A Star Is Born

1. A Star Is Born Looking at the Interstellar Medium

2. The Stuff in Space • Material exists between the stars and planets • “Building Blocks” for more stars and planets • Interstellar Medium • Composed of Gas and Dust

3. Interstellar Medium • Seen in telescopes • Large clouds of gas and dust are stellar “nurseries” • ISM is clumpy! • Obscures objects beyond it

4. Interstellar Gas • Gas is mostly individual atoms • Individual neutral atoms and ions • Molecules • Electrons • Hydrogen is main component • Very tenuous, doesn’t block light • Some parts cool, some parts hot

5. Interstellar Dust • More complex composition • 1% of ISM • Clumps of atoms and molecules • Much larger than gas particles so… • Can block light • Composition is not well known

6. Interstellar Dust • Light passes through dust cloud • Extinction-dimming of light • Longer Wavelengths pass through • Short Wavelengths get absorbed • Reddening- scattering of blue light • Red light makes it through • Changes star’s apparent color

7. Dust and Gas • Dust is believed to come from mass loss winds in stars (like solar wind) • Gas of wind can cool and solidify • Dust grains provide coagulation seeds for molecules

8. Evidence of Gas and Dust • Observed Nebulae (clouds) • Dust and Gas form different types of nebulae • 4 basic types • Emission (Bright) nebula • Dark nebula • Reflection nebula • Molecular Clouds

9. Emission Nebula • Example: Orion Nebula • Spectra has emission lines (hot gas) • Does not shine under it’s own light • Powered by hot stars • H II region (ionized hydrogen)

10. Emission Nebula • OB associations • Form H II region hot spot on molecular cloud • Drives new star formation • Reddish hue is from Hydrogen • H II regions are star “nurseries” • Very bright!

11. Emission Nebula Heart and Soul Nebulae Astronomy Picture of the Day

12. Emission Nebula The Eagle Nebula and a close up of a star forming region Astronomy Picture of the Day

13. Molecular Clouds • Composed mostly of H2 (hard to detect) • Use other molecules as tracers • 80 known ISM molecules • Many are organic molecules • Associated with H II regions • Majority of ISM is here

14. Molecular Clouds • 10’s of ly across (6 trillion miles= 1ly) • Cool clouds (Dark) • Occur in huge complexes • Contain enough gas to make millions of stars • 1000 + complexes in our Galaxy

15. Molecular Cloud Barnard 68 APOD

16. Molecular Cloud Horsehead Nebula Note: there are several types of nebulae in this panorama APOD

17. Dark Nebula • Example: Snake Nebula • Contain gas and dust • Dust blocks light • Cool (10’s K) • Larger than our Solar System • Also, dust lanes w/ H II regions

18. Dark Nebula Snake Nebula APOD

19. Reflection Nebula • Gas and Dust • Absorption line spectra (stars) • Doesn’t generate own light • Scatters blue light from starlight passing through • Nebula appears blue (like sky) • Example: Pleiades Nebula

20. Reflection Nebula Witch Head Nebula APOD

21. A Panorama in Orion Can you ID the different types of nebula here? APOD

22. Neutral Hydrogen • Presence was suspected • H II regions come from it • Not observed until 1951 • Need to observed from its own radiation • Low-energy Radio emissions from the gas itself

23. 21-cm Radiation • From single electron orbiting nucleus • Not from excitation, from spin of electron • 2 possible configurations • Parallel • Anti-parallel • Lower energy one preferred • Spin flipping emits 21-cm photon

24. Coronal Interstellar Gas • Very hot gas • Highly Ionized • Very low density • Exists between clouds • Why?

25. Star Birth • Star’s life is a dance between gravity and radiation pressure • All stars have a similar origin • Cold, dark molecular clouds • Collapse to form stars

26. Cloud Collapse • Giant molecular cloud • Something to trigger collapse (shockwave) • Cloud begins to collapse under it’s own weight • Jean’s Instability • Cloud will begin to heat up as it collapses

27. Cloud Collapse • Heating causes outward pressure • Balances out gravity pushing inward • Collapse is “lumpy”, fragmentation • Pockets collapse faster (denser) • Centrally dense region is where star will eventually form

28. Cloud Collapse • Fragmentation occurs in several ways • Dozens of Massive Stars • Hundred or Thousands of Sun-like Stars • No evidence for single star formation • Single stars must escape after formation • Collapse can occur with or without rotation

29. Cloud Fragmentation • Sun sized star comes from • 2 solar mass fragment • 100 times radius of Solar System • Less than 100K • Fragmentation ceases as density of each fragment increases • Interior becomes opaque • Radiation is trapped

30. Fragmentation to Protostar • 10,000 + years passed • Central part 10,000K • Outer part cool • Dense, opaque central region=Protostar • Still contracting, material raining down on it

31. Protostar (Sun predecessor) • 1,000,000 years have passed • Not hot enough for P-P chain • Still about size of Mercury’s orbit • More luminous than Sun (bigger) • Surrounded by a dusty shroud • Vaporizes nearby dust

32. Protostar Protostar Dust Free Zone IR photon Outer Envelope of Gas and Dust

33. Protostar to Star • Protostar first appears on HR diagram • Protostar loses shroud • Vaporizes • Falls onto Star • Blown away by wind • Contracts, Luminosity , Temp  • Hayashi track on HR Diagram • Violent Surface Wind (T Tauri Star)

34. T-Tauri Stars • Exhibit strong winds • Bipolar flows • Clear gas and dust away from young star so it is at last visible • Where the outrushing gas impacts stationary gas in the ISM a bright hot spot appears • Herbig-Haro object

35. T-Tauri Stars • Also appear to vary in brightness • Likely associated with star’s magnetic field, much like the active Sun • Have “star spots” like sun spots • Are still collapsing to final size • Larger in size and therefore brighter than they will be as main sequence stars

36. T-Tauri Star A false color image of the T-Tauri system Note jet APOD

37. T Tauri Tantrum

38. HH object APOD Hubble Heritage

39. Pre-Main Sequence Star • 10,000,000 Years • Now a “True Star” • P-P Chain has begun • Larger and Cooler than Main Sequence Star • Still slowly contracting

40. At last, the Sun! • Contraction continues • Central Temp=15,000,000K • Outward pressure balances inward gravity • Hydrostatic Equilibrium (HSE) • Contraction Stops, Balance Reached • Main Sequence Star! • All main sequence stars are in HSE

41. Massive Stars • Steps occur faster • Collapse of Cloud occurs in similar way • Central part still collapses faster • Fragments are larger • No T Tauri phase • Chain Reaction from OB association • >100 M Gravity can’t hold together

42. Failed Stars • Some cloud fragments are too small • Don’t get hot enough • H-fusion never occurs • Warm due to collapsing • Brown dwarfs • Cooling objects • <0.08M (Jupiter)

43. Collapse with Rotation • Rotation during collapse leads to orbiting clumps around protostar • These clumps can attract more material via gravity and form planetismals • These could be a future solar system

44. Summary • All stars have a “cloudy” beginning • Different types of nebula • Stars collapse from clouds