Lecture Notes: Understanding Stars and Their Sizes

Introduction to Stars

• Stars are massive celestial bodies made mostly of hydrogen and helium.
• They undergo nuclear fusion in their cores, releasing energy that makes them shine.

Scale of Celestial Bodies

• Gas Giants & Sub-Brown Dwarfs: Like Jupiter, are large but not star-like.
• Brown Dwarfs:
  • Between 13 and 90 times the mass of Jupiter.
  • Not true stars; they glow dimly due to some nuclear fusion.

Main Sequence Stars

• Formation: When large gas balls reach a mass threshold, they ignite hydrogen into helium.
• Characteristics:
  • More mass = hotter, brighter, shorter lifespan.
  • Example: Sun, Sirius, Beta Centauri.
• Types:
  • Red Dwarfs: Small, long-lived stars. Example: Barnard's Star.
  • Sun-like Stars: More massive and hotter than red dwarfs.
  • Blue Giants: Hot, massive, short-lived.

The Largest Stars

• R136a1: Most massive known star.
  • 315 solar masses, 9 million times brighter than the Sun.
  • Formed likely by several high mass star mergers.
• Formation Limit: Around 150 solar masses due to mass loss through stellar winds.

Giant and Hypergiant Stars

• Red Giants:
  • Stars swell in size as they exhaust core hydrogen.
  • Example: Gacrux, Sun in its future phase.
• Hypergiants:
  • Blue Hypergiants: Large with massive energy output, e.g., Pistol Star.
  • Yellow Hypergiants: Short-lived, intermediate phase; e.g., Rho Cassiopeiae.
  • Red Hypergiants: Largest known stars like Stephenson 2-18.

Largest Star Discussion

• Stephenson 2-18:
  • Possibly the largest known star with 2150 solar radii.
  • Size estimates are uncertain due to their brightness and distance.
• Scale: Unimaginably large; highlights the vastness of the universe.

Conclusion

• Stars vary widely in size and lifespan.
• The universe contains many massive stars, continually cycling through birth and death.

Additional Resources

• App - Universe In A Nutshell:
  • Explore the scale of the universe from the smallest to largest objects.
  • No ads, future updates included. Available in app stores.

Remarks

• Viewing stars and understanding their scale showcases the grandeur of the universe.
• Feedback is encouraged for the app to improve future digital projects.