STAR FORMATION

1. STAR FORMATION Still somewhat mysterious, stars are born inside dark clouds and then revealed in all their beauty.

2. Most stars form in GIANT MOLECULAR CLOUDS • GMCs have masses from 105 to above 106 solar masses (M) • typical densities above 1000 cm-3 • initial T < 10 K, since their cores are well shielded from external starlight and other heat sources • typical size of a GMC > 5 pc • If triggered to collapse, these clouds yield entire STAR CLUSTERS (currently Open Clusters) • In both GMCs and regular Molecular Clouds the most abundant molecules are: H2 , He, CO, CO2, OH, H2O, but many others are detected.

3. Galactic and Extragalactic SF • Star formation (SF) is ongoing in the Milky Way but also seen in distant galaxies • Clouds collapse, heat, start to fuse -- ignite as a star • Why didn’t this all finish happening long ago? Galaxy M33 (left): SF region NGC 604 ~500 pc across

4. Observing Newborn Stars • Visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed

5. Observing Newborn Stars • Observing the infrared light from a cloud can reveal the newborn star embedded inside it • Orion Star Forming Region Applet

6. A Star’s Interior: A PERMANENT BATTLEGROUND • The combatants: • GRAVITY pulling inwards (with blob collisions helping push inwards) • and PRESSURE pushing outwards • Types of PressureThermal or Gas pressure (most common)Radiation PressureDegeneracy Pressure (White Dwarfs and Brown Dwarfs)Magnetic RotationalTurbulent

7. Gas Pressure • Gas pressure is proportional to the product of density and temperature: • P  n T • compressing a cloud always increases n • compressing a cloud sometimes increases T • so P always goes up with compression. • T1 = 10 K & n1 =106 cm-3 ; T2=100 K & n2= 1012 cm-3

8. Self-Gravity Fights Back • BUT self-gravity also goes up with compression and gravity is independent of T. • for a gas cloud: very roughlyFg n2. • So Fg rises faster with density than does P if only density rises

9. If a cloud is squeezed it can: • collapse, with Fg >> P ( Area), OR • contract, with Fg just barely winning over P • OR remain stable, with them in balance • GMCs (Giant Molecular Clouds) are also supported by rotation, magnetic fields and turbulence, so a small squeeze usually isn't enough to trigger star formation. • Therefore only a small fraction of clouds are forming stars at any given time.

10. Complications are Important • Gravity vs pure gas pressure is pretty easy • Most MCs are rotating: support against collapse in “equator” and encourages fragmentation • Magnetic fields funnel collapsealong field lines if B strong enough

11. Fragmentation of a Cloud • This simulation begins with a turbulent cloud containing 50 solar masses of gas • Real giant molecular clouds start with >105 solar masses

12. Fragmentation of a Cloud • The random motions of different sections of the cloud cause it to become lumpy • Cloud Collapse Applet

13. Fragmentation of a Cloud • Each lump of the cloud in which gravity can overcome pressure can go on to become a star • A large cloud can make a whole cluster of stars

14. Thought Question What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change B. Its mass would increase C. Its internal pressure would increase

15. Thought Question What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change B. Its mass would increase C. Its internal pressure would increase

16. TRIGGERS OF STAR FORMATION Squeezing of a GMC by supernova remnant: the shock wraps around the cloud and compresses it. Compression of a GMC by the ionization front at the edge of a H II region. BOTH of the above rely on the existence of nearby massive, hot (O and B) stars.

17. Triggers of SF, 2 • ALSO, density waves can cause compression: these are due to non-symmetric gravitational distributions near the centers of galaxies and produce SPIRAL ARMS -- more about this when we talk about Milky Way structure later.

18. SIGNPOSTS OF STAR FORMATION • MASERs (Microwave Amplification through Stimulated Emission of Radiation) from molecules like OH, H2O, CO; • excited by energy from buried stars, they arise from clumps of gas near those stars being born and shine very brightly in the microwave bands. • MASERs are produced from molecular rotational/vibrational levels being stimulated, while • LASERs (Light Amplification through Stimulated Emission of Radiation) come from electronic energy levels in atoms or molecules.

19. Signposts, 2 • HERBIG-HARO OBJECTS: emission line clouds moving away from molecular clouds • H-H objects are understood to be shocks in jets speeding away in opposite directions from a forming star (still buried in the molecular cloud). • More generally: BIPOLAR NEBULAE -- gas flows away from the forming star in opposite directions. HH30

20. Herbig-Haro Objects: HH1 & 2 in Orion

21. Signposts of SF, 3 • BOK GLOBULES: small molecular clouds, perhaps forming one or a few stars. • PROTOSTARS: Emitting much IR radiation from infalling matter, usually in a flattened disk

22. Protostars in Orion

23. Signposts, 4: T Tauri Stars • Last stage of a PROTOSTAR's life before it becomes a real star, with Hydrogen fusion in its core. • T Tauri's are: very variable in an irregular way (not like eclipsing binaries or pulsating stars); very red, and emit lots of IR radiation; sources of powerful winds.

24. The First Stars • Elements like carbon and oxygen had not yet been made when the first stars formed • Without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably warmer than today’s molecular clouds • The first stars should therefore have been more massive than most of today’s stars, so that gravity could overcome the higher pressure

25. Simulation of the First Star • Simulations of early star formation suggest the first molecular clouds never cooled below 100 K, making stars of ~100MSun

26. THE ROAD FROM CLOUD TO STAR • When a (Giant) Molecular Cloud is triggered to collapse, it will fragment and re-fragment. • The original 105 -- 3x106 M cloud will typically form 10’s--1000's of stars, but only somewhere between 5 and 25% of the mass of the cloud eventually winds up in stars; the rest is re-dispersed into the ISM. • The first stage is ISOTHERMAL COLLAPSE. The fragment is at first of sufficiently low density that the heat generated by compression of the cloud can escape as microwave radiation, thus keeping the Temperature around only 10 K -- thus ISO(equal)THERMAL(temperature). • Since gravity wins over pressure by a large margin if only n and not T too goes up, this is a COLLAPSE.

27. Isothermal Collapse: An Economic Analogy to Reagonomics/Bushonomics • The denser regions at the center collapse faster (the rich get richer quickly), • the medium density regions collapse slower and might become part of the star (the middle classes get a little richer, if they are lucky), but are more likely to never make it in. • the lower density outskirts get blown away and dispersed (the lower middle class and the poor get poorer). • Basically what happens to newly forming stars is what happened to the American economy in the 1980s with Reagonomics, and happened in the 2000s with Bushonomics.

28. Nobel Prize in Physics 2009 • Willard S. Boyle and George E. Smith who were at Bell Labs in 1969 share half the prize for the invention of the Charge Coupled Device sensor: CCDs were used first in spy satellites, then by astronomers and today in digital cameras. • The other half went to Charles K. Kao, who while working in England in 1966 demonstrated pure enough glass would allow fiber optic cables to work; hence the internet. • All are Americans, though Kao is also British and Boyle also Canadian

29. KELVIN-HELMHOLTZ CONTRACTION • Once the density at the center of the cloudlet gets high enough, it becomes OPAQUE and the photons are scattered or absorbed and reradiated many times before their descendents escape. • Then the temperature as well as the density rises. P n T, rises fast and P can nearly balance gravity. • We call this KELVIN-HELMHOLTZ CONTRACTION a slower reduction in size, accompanied by heat generation. • Actually, just about 1/2 of the heat produced from gravity is radiated in the microwave and IR bands, while 1/2 is trapped and raises the temperature of the gas.

30. More Collapse & Contraction • Dissociation of H2 molecules into H atoms yields an inner isothermal core within the contracting outer core until that core too becomes opaque • Rotation and magnetic fields will prevent the collapse from being spherical -- they spread the outer parts into a disk, part of which accretes onto the forming star, part of which is launched into winds and jets (bipolar nebulae, Herbig-Haro objects), part of which can form smaller companion star(s) or planets. • So the inner core contracts slowly, but the outer layers are in free-fall onto that core. This produces a STANDING SHOCK which generates much additional heat and light.

31. 2nd K-H Contraction on H-R Diagram

32. Star Formation Illustrated

33. FINAL STAGES OF STAR FORMATION • The core of the contracting cloudlet heats up -- but still not hot enough to begin nuclear fusion. • This protostellar period lasts for < 1 percent of the star's total life on the Main Sequence (i.e. ~3x107 yr for the Sun, whose total lifespan is ~ 10 billion yr.) • Much luminosity is generated in the collapse of the outer layers onto the opaque core: this accretion generated heat makes the protostar some 10's or 1000's of times as luminous as it will be when it gets to the Main Sequence • Protostars are 10's to 100's of times as large as they will be when on the MS; the surface temperature of these protostars will be ~5000 K (higher for higher masses, lower for lower masses, than the Sun).

34. FINAL STAGES, 2 • On the H-R diagram the protostars move from the very lower right (way off usual plots): T = 10K, L << L to moderate T's and high L's -- above the MS. • BUT the observed T is much less than protostellar surface T, since the visible radiation is absorbed and reemitted by dust in the surrounding cloud -- the protostar looks much cooler than it is for a long time. • Eventually, all the nearby gas has fallen onto the core so the protostar's accretion generated luminosity falls. • The star then enters the HAYASHI TRACK, a nearly vertical decline in the H-R diagram and gets very close to the MS -- such protostars are fully convective. • Often the outer layers of gas are dispersed by winds or bi-polar outflows while the inner layers are accreted.

35. Protostars on H-R DiagramHayashi Track (4-6)Evolution slows as the core gets hotter, fighting off gravity more efficiently

36. Final Stages, 3 • When the core temperature reaches about 1 x 106 K, it is hot enough for deuterium (and tritium) to fuse. • But these are rare isotopes of hydrogen and are used up quickly. • However they can cause the L to rise while Ts also goes up and the protostar gets a little brighter for a while (6 to 7 on H-R diagram). • L also increases due to shift from convective to radiative transport of enegy. • T Tauri stars are found in this final stage of protostellar evolution, just above the MS.

37. Conservation of Angular Momentum: Evidence from the Solar System • The nebular theory of solar system formation illustrates the importance of rotation • The rotation speed of the cloud from which a star forms increases as the cloud contracts

38. Flattening • Collisions between particles in the cloud cause it to flatten into a disk • Protostar Track Applet

39. Formation of Jets • Rotation also causes jets of matter to shoot out along the rotation axis • These jets can yield the H-H objects seen earlier

40. Thought Question What happen to a protostar that formed without any rotation at all? A. Its jets would go in multiple directions B. It would not have planets C. It would be very bright in infrared light D. It would not be round

41. Thought Question What happen to a protostar that formed without any rotation at all? A. Its jets would go in multiple directions B. It would not have planets C. It would be very bright in infrared light D. It would not be round

42. A STAR IS BORN • When the center of the contracting protostar gets to T > 6 x 106 K then ordinary H fusion can begin. • This is official definition of stellar birth -- the star is on the Zero Age MS (ZAMS) now. • The star's location on the ZAMS is determined almost completely by its MASS (there are lesser effects from composition and rotation that you should know exist, but needn't worry about). • During the majority of its life on the MS, the star does not move very much at all on the H-R diagram -- the particular place on the ZAMS is very close to the H-R diagram location where an old MS star of the same mass is found.

43. Pre-MS Tracks & ZAMS for Different Mass Stars

44. Limits to Stellar Masses • If the protostar's mass is less than about 8% of the Sun's mass it is insufficient to compress the center to temperatures and densities adequate to allow ordinary fusion -- THE LOWER MASS LIMIT • Such failed stars are calledbrown dwarfs. • Most astronomers make a further distinction between brown dwarfs and even lower mass objects, with less than about 1.3% of M (or about 13 times Jupiter's mass): these can't even trigger deuterium or tritium fusion and are classified as giant planets. • Over the past decade several dozen brown dwarfs and over 200 giant planets have been found, most through very careful spectroscopic studies of single-line spectroscopic binaries with tiny (m/s) velocities.

45. Fusion and Contraction • Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 107 K. • Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation • Is there another form of pressure that can stop contraction?

46. Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying same state in same place

47. Thermal Pressure: Depends on heat content: P  T The main form of pressure in most stars Degeneracy Pressure: Particles can’t be in same state in same place; quantum mechanics Doesn’t depend on heat content: P  5/3

48. Brown Dwarfs • Degeneracy pressure halts the contraction of objects with <0.08MSun before core temperature become hot enough for fusion • Starlike objects not massive enough to start fusion are brown dwarfs

49. Images of Brown Dwarfs HST image of Gliese 623 w/ M ~ 0.1 M; IR and HST images of Gliese 229 w/ M ~ 0.04 M

50. Brown Dwarfs in Orion • Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous