Defining Stars and Planets
A star is a massive, luminous ball of plasma, held together by its own gravity, that generates and emits light and heat through nuclear fusion in its core. In contrast, a planet is a celestial body that orbits a star, is massive enough to be rounded by its own gravity, but does not undergo nuclear fusion to produce light.
Energy Production and Composition
The primary distinction lies in energy production. Stars are natural nuclear reactors, converting hydrogen into helium through fusion, releasing immense energy. Planets, however, only reflect light from their parent star and do not have internal fusion. Their composition varies, with stars being predominantly hydrogen and helium, while planets can be rocky, gaseous, or icy.
The Sun and Earth as Examples
Our Sun is a perfect example of a star, generating the light and warmth that sustains life on Earth. It is a main-sequence star undergoing constant fusion. Earth, on the other hand, is a planet; it orbits the Sun, is spherical due to its gravity, and reflects the Sun's light, but does not produce its own.
Gravitational Influence and Role
Stars are typically much more massive than planets, exerting the dominant gravitational pull in a star system, which keeps planets in orbit. Planets, while having their own gravitational fields, are gravitationally bound to their stars. This fundamental difference in mass and energy output dictates their distinct roles in the formation and dynamics of galaxies and solar systems.