Stars, Their Origin and Evolution Notes

The Birth of a Star: From Nebula to Protostar

1. Nebulae are vast clouds of gas and dust, primarily composed of hydrogen.
2. Gravitational forces cause denser regions within these clouds to collapse, forming cloudlets.
3. As the cloudlet contracts, it heats up due to friction and compression, eventually forming a protostar.

A Star Is Born: The Onset of Nuclear Fusion

1. When the core temperature of a protostar reaches 10 million K, nuclear fusion begins.
2. Hydrogen nuclei fuse to form helium, releasing energy according to Einstein's equation: \$E = mc^2\$.
3. This energy creates outward pressure that balances the inward pull of gravity, stabilizing the star.

The Main Sequence: A Star's Longest Phase

1. Main sequence stars fuse hydrogen into helium in their cores.
2. The position of a star on the Hertzsprung-Russell (H-R) diagram depends on its mass:
3. High-mass stars: Hot, blue, and luminous.
4. Low-mass stars: Cool, red, and dim.

The Evolution of Stars: Mass Determines Destiny

1. Low-Mass Stars (e.g., Red Dwarfs)

• Mass: Less than 0.5 solar masses.
• Fusion Rate: Slow, allowing them to last for trillions of years.
• End Stage: Gradually cool into white dwarfs without becoming giants.

2. Sun-Like Stars

• Mass: 0.5 to 8 solar masses.
• Red Giant Phase:
  • Hydrogen in the core is exhausted.
  • The core contracts and heats up, igniting hydrogen in a shell around the core.
  • The outer layers expand, and the star becomes a red giant.
• Helium Fusion:
  • The core temperature reaches 100 million K, allowing helium to fuse into carbon.
• End Stage:
  • The outer layers are ejected as a planetary nebula.
  • The core remains as a white dwarf, which eventually cools into a black dwarf.

3. High-Mass Stars

• Mass: Greater than 8 solar masses.
• Supergiant Phase:
  • These stars fuse heavier elements in their cores, forming layers like an onion.
• Supernova:
  • When iron forms in the core, fusion stops, and the core collapses.
  • The outer layers explode in a supernova, dispersing elements into space.
• End Stage:
  • The core becomes a neutron star or, if massive enough, a black hole.

The Hertzsprung-Russell Diagram: A Map of Stellar Evolution

1. The H-R diagram plots stars based on their luminosity and surface temperature.
2. Key regions include:
3. Main Sequence: Where stars spend most of their lives.
4. Red Giants and Supergiants: Cool but luminous stars in later stages.
5. White Dwarfs: Hot but dim remnants of low- to medium-mass stars.

Why Study Stellar Evolution?

1. Element Formation: Stars create elements like carbon, oxygen, and iron through fusion.
2. Galactic Recycling: Supernovae distribute these elements, enriching the interstellar medium and enabling the formation of new stars and planets.
3. Cosmic History: Understanding stars helps us trace the history and future of the universe.