18. Stellar Birth

1. 18. Stellar Birth • Star observations & theories aid understanding • Interstellar gas & dust in our galaxy • Protostars form in cold, dark nebulae • Protostars evolve into main-sequence stars • Protostars both gain & lose mass • Star clusters reveal formation & evolution details • Protostars can form in giant molecular clouds • Supernovae can trigger star birth

2. Stellar Observations & Theories • Fundamental observational difficulties • Stars exist far longer than astronomers • Star lifetimes range from millions to billions of years • Stellar birth, life & death observed as stages • Each observation is an extremely brief snapshot • Fundamental observational simplicity • Every star is far simpler than any living organism • The materials are very simple • The processes are very simple • Basic physical processes • Gravity tends to gather matter closer together • Gravity is determined by distance between atoms • Pressure tends to disperse matter farther apart • Pressure is determined by temperature of atoms

3. Interstellar Gas & Dust in Our Galaxy • Emission nebulae • Fluorescence similar to common light bulbs • Emission lines depend on material & temperature • Reflection nebulae • Characteristic blue color • Selective scattering of continuous spectra from stars • Dust particles comparable in size to blue wavelengths • Dark nebulae • Characteristic blocking of background light • May be partial or total blocking • Thermal infrared can penetrate some dark nebulae

4. Initiation of Star Formation • Compression of interstellar medium is essential • Gentle mechanisms from low-mass star death • Gently expanding shell of gas called a “planetary nebula” • Weak shock wave may initiate compression • Gas adds low-mass elements to the forming stars • Usually limited to Carbon & Silicon • Violent mechanisms from high-mass star death • Rapidly expanding gas shell is a “supernova remnant” • Strong shock wave will initiate formation of O & B stars • Gas adds high-mass elements to the forming stars • May include elements as heavy as Uranium

5. The Orion Nebula: A Close-Up View

6. Emission, Reflection & Dark Nebulae

7. Reflection Nebula In Corona Australis

8. Interstellar Reddening by Dust Grains Strongly scattered Weakly scattered

9. Spiral Galaxies: Two Perspectives …Face-on Edge-on…

10. Protostars Form in Cold Dark Nebulae • Basic physical processes • Gravity effects must exceed pressure effects • Highest probability for star formation • Extremely low temperatures minimize pressure • Extremely close atoms maximize gravity • Only dark nebulae have high enough density • Large Barnard objects • A few thousand M☉& ~ 10 pc in diameter • Small Bok globules • Resembles the core of a Barnard object • Basic chemical composition (by mass) • ~ 74% hydrogen • ~ 25% helium • ~ 1% “metals”All elements heavier than helium

11. Anglo-Australian Observatory Bok Globules: Opaque Dust & Gas

12. Protostar Details • Earliest model • Henyey & Hayashi 1950’s • Stage 1 Cool nebula several times Solar System size • Stage 2 Continued contraction raises the temperature Kelvin-Helmholtz contraction • Stage 3 Still quite large, the cloud begins to glow Convection move heat outward Low temperature + Huge surface = Very bright • A protostar the mass of the Sun • After 1,000 years of contraction… • Surface temperature is ~ 2,000 K to 3,000 K • Diameter is ~ 20 times > the Sun • Luminosity is ~ 100 times > the Sun

13. Evolutionary Track of Protostars • High- mass stars • Approximately a horizontal line on an H-R diagram • Progression is toward the left Cool to hot • Solar- mass stars • Approximately a V-shaped line on an H-R diagram • Progression is toward the left Cool to hot • Low- mass stars • Approximately a vertical line on an H-R diagram • Progression is toward the bottom Bright to dim

14. Pre-Main-Sequence Evolutionary Tracks

15. Progress of Star Formation • A positive feedback process • Gravity & pressure increase as the nebula shrinks • Pressure increases µ d • Gravity increases µ d2 Gravity overwhelms pressure • Magnetism could disrupt this in the earliest stages • Additional characteristics • Angular momentum is conserved • The shrinking nebula spins faster & faster • Original 3-D cloud deforms into a donut-like disk • Material spins inward very rapidly • Much of this material is ejected at the protostar’s poles

16. Culmination of Star Formation • A negative feedback process • High core pressure & temperature sustain H fusion • A new & intense source of heat energy • Core pressure rises dramatically • Gravitational collapse ends • Thermal & hydrostatic equilibrium established • A new star stabilizes on the main sequence

17. Protostars Become Main-Sequence Stars • Protostar temperature changes • Surface Little temperature change • Minimal increase for 15 M☉ protostars • Slight increase for 5 M☉protostars • Slight decrease for 2 M☉protostars • Significant decrease for 1 M☉protostars • Dramatic decrease for 0.5 M☉protostars • Core Dramatic temperature increase • Increasing temperature ionizes the protostar’s interior • Energy is transmitted outward by radiation • Temperatures > several million kelvins initiate fusion • This event marks the “birth” of a true star

18. Protostar Evolution is Mass-Dependent • Very-low- mass starsM < 0.8 M☉ • Core temperatures too low to ionize interior • Convection characterizes the entire interior of the star • Low- mass stars0.8 MSun < M < 4 M☉ • Core temperatures high enough to ionize interior • Radiation characterizes the region surrounding the core • Convection characterizes the region near the surface • High- mass starsM > 4 M☉ • Hydrogen fusion begins very early • Convection characterizes the region surrounding the core • Radiation characterizes the region near the surface

19. Main-Sequence Stars of Different Mass

20. Brown Dwarfs: Failed Stars • A minimum mass is required for fusion • Pressure & temperature cannot get high enough • Minor lithium fusion can occur • Surface temperature may reach ~ 2,000 K • Brown dwarf characteristics • Mass between 1028 kg & 84 . 1028 kg • ~ 10 to 84 times the mass of Jupiter • The lower mass limit is sometimes set at ~ 14 times MJup • Continues to cool & contract • Detectable only at thermal infrared wavelengths • Many brown dwarfs exhibit irregular brightness changes • Possible storms far more violent than on Jupiter

21. Protostars Both Gain & Lose Mass • Protostar formation is extremely dynamic • Matter is drawn inward along an accretion disk • Matter is hurled outward perpendicular to this disk • T Tauri stars • 20th brightest star in the constellation Taurus • Exhibit both emission & absorption spectral lines • Surrounded by hot low-density gas • Doppler shift indicates a velocity of 80 km . sec-1 • Luminosity varies irregularly over several days • Mass ~ 3 M☉ • Herbig-Haro objects • Bipolar outflow compresses & heats interstellar gas • May last only ~ 10,000 to 100,000 years

22. Herbig-Haro Objects: Bipolar Outflow

23. Clusters Reveal Formation & Evolution • Star clusters never have stars of uniform mass • High-mass stars evolve very quickly • O & B spectral class stars emit abundant UV radiation • Low-mass stars evolve very slowly • K & M spectral class stars emit abundant IR radiation • The destiny of excess gas & dust • H II regions • H I regions are neutral (non-ionized) hydrogen • H II regions are singly-ionized hydrogen • Hydrogen has only 1 electron ⇒ Result is free protons & electrons • Produce red emission nebulae • Dust regions • Resist dissipation by strong UV radiation from O & B stars • Produce blue reflection nebulae

24. A Star Cluster With An H II Region

25. H-R Diagram of a Young Star Cluster

26. The Pleiades & Its H-R Diagram

27. Protostars In Giant Molecular Clouds • Characteristics of molecular clouds • 195 different molecules identified in space • ~ 10,000 H2 molecules for every CO molecule • The Milky Way contains ~ 5,000 molecular clouds • These include several star-forming regions • 17 molecular clouds outline the local arm of our galaxy • Orion nebula’s parent cloud contains ~ 500,000 M☉ • Spectral emission lines • Cold dark interstellar hydrogen clouds • Emission in the UV, visible & IR regions of the spectrum • Molecular interstellar gas clouds • Emission in the microwave region of the spectrum

28. Carbon Monoxide Molecular Clouds

29. Molecular Clouds in the Milky Way

30. O & B Stars Trigger Star Formation

31. Supernovae Can Trigger Star Birth • Supernova remnants are common • High-mass stars exhaust their H2 supply very quickly • Many old star clusters have supernova remnants • Supernova remnants are violent • High-mass stars die in tremendous explosions • Spherical shock wave goes outward at supersonic speeds • This compresses interstellar gas & dust clouds • Often results in associations rather than clusters • New stars are moving too fast to stay gravitationally bound • New stars quickly disperse in various directions • Probably the situation when our Sun formed

32. Supernova Remnant in the Cygnus Loop

33. Interstellar gas & dust Emission, reflection & dark nebulae Potential birthplace of stars Stages of star formation Initiation Coldest & densest regions are ideal Contest between gravity & pressure Compression mechanism required Progress Positive feedback: Gravity > Pressure Collapse accelerates until fusion Culmination Heat from fusion increases pressure Equilibrium is established Protostar evolution depends on mass Very-low- mass < 0.8 times MSun Low- mass < 4 times MSun High- mass > 4 times MSun Mass gain & loss in protostars Circumstellar accretion disk inflows Bipolar outflows T Tauri [variable] stars Herbig-Haro objects Star clusters give evolution details Few clusters have same-age stars Luminosity & color on H-R diagram Stellar models fit observations well Star formation in molecular clouds ~ 5,000 in the Milky Way galaxy 17 define our galactic spiral arm Compression mechanisms UV emissions from OB associations Supernova explosions Important Concepts