases. This is because main-sequence stars undergo nuclear fusion at their cores, converting hydrogen into helium and releasing energy in the process. Massive stars on the main-sequence have high temperatures and high luminosities, while low-mass stars have low temperatures and low luminosities.
However, once main-sequence stars expend their nuclear fuel, they leave the main sequence. For example, a star like our Sun eventually becomes a red giant, a stage characterized by an increase in luminosity and a decrease in surface temperature. At this point, the star moves on the H-R diagram upward (becoming brighter) and to the right (indicating cooler surface temperature), eventually evolving into red supergiants or giants, with contracting cores and expanding outer layers.