8.1.4 The Life Cycle of Solar Mass Stars

Stars go through several stages in their lifecycle, starting from clouds of dust and gas in a galaxy. Here are the main stages of a star's life:

1. Formation from Dust and Gas Clouds

• Stars begin as large clouds of dust and gas, often called nebulae, found within galaxies.
• The gravitational attraction between the dust and gas particles causes them to draw closer together, making the cloud more concentrated.
• As the particles get closer, the temperature and pressure within the cloud increase due to compression.

2. Birth of a Star

• Eventually, the pressure and temperature become so high that the hydrogen gas within the cloud begins to undergo nuclear fusion, where light hydrogen nuclei fuse to form helium nuclei.
• This fusion process releases a tremendous amount of energy, which counteracts the gravitational collapse of the cloud.
• An equilibrium is reached when the outward pressure from the energy produced by fusion balances the inward pull of gravity.
• At this point, a protostar forms, which then matures into a main sequence star. The star can remain in this stable state, fusing hydrogen into helium, for billions of years.

3. Evolution After Hydrogen Depletion

• Eventually, the star exhausts its hydrogen fuel. Without fusion to produce outward pressure, the star becomes unstable and begins to collapse under its own gravity.

• The next stages of the star's life depend on its initial mass: For Massive Stars:

• The core collapse increases the temperature and pressure to levels where heavier elements can fuse.

• After the fusion of heavier elements, the star becomes too massive to maintain stability and undergoes a dramatic collapse.

• This collapse causes the star to explode in a supernova, a massive explosion that expels the outer layers of the star into space.

• The core that remains after the supernova may form a neutron star (extremely dense core) or, if massive enough, collapse further into a black hole. For Medium-sized Stars (like the Sun):

• As the hydrogen runs out, the star begins to fuse helium and other elements in its core, but less vigorously compared to massive stars.

• The outer layers are expelled gently, forming a planetary nebula—a shell of gas that surrounds the remaining core.

• The remaining core cools and contracts into a white dwarf, a small, dense, and slowly cooling remnant.