Final answer:
The star-gas-star cycle describes how stars are born from gas and dust, live through nuclear fusion, and expire to contribute to the birth of new stars. This process, observed in the Milky Way and other galaxies, is driven by gravitational forces, nuclear fusion, and the explosive deaths of massive stars, which trigger further star formation.
Step-by-step explanation:
The Star-Gas-Star Cycle
The star-gas-star cycle is a fundamental concept in astrophysics that describes how stars are formed, live out their lives, and die, only to contribute to the formation of new stars. The process begins with large molecular clouds, known as nebulae, which consist of gas and dust. Under gravitational forces, these clouds collapse to form protostars, which may form single or multiple star systems. As protostars collapse, they spin and flatten into disks, where planets can form.
When a star like the Sun exhausts its hydrogen fuel, it undergoes a change, expanding into a red giant and eventually shedding its outer layers to form a planetary nebula. This leaves behind a hot core that is rich in helium, which can fuse into heavier elements like carbon and oxygen, although for a Sun-like star, this marks the nearing of its life's end. In more massive stars, however, the carbon-nitrogen-oxygen (CNO) cycle can become the dominant process for energy production until these stars exhaust their fuel as well and explode as supernovae.
These explosive deaths of stars trigger the formation of new stars by compressing the surrounding molecular clouds, increasing their density. This can set off a chain reaction, facilitating the formation of new stars in neighboring regions. The materials expelled by dying stars, including heavy elements, enrich the interstellar medium, providing the ingredients for future celestial bodies. This ongoing cycle is a cornerstone of galactic evolution, making it possible for the Milky Way Galaxy and other galaxies to continue hosting stellar generations. Evidences of such processes can be observed by astronomers using advanced telescopes like the Hubble Space Telescope.