Interstellar Medium and Star Formation

Hydrogen is the most abundant element in the universe and makes up about 75% of the mass of the interstellar medium.

Helium is the second most abundant element in the universe and makes up about 23% of the mass of the interstellar medium.

Stars form when a large cloud of gas and dust collapses under its own gravity.

A protostar is a dense core of gas and dust that forms at the center of a collapsing cloud and is the earliest stage in the formation of a star.

A T Tauri star is a young star that is still in the process of forming and is characterized by strong magnetic activity and variability.

A Herbig Ae/Be star is a more evolved young star that is still in the process of forming and is characterized by a strong stellar wind and a circumstellar disk.

A main sequence star is a star that is in the main phase of its life and is characterized by a stable balance between gravitational collapse and nuclear fusion.

A circumstellar disk is a region of space around a young star where planets form.

Planets form by the process of planetary accretion, in which small particles of dust and gas collide and stick together to form larger and larger bodies.

Terrestrial planets are composed primarily of rock and metal and include planets like Earth, Mars, Venus, and Mercury.

Gas giants are composed primarily of gas and include planets like Jupiter, Saturn, Uranus, and Neptune.

Ice giants are composed primarily of ice and include planets like Uranus and Neptune.

Dwarf planets are smaller than planets but larger than asteroids and include objects like Pluto, Eris, and Ceres.

The Sun is the closest star to Earth.

Earth is located in the Milky Way galaxy.