Our Sun and its planets were formed about 4.5 billion years ago.
Stars form by gravitational collapse.
Part of the gas and dust material accretes onto the circumstellar disk, in which planets can form by fragmentation and coagulation. Young stars can be observed as infrared protostars and also optically as so-called T Tauri stars. They can be identified as being young by their location in the Hertzsprung-Russell diagram and by some emission and absorption lines in the spectrum. We will also include the physics of brown dwarfs, substellar objects, which cannot sustain stable fusion of hydrogen and which lie in between stars and planets in terms of their mass.