The Formation and Structure of Stars Chapter 9 The Formation and Structure of Stars

The Interstellar Medium (ISM) The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky. We are interested in the interstellar medium because: a) Dense interstellar clouds are the birth place of stars. b) Dark clouds alter and absorb the light from stars behind them.

Three kinds of nebulae 1) Emission Nebulae (HII Regions) 1) Emission Nebulae (HII Regions) A hot star illuminates a gas cloud; excites and/or ionizes the gas (electrons kicked into higher energy states); electrons recombining, falling back to ground state produce emission lines The Fox Fur Nebula The Trifid Nebula NGC 2246

2) Reflection Nebulae Star illuminates gas and dust cloud Star illuminates gas and dust cloud star light is reflected by the dust reflection nebula appears blue because blue light is scattered by larger angles than red light same phenomenon makes the day sky appear blue (if it’s not cloudy)

Emission and Reflection Nebulae

3) Dark Nebulae Dense clouds of gas and dust absorb the light from the stars behind; appear dark in front of the brighter background Barnard 86 Horsehead Nebula

Interstellar Reddening Blue light is strongly scattered and absorbed by interstellar clouds. Red light can more easily penetrate the cloud, but it is still absorbed to some extent. Infrared radiation is hardly absorbed at all. Barnard 68 Interstellar clouds make background stars appear redder. Visible Infrared

Interstellar Absorption Lines The interstellar medium produces absorption lines in the spectra of stars. These can be distinguished from stellar absorption lines through: a) Absorption from wrong ionization states Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple components b) Small line width (too low temperature; too low density) c) Multiple components (several clouds of ISM with different radial velocities)

Structure of the ISM HI clouds: Hot intercloud medium: The ISM occurs in two main types of clouds: HI clouds: Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 – a few hundred atoms/cm3); size: ~ 100 pc Hot intercloud medium: Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm3); gas can remain ionized because of very low density

Shocks Triggering Star Formation The gas in the ISM needs to be compressed in order to collapse and form stars: Shocks traveling through interstellar space can do this.

Shocks Triggering Star Formation

Compression of the ISM by Winds from Hot Stars

The Contraction of a Protostar

From Protostars to Stars Star emerges from the enshrouding dust cocoon Ignition of H → He fusion processes

Evidence of Star Formation Nebula around S Monocerotis: Contains many massive, very young stars, including T Tauri Stars: strongly variable; bright in the infrared.

T Tauri Stars Very young stars, still in the forming stage Very young stars, still in the forming stage Typically 100,000 – 10 million years old

Protostellar Disks and Jets – Herbig Haro Objects Disks of matter accreted onto the protostar (“accretion disks”) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig Haro Objects

Contracting to form protostars Globules Bok Globules: ~ 10 – 1000 solar masses; Contracting to form protostars

Globules Evaporating Gaseous Globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars.

Winds from Hot Stars Very young, hot stars produce massive stellar winds, blowing parts of it away into interstellar space. Eta Carinae

The Orion Nebula An Active Star-Forming Region

The Trapezium The 4 trapezium stars: Brightest, very young (less than 2 million years old) stars in the central region of the Orion nebula Only one of the trapezium stars is hot enough to ionize hydrogen in the Orion nebula. Infrared image: ~ 50 very young, cool, low-mass stars X-ray image: ~ 1000 very young, hot stars The Orion Nebula

Spectral types of the trapezium stars Kleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infrared The Becklin-Neugebauer Object (BN): Hot star, just reaching the main sequence Spectral types of the trapezium stars B3 B1 B1 O6 Visual image of the Orion Nebula Protostars with protoplanetary disks

The Source of Stellar Energy Recall from our discussion of the sun: Stars produce energy by nuclear fusion of hydrogen into helium In the sun, this happens primarily through the proton-proton (PP) chain.

The CNO Cycle The CNO Cycle In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over. The CNO Cycle

Fusion into Heavier Elements Fusion into heavier elements than C, O: requires very high temperatures; occurs only in very massive stars (more than 8 solar masses)

Hydrostatic Equilibrium Imagine a star’s interior composed of individual shells. Within each shell, two forces have to be in equilibrium with each other: Gravity, i.e. the weight from all layers above Outward pressure from the interior

Hydrostatic Equilibrium Outward pressure force must exactly balance the weight of all layers above everywhere in the star. This condition uniquely determines the interior structure of the star. This is why we find stable stars on such a narrow strip (Main Sequence) in the Hertzsprung-Russell diagram.

Energy Transport Radiative energy transport Convection Energy generated in the star’s center must be transported to the surface. Inner layers of the sun: Radiative energy transport Outer layers of the sun (including photosphere): Convection

Stellar Structure Sun Flow of energy Energy transport via convection Energy transport via convection Sun Energy transport via radiation Flow of energy Energy generation via nuclear fusion Basically the same structure for all stars with approx. 1 solar mass or less. Temperature, density and pressure decreasing

The structure and evolution of a star is determined by the laws of: Stellar Models The structure and evolution of a star is determined by the laws of: Hydrostatic equilibrium Energy transport Conservation of mass Conservation of energy A star’s mass (and chemical composition) completely determines its properties. That’s why stars initially all line up along the main sequence.

Interactions of Stars and their Environment Supernova explosions of the most massive stars inflate and blow away remaining gas of star forming regions. Young, massive stars excite the remaining gas of their star forming regions, forming HII regions.

The Life of Main Sequence Stars Stars gradually exhaust their hydrogen fuel. In this process of aging, they are gradually becoming brighter, evolving off the zero-age main sequence.

The Lifetimes of Stars on the Main Sequence