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.

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

Factors in Nuclear Fusion Hydrogen atoms are ionized (bare nuclei) Nuclei repel each other (Coulomb barrier) High enough temperature means a small percentage will have a high enough energy to get close enough for strong interaction to occur (Maxwell distribution of velocities) Sufficiently high pressure ensures that enough reactions occur to supply energy needs of star

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 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.

Energy Source 1H + 1H  2H + e+ + ν 2H + 1H  3He + γ 2H moving fast e+ annihilates an electron producing Gamma rays Neutrino escapes from sun 2H + 1H  3He + γ 3He + 3He  4He + 1H + 1H

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.

Regulation of Energy Production If the energy production were to be insufficient then temp of core would decrease. Pressure would decrease which would cause star to contract causing temp to increase again because of energy release from gravity. If energy production were to be too much then the steps would occur in reverse.

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