Here's why:

* Main Sequence: Main sequence stars are stars that are fusing hydrogen into helium in their core. This is the longest and most stable phase of a star's life.

* Temperature and Size: The temperature of a main sequence star is directly related to its mass. More massive stars are hotter, smaller stars are cooler.

* The Sun: Our Sun is a medium-sized, average-temperature main sequence star.

Therefore:

* Hotter than the Sun: Stars with larger masses than the Sun are hotter. They burn hotter and have a shorter lifespan.

* Cooler than the Sun: Stars with smaller masses than the Sun are cooler. They burn cooler and have a longer lifespan.

Examples:

* Hotter: Blue giants like Rigel are much hotter than our Sun.

* Cooler: Red dwarfs like Proxima Centauri are much cooler than our Sun.