Lecture Notes: Life Cycle of Stars and Formation of Elements

Introduction

• Understanding the first billion years of the universe leads to understanding stars and galaxies.
• Need to explore the creation of elements beyond hydrogen and helium.
• Stars play a crucial role from birth to death in creating new elements and celestial bodies.

The Life Cycle of Stars

Birth of a Star

• Stars form from clouds of gas and dust, primarily hydrogen and helium.
• Gravity causes the cloud to contract, heating up over millions of years.
• Nuclear fusion begins when sufficient heat is achieved, forming a main sequence star.

Low-Mass Stars

• Range from 13 Jupiter masses to around our Sun's mass.
• Main sequence stars fuse hydrogen into helium, maintaining size and luminosity for billions of years.

Red Giant Phase

• Occurs once hydrogen is depleted.
• The core shrinks and heats up, outer layers expand and cool.
• Star becomes a red giant.

Helium Flash and Beyond

• Helium burning in the core through the triple alpha process.
• Formation of elements like carbon and oxygen.
• Final stages include the horizontal branch and asymptotic giant branch.
• Ends as a white dwarf surrounded by a planetary nebula.

High-Mass Stars

• Form from larger gas clouds with more mass.
• Main sequence stars that are hotter, brighter, and burn fuel faster.

Supernova and Element Formation

• High-mass stars end in supernovae, ejecting heavy elements into space.
• Supernovae enable formation of elements heavier than iron.
• Supernovae are extremely bright and visible across galaxies.

Remnants

• White Dwarfs: Stars below 1.4 solar masses.
• Neutron Stars: Stars with core mass between 1.4 and 3 solar masses.
• Black Holes: Stars with core mass above 3 solar masses.

Black Holes

• Form from stars whose core mass exceeds the neutron degeneracy pressure limit.
• Infinite density where even light cannot escape, creating a singularity.
• Black holes significantly warp spacetime.

Conclusion

• The mass of a star determines its life cycle and the remnants it leaves behind.
• Understanding stars is crucial to understanding elemental formation and the universe's development over billions of years.

Upcoming Topics

• Detailed exploration of black holes.