pääketjua

Pääketjua is a Finnish term that translates to "main chain" or "main sequence" in English, primarily used in the context of stellar evolution. It refers to the longest and most stable phase of a star's life, during which it fuses hydrogen into helium in its core. This process releases a significant amount of energy, which provides the outward pressure that counteracts the inward pull of gravity, keeping the star in a state of hydrostatic equilibrium.

Stars spend the vast majority of their existence on the main sequence. The position of a star on the main sequence is determined by its mass. More massive stars are hotter, brighter, and have shorter lifespans on the main sequence compared to less massive stars. Our Sun, for instance, is a main-sequence star. The color of a main-sequence star is also indicative of its temperature and mass, with hotter, more massive stars appearing blue or white, and cooler, less massive stars appearing red or orange. When a star exhausts the hydrogen fuel in its core, it leaves the main sequence and begins the next phase of its evolution, which depends on its initial mass.