ASTR L

ASTR 1P02 Lecture Notes

Lecture Info

• Course: ASTR 1P02

• University: Brock University

• Instructor: Prof. Barak Shoshany

Lecture 14: Star Formation and Evolution

Objectives

• Study interstellar matter and nebulae.

• Explore star formation processes.

• Analyze the evolution of stars throughout their lifetimes.

• Examine the star-forming region NGC 3324 in the Carina Nebula (infrared).

Interstellar Medium (ISM)

• Definition: Matter existing in the space between stars.

• Nebulae: Massive clouds of interstellar matter; can glow or reflect light.

  • Plural term: nebulae (pronounced "NEH-byoo-lee").

• Characteristics of ISM:

  • Constantly shifts, merges, grows, and disperses like Earth's clouds.

  • Stars form when clouds collapse under gravity, and die by ejecting material back into space.

• Composition:

  • Gas: ~99% (mainly hydrogen and helium, plus heavier elements).

  • Dust: ~1% (solid particles like silicates or graphite).

Density of Interstellar Matter

• Interstellar gas: low density (~1 atom/cm³), compared to Earth air (~10^{19} atoms/cm³).

• Interstellar dust: extremely low density (~1,000 grains/km³), average size <0.1 micrometers.

• Covers more volume than stars, e.g., distance from Sun to Proxima Centauri (4.2 light-years).

Properties of Interstellar Gas

• Temperature: Ranges from near absolute zero to millions of degrees (e.g., heated by hot stars).

• Regions can be characterized as:

  • H I Region: Neutral hydrogen.

  • H II Region: Ionized hydrogen (ionization caused by UV light).

Fluorescence in H II Regions

• Electrons returning to lower energy states emit photons in visible light (from originally UV light).

• H II regions emit their own light (emission nebulae).

Spectral Lines: Balmer Series

• Discrete lines from electron transitions in hydrogen (e.g., Hα line at ~656 nm).

• Responsible for red glow seen in some nebulae.

Observations of Nebulae

• Example: Orion Nebula displays red glow from hot stars, with blue from dust scattering starlight.

• Simulations available for visualizing nebula structures (e.g., flying through the Orion Nebula).

Molecular Clouds

• Large clouds (>1 million M⊙) primarily of molecular hydrogen (H₂) and other molecules.

• High density compared to average interstellar gas; blocks UV light, resulting in cold temperatures (~10 K).

Interstellar Dust

• Dark Nebulae: Seen by blocking star light; visible as dark patches.

• Absorb visible/UV light; warm up to emit infrared radiation.

• Reflection Nebulae: Nearby luminous stars reflect blue light more than red.

Cosmic Rays

• High-speed charged particles, ~90% protons, some helium nuclei.

• Likely sources: supernovae; charged particles can be trapped in shock waves.

Life Cycle of Interstellar Matter

• Dynamic cycle involving gas accreting from intergalactic space and matter being ejected during stellar deaths.

• The baryon cycle describes this ongoing transformation.

Superbubbles

• Formed by supernovae; hot, low-density regions that expand and can be detected via X-ray emissions.

• Local Bubble: Contains our solar system; formed ~14 million years ago.

Star Formation Processes

• Stars form in stellar nurseries within molecular clouds.

• Clumps and cores within these clouds lead to the protostar stage.

• Collision and gravitational collapse lead to protostar formation, often surrounded by dust and gas.

Protostar Evolution

• Initial non-fusion processes convert gravitational energy to heat.

• Supports the conservation of angular momentum causing forming disks.

• The emergence of jets and outflows visualizes material expulsion from developing stars.

Stellar Winds and Herbig-Haro Objects

• Stellar winds initiate jets from protostars; collisions with surrounding gas produce visible light (HH objects).

• T Tauri Stars: Transitional phase before stars reach the main sequence.

Main Sequence Stars

• Reached when hydrogen fusion initiates (~12 million K); remains stable and in equilibrium.

• Evolutionary tracks visualized on the Hertzsprung-Russell (H-R) diagram, illustrating stellar evolution stages.

Evolving Giants

• Post-main-sequence, stars undergo structural changes, expanding and cooling while attempting to stabilize. Relationship between core compositions and fusion processes is critical.

Planetary Nebula and Stellar Death

• Transition processes leading to ejected shells result in luminous concentrations of material.

• Planetary Nebulae: Appearance derives from interactions of nuclear processes with outer materials, misnamed historically.

Massive Star Development

• Continue fusion processes beyond carbon-oxygen core, creating heavier elements (up to iron).

• Fusion capabilities dictate subsequent evolutionary paths, leading to supernovae or black holes upon collapse.

Summary

• Stars form from molecular clouds, evolve significantly over their lifetimes, eventually leading to a wide variety of end stages.

• Next lecture will cover stellar death processes.