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

The Interstellar Medium (ISM) Gas: ~75% H, 25% He, traces of “metals” 1% “dust” (silicates, carbon, heavy elements coated with ice, About the size of the particles in smoke) 150 m average distance between dust grains “Dense” => ~10 to 1000 atoms/cm3 “Thin” ~ 0.1 atoms/cm3

Structure of the ISM HI clouds: Hot intercloud medium: The ISM occurs mainly in two 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.

3 types of nebula Emission Reflection Dark Q: Why do emission nebula look red and reflection nebula blue?

Evidence for the ISM We see absorption in elements where the background stars are too hot to form these lines Narrow width (low temperature; low density) Multiple components (several clouds of ISM with different radial velocities) => Comes from the ISM

Interstellar reddening Q: Why do astronomers rely heavily on IR observations?

Q: How do we know the ISM exists?

The Various Components of the Interstellar Medium Infrared observations reveal the presence of cool, dusty gas. X-ray observations reveal the presence of hot gas.

Stellar formation from the ISM: Must be triggered by high mass stars – Give off intense radiation Explode as SNs Collapsing cloud can form 10 to 1000 stars Association Cluster

The Contraction of a Protostar Q: Why do you think there’s a lower limit on the mass of a main-seq. star?

The Contraction of a Protostar Sun: ~30 million years 15 M: 160,000 years 0.2 M: 1 billion years

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

Protostellar Disks and Jets – Herbig-Haro Objects Q: What are the bipolar flows evidence of? Herbig-Haro Object HH34

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

Observations of star formation: Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars

200 solar mass star

N 11B

V838 Mon Trifid

Tarantula N 49

The Source of Stellar Energy Stars produce energy by nuclear fusion of hydrogen into helium. Q: How does the sun fuse H to He? In the sun, this happens primarily through the proton-proton (P-P) chain

The CNO Cycle Happens in stars > 1.1 M Happens in stars > 1.1 M More efficient that the P-P chain. Requires high T (>16 million K) Q: Why does the CNO require a higher temp. than the P-P chain?

Fusion into Heavier Elements Fusion into elements heavier than C, O: requires high temperatures (>600 million K); occurs only in very massive stars (more than 8 solar masses).

Stellar structure Conservation of mass: Weight of each shell = total weight Conservation of energy: E(out) = E(from within) Hydrostatic equilibrium: Pressure balances gravity Energy transport: Describes flow of energy

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 (II) Outward pressure force must exactly balance the weight of all layers above, everywhere in the star. This is why we find stable stars on such a narrow strip (main sequence) in the Hertzsprung-Russell diagram. Pressure-temperature thermostat Q: How does the P-T thermostat control the reactions in stars?

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 Basically the same structure for all stars close to 1 solar mass. Q: Why is convection in stars important?

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. …why stars initially all line up along the main sequence, and why there’s a mass-luminosity relation….

The Life of Main-Sequence Stars Stars gradually exhaust their hydrogen fuel. They gradually becoming brighter, evolving off the zero-age main sequence (ZAMS). Lifetime of a main-sequence star (90% of total life is on main-seq.)

The Lifetimes of Stars on the Main Sequence

The Orion Nebula: An Active Star-Forming Region

The Trapezium Infrared image: ~ 50 very young, cool, low-mass stars less than 2 million years old 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 Gas blown away from protostars 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 IR IR + visual Spectral types of the trapezium stars B3 B1 B1 O6 Protostars with protoplanetary disks