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 Thematerialsare very simple Theprocessesare very simple Basic physical processes –Gravitytends togathermattercloser together Gravityis determined bydistance between atoms –Pressuretends todispersematterfarther apart Pressureis determined bytemperature of atoms

3. Interstellar Gas & Dust in Our Galaxy Emissionnebulae –Fluorescence similar to common light bulbs Emission lines depend on material & temperature Reflectionnebulae –Characteristic blue color Selective scattering of continuous spectra from stars Dust particles comparable in size to blue wavelengths Darknebulae –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 –Gentlemechanisms 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 –Violentmechanisms 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 Weakly scattered Strongly 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 Extremelylow temperaturesminimizepressure Extremelyclose atomsmaximizegravity –Only dark nebulae have high enough density LargeBarnard objects –A few thousand M ☉ & ~ 10 pc in diameter SmallBok 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. Bok Globules: Opaque Dust & Gas Anglo-Australian Observatory

12. Protostar Details Earliest model –Henyey & Hayashi1950’s Stage 1Cool nebula several times Solar System size Stage 2Continued contraction raises the temperature Kelvin-Helmholtz contraction Stage 3Still 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 temperatureis ~ 2,000 K to 3,000 K Diameter is ~ 20 times > the Sun Luminosityis ~ 100 times > the Sun

13. Evolutionary Track of Protostars High-mass stars –Approximately ahorizontalline on an H-R diagram Progression is toward the leftCool to hot Solar-mass stars –Approximately aV-shapedline on an H-R diagram Progression is toward the leftCool to hot Low-mass stars –Approximately averticalline on an H-R diagram Progression is toward the bottomBright to dim

14. Pre-Main-Sequence Evolutionary Tracks

15. Progress of Star Formation A positive feedback process –Gravity & pressure increase as the nebula shrinks Pressureincreases  d Gravityincreases  d 2 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 –SurfaceLittle temperature change Minimalincreasefor 15 M ☉ protostars Slightincreasefor 5 M ☉ protostars Slightdecreasefor 2 M ☉ protostars Significantdecreasefor 1 M ☉ protostars Dramaticdecreasefor 0.5 M ☉ protostars –CoreDramatic 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 temperaturestoo lowto ionize interior Convection characterizes the entire interior of the star Low-mass stars 0.8 M Sun < M < 4 M ☉ –Core temperatureshigh enoughto 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 10 28 kg & 84. 10 28 kg ~ 10 to 84 times the mass of Jupiter The lower mass limit is sometimes set at ~ 14 times M Jup –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 drawninwardalong an accretion disk –Matter is hurledoutwardperpendicular to this disk T Tauri stars –20 th 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 Produceredemissionnebulae –Dust regions Resist dissipation by strong UV radiation from O & B stars Producebluereflectionnebulae

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 H 2 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 theUV, visible & IR regionsof the spectrum –Molecular interstellar gas clouds Emission in themicrowave regionof 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 H 2 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 M Sun –Low-mass< 4 times M Sun –High-mass> 4 times M Sun 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