Final answer:
A main-sequence star is one that generates its energy from the nuclear fusion of hydrogen into helium in its core, maintaining an equilibrium. Most stars, including the Sun, are main-sequence stars, which is a stage that lasts for the majority of a star's life. The defining feature of these stars is their stable balance between gravitational forces and the energy produced by fusion.
Step-by-step explanation:
A main-sequence star is a star that derives its energy from the nuclear fusion of hydrogen into helium in its core. This process of fusion provides the energy needed to maintain the star's equilibrium, preventing further condensation and making the star stable. Main-sequence stars, such as our Sun, are characterized by a delicate balance between gravitational collapse and the outward pressure from nuclear fusion, which is the defining feature of this phase in a star's life.
During the main-sequence stage, a star will spend the majority of its lifetime in a relatively stable state. Approximately 90% of all stars are found on the main sequence of the Hertzsprung-Russell diagram, which represents a continuum of stars from the hottest and most luminous to the coolest and dimmest. Moreover, the Sun fuses about 620 million metric tons of hydrogen into helium every second, illustrating the immense scale of energy production within these stellar powerhouses.
Main-sequence stars vary greatly in size, mass, luminosity, and color. However, they all share the common characteristic of producing energy through the nuclear fusion of hydrogen. The main sequence can be visualized on the Hertzsprung-Russell diagram, which provides a graphical representation of the relationship between a star's luminosity and its surface temperature or color.