Final answer:
High-mass stars are those with initial masses significantly greater than the Sun's, usually over 8.0 Msun, and they have unique evolutionary paths and behaviors compared to low-mass stars.
Step-by-step explanation:
In the context of understanding stellar lives, high-mass stars are defined as those with an initial mass significantly greater than our Sun. Specifically, high-mass stars are often considered to be those with initial masses that exceed approximately 8.0 solar masses (Msun). These stars can lose quite a bit of mass as they age and approach the end of their lifecycles, potentially reducing to around 1.4 Msun by the time they die. Nevertheless, these stars have unique evolutionary paths that lead to different behaviors and endpoints compared to low-mass stars, such as our Sun. The study of these high-mass stars is essential because they evolve more quickly and have a more varied range of possible behaviors at the end of their lives, including some of the most energetic phenomena in the universe like supernovae.