The Life Cycle of Solar Mass Stars Revision Notes for OCR GCSE Physics A (Gateway Science Suite)

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:

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.

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.

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.