2 GuidepostPrevious chapters have used the basic principles of physics as a way to deduce things about stars and the interstellar medium. All of the data we have amassed will now help us understand the life stories of the stars in this chapter and those that follow.In this chapter, we use the laws of physics in a new way. We develop theories and models based on physics that help us understand how stars work. For instance, what stops a contracting star and gives it stability? We can understand this phenomenon because we understand some of the basic laws of physics.Throughout this chapter and the chapters that follow, we search for evidence. What observational facts confirm or contradict our theories? That is the basis of all science, and it must be part of any critical analysis of what we know and how we know it.
3 Outline I. Making Stars from the Interstellar Medium A. Star Birth in Giant Molecular CloudsB. Heating By ContractionC. ProtostarsD. Evidence of Star FormationII. The Source of Stellar EnergyA. A Review of the Proton-Proton ChainB. The CNO CycleIII. Stellar StructureA. Energy TransportB. What Supports the Sun?C. Inside StarsD. The Pressure-Temperature Thermostat
4 Outline (continued)IV. The Orion NebulaA. Evidence of Young Stars
5 Young stars, still in their birth nebulae The Life Cycle of StarsDense, dark clouds, possibly forming stars in the futureAging supergiantYoung stars, still in their birth nebulae
6 Giant Molecular Clouds Barnard 68InfraredVisibleStar formation from the collapse of the cores of giant molecular clouds: dark, cold, dense clouds obscuring the light of stars behind them.(More transparent in infrared light.)
7 Parameters of Giant Molecular Clouds Size: r ~ 50 pcMass: > 100,000 MsunTemp.: a few 0KDense cores:R ~ 0.1 pcM ~ 1 MsunMuch too cold and too low density to ignite thermonuclear processesClouds need to contract and heat up in order to form stars.
8 Contraction of Giant Molecular Cloud Cores Horse Head NebulaThermal Energy (pressure)Magnetic FieldsRotation (angular momentum)TurbulenceExternal trigger required to initiate the collapse of clouds to form stars.
9 Shocks Triggering Star Formation Trifid NebulaGlobules are sites where stars are being born right now!
10 Sources of Shock Waves Triggering Star Formation (1) Previous star formation can trigger further star formation through:a) Shocks from supernovae (explosions of massive stars):Massive stars die young. Supernovae tend to happen near sites of recent star formation
11 Sources of Shock Waves Triggering Star Formation (2) Previous star formation can trigger further star formation through:b) Ionization fronts of hot, massive O or B stars which produce a lot of UV radiation:Massive stars die young. O and B stars only exist near sites of recent star formation
12 Sources of Shock Waves Triggering Star Formation (3) Giant molecular clouds are very large and may occasionally collide with each otherc) Collisions of giant molecular clouds.
13 Sources of Shock Waves Triggering Star Formation (4) d) Spiral arms in galaxies like our Milky Way:Spirals’ arms are probably rotating shock wave patterns.
14 ProtostarsProtostars = pre-birth state of stars:Hydrogen to Helium fusion not yet ignitedStill enshrouded in opaque “cocoons” of dust, so barely visible in the optical, but bright in the infrared.
15 Heating By Contraction As a protostar contracts, it heats up:Free-fall contraction→ Heating
16 Protostellar DisksConservation of angular momentum leads to the formation of protostellar disks – the birth place of planets and moons
17 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
20 From Protostars to Stars Star emerges from the enshrouding dust cocoonIgnition of hydrogen (H) to helium (He) fusion processes
21 Evidence of Star Formation Nebula around S Monocerotis:Contains many massive, very young stars,including T Tauri Stars: strongly variable; bright in the infrared.
22 Evidence of Star Formation (2) Smaller, sunlike stars, probably formed under the influence of the massive starYoung, very massive starOpticalInfraredThe Cone Nebula
23 Evidence of Star Formation (3) Star Forming Region RCW 38
24 Globules Bok Globules: about 10 to 1000 solar masses; Contracting to form protostars
25 Globules (2)Evaporating Gaseous Globules (“EGGs”) are newly forming stars exposed by the ionizing radiation from nearby massive stars
26 Open Clusters of StarsLarge masses of Giant Molecular Clouds means stars do not form isolated, but in large groups, called Open Clusters of Stars.Open Cluster M7
27 Open Clusters of Stars (2) Large, dense cluster of (yellow and red) stars in the foreground; about 50 million years old.Scattered individual (bright, white) stars in the background; only about 4 million years old.
28 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
29 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.
30 Energy Transport Radiative energy transport Convection Energy generated in the star’s center must be transported to the surface.Inner layers:Radiative energy transportOuter layers (including photosphere):ConvectionBubbles of hot gas rising upCool gas sinking downGas particles of solar interiorg-rays
31 Conduction, Convection, and Radiation (SLIDESHOW MODE ONLY)
32 Stellar Structure Sun Flow of energy Energy transport via convection Energy transport via radiationFlow of energyEnergy generation via nuclear fusionBasically the same structure for all stars with approx. 1 solar mass or less.Temperature, density and pressure decreasing
34 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 aboveOutward pressure from the interior
35 Hydrostatic Equilibrium (2) 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.
37 Energy Transport Structure Inner convective, outer radiative zoneInner radiative, outer convective zoneCNO cycle dominantPP chain dominant
38 Summary: Stellar Structure Convective Core, radiative envelope;Energy generation through CNO CycleSunMassRadiative Core, convective envelope;Energy generation through PP Cycle
39 The Orion Nebula: An Active Star-Forming Region
40 In the Orion NebulaThe Becklin-Neugebauer Object (BN): Hot star, just reaching the main sequenceKleinmann-Low nebula (KL): Cluster of cool, young protostars detectable only in the infraredB3B1B1O6Visual image of the Orion NebulaProtostars with protoplanetary disks