The distinction between a star and a planet goes beyond just size and mass, encompassing various characteristics and processes. Here are some key differences between stars and planets:
Formation: Stars are formed through the gravitational collapse of large clouds of gas and dust called nebulae. The intense gravitational forces at their cores trigger nuclear fusion, leading to the ignition of hydrogen and the release of tremendous amounts of energy. Planets, on the other hand, form from the leftover material in the protoplanetary disk surrounding a young star. These materials come together through processes such as accretion and eventually form planets.
Energy Source: Stars generate energy through the process of nuclear fusion. In their cores, hydrogen nuclei combine to form helium, releasing vast amounts of energy in the form of light and heat. Planets do not have internal sources of energy like stars; instead, they primarily rely on the radiation they receive from their parent star (in our case, the Sun) for their heat and light.
Size and Mass: Stars are typically much larger and more massive than planets. They can range from small, low-mass stars called red dwarfs to massive, high-mass stars known as blue giants. Planets are comparatively smaller and have significantly less mass. For instance, Earth has a mass roughly 330,000 times smaller than that of the Sun.
Composition: Stars are primarily composed of hydrogen and helium, with trace amounts of other elements. They have a plasma state due to the high temperatures and pressures in their cores. In contrast, planets are made up of various materials such as rock, metal, and in some cases, gas. The composition of a planet can vary depending on factors like distance from the star, formation history, and elemental abundances in the protoplanetary disk.
Light Emission: Stars emit their own light, which is produced as a result of the nuclear fusion reactions occurring in their cores. Planets, however, do not generate their own light. Instead, they reflect the light from their parent star and appear as illuminated objects in the sky.
Galaxies and planets like Earth exist on vastly different scales. Galaxies are enormous structures containing billions or even trillions of stars, as well as other celestial objects like nebulae, star clusters, and dust. They can span hundreds of thousands to millions of light-years in size. In contrast, planets like Earth are relatively small rocky bodies orbiting stars within galaxies. While Earth is about 12,742 kilometers in diameter, the Milky Way galaxy, our home galaxy, has a diameter of about 100,000 light-years. Thus, the comparison between a galaxy and one of its constituent planets highlights the significant disparity in size and complexity of these astronomical objects.