Define a star, a stellar, a solar system, a galaxy.

- Star: A star is a massive, luminous sphere of plasma held together by its own gravity. Stars emit light and heat through nuclear fusion reactions in their cores, which convert hydrogen into helium. The Sun is an example of a star.

- Stellar: "Stellar" is an adjective that refers to anything related to stars or resembling a star. For example, a stellar explosion is a powerful release of energy from a star, such as a supernova.

- Solar system: A solar system is a collection of planets, moons, asteroids, comets, and other celestial bodies that orbit a star, typically in a flat plane. Our solar system is centered around the Sun and includes eight planets, including Earth, as well as numerous smaller objects.

- Galaxy: A galaxy is a massive system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. Galaxies come in various shapes and sizes, ranging from small dwarf galaxies to massive spiral and elliptical galaxies. The Milky Way is the galaxy that contains our solar system.

Star:
- A self-luminous celestial body formed from the gravitational collapse of a cloud of hydrogen and helium gas.
- Characterized by nuclear fusion reactions at its core, releasing immense amounts of energy.

Stellar:
- Related to or characteristic of stars.
- Used to describe objects, phenomena, or processes associated with stars.

Solar System:
- A gravitationally bound system consisting of a star (usually the Sun) and any orbiting celestial bodies.
- Includes planets, moons, dwarf planets, asteroids, comets, and other celestial objects.

Galaxy:
- A vast collection of stars, gas, and dust bound together by gravity.
- Contains billions to trillions of stars, organized into various structures such as spiral arms, bars, and bulges.
- Galaxies vary in size, shape, and age, and are classified into different types (e.g., elliptical, spiral, irregular).