star formation lecture powerpoint presentation
- 1. The Formation and Structure of Stars Chapter 9
- 2. 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
- 3. Structure of the ISM • HI clouds: • Hot intercloud medium: The ISM occurs mainly in two types of clouds: Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 – a few hundred atoms/cm3); size: ~ 100 pc 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.
- 4. 3 types of nebula 1. Emission 2. Reflection 3. Dark Q: Why do emission nebula look red and reflection nebula blue?
- 5. 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 Evidence for the ISM
- 6. Interstellar reddening Q: Why do astronomers rely heavily on IR observations?
- 7. Q: How do we know the ISM exists?
- 8. 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.
- 9. 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
- 11. The Contraction of a Protostar Q: Why do you think there’s a lower limit on the mass of a main-seq. star?
- 12. The Contraction of a Protostar Sun: ~30 million years 15 M: 160,000 years 0.2 M: 1 billion years
- 13. From Protostars to Stars Ignition of H He fusion processes Star emerges from the enshrouding dust cocoon
- 14. Protostellar Disks and Jets – Herbig-Haro Objects Herbig-Haro Object HH34 Q: What are the bipolar flows evidence of?
- 15. Globules Bok globules: ~ 10 – 1000 solar masses; Contracting to form protostars
- 16. Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars Observations of star formation:
- 17. 200 solar mass star
- 21. N 11B
- 22. Trifid V838 Mon
- 23. Tarantula N 49
- 25. The Source of Stellar Energy Stars produce energy by nuclear fusion of hydrogen into helium. In the sun, this happens primarily through the proton-proton (P-P) chain Q: How does the sun fuse H to He?
- 26. The CNO Cycle 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?
- 27. 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).
- 28. 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 2 4 dM r dr 2 4 dL r e dr 2 dP GM dr r 3 2 3 16 dT L dr ac T r
- 29. Hydrostatic Equilibrium Imagine a star’s interior composed of individual shells Within each shell, two forces have to be in equilibrium with each other: Outward pressure from the interior Gravity, i.e. the weight from all layers above
- 30. 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?
- 31. Energy Transport 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?
- 33. 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….
- 34. The Life of Main-Sequence Stars Stars gradually exhaust their hydrogen fuel. They gradually becoming brighter, evolving off the zero-age main sequence (ZAMS). 3.5 2.5 fuel 1 rate of consumption M M M Lifetime of a main-sequence star (90% of total life is on main-seq.)
- 35. The Lifetimes of Stars on the Main Sequence
- 38. The Orion Nebula: An Active Star-Forming Region
- 39. The Trapezium The Orion Nebula Infrared image: ~ 50 very young, cool, low- mass stars X-ray image: ~ 1000 very young, hot stars less than 2 million years old
- 40. The Becklin- Neugebauer object (BN): Hot star, just reaching the main sequence Kleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infrared Spectral types of the trapezium stars Protostars with protoplanetary disks B3 B1 B1 O6 IR + visual IR Gas blown away from protostars