ASTR L

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).

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).

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).

Electrons returning to lower energy states emit photons in visible light (from originally UV light).
H II regions emit their own light (emission nebulae).

Discrete lines from electron transitions in hydrogen (e.g., Hα line at ~656 nm).
Responsible for red glow seen in some 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).

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).

Dynamic cycle involving gas accreting from intergalactic space and matter being ejected during stellar deaths.
The baryon cycle describes this ongoing transformation.

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.

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.

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 initiate jets from protostars; collisions with surrounding gas produce visible light (HH objects).
T Tauri Stars: Transitional phase before stars reach the main sequence.

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.

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

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.

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.

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.