1. February 14, 2006 Astronomy 2010 1 Chapter 20: The Birth of Stars and the Discovery of Planets Outside the Solar System

2. February 14, 2006Astronomy 20102 Questions About Star Formation Are new stars still being created, or did creation cease billions of years ago? Where are new stars being created? Are planets a natural result of star formation or is our solar system unique in the universe? How can we observe planets around distant stars?

3. February 14, 2006Astronomy 20103 Basics About Stars (Table 20.1) Stable (main-sequence) stars maintain equilibrium by producing energy through nuclear fusion in their cores. Generating energy by fusion defines a star. Hydrogen is being converted to helium, but eventually the supply of hydrogen will run out. Stars range in mass from about 1/12 M sun to 200 Stars range in mass from about 1/12 M sun to 200 M sun. Low mass stars are more common. For main sequence stars, mass and luminosity are related such that high mass stars have high luminosity and low mass stars have low luminosity. Galaxies, like the Milky Way, contain enough gas and dust to create billions of new stars.

4. February 14, 2006 Astronomy 2010 4 Life Cycle of Stars: from Birth to Maturity Stage 1: Giant Molecular Cloud – cold dust clouds in space Clumps (dust bunnies) accrete matter from cloud to form protostar Stage 2: Protostar – energy generated by gravitational collapse Stage 3: Wind formation – protostar produces strong solar winds winds eject much of the surrounding cocoon gas and dust winds blow mostly along the rotation axes Stage 4: Main Sequence -- the new star becomes stable Equilibrium: hydrogen fusion into helium in the core balances gravity. Stage continues until most of the hydrogen in the core is used up.

5. February 14, 2006Astronomy 20105 Stage 1: Giant Molecular Cloud giant molecular cloud large, dense gas cloud with dust cold enough for molecules to form thousands of giant molecular clouds in the galactic disk each giant molecular cloud contains vast amount of material for star formation about one million solar masses

6. February 14, 2006Astronomy 20106 Star Formation (Reference Slide) matter in part of giant molecular cloud begins to collapse tens to hundreds of solar masses tens to hundreds of solar masses collapse can start by itself if matter is cool and massive enough shock waves can trigger collapse by compressing the gas clouds into clump the explosion of a nearby massive star – supernova gravity from nearby stars or groups of stars gravity pulls more matter to form sufficiently massive clumps whatever the reason, the result is the same: gas clumps compress to become protostars

7. February 14, 2006Astronomy 20107 Eagle Nebula M16 nearby star shines light on gas cloud revealing protostar formation

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10. Evaporating Gas Globules

12. February 14, 2006Astronomy 201012 giant molecular cloud shock wave creates clumps in giant molecular cloud clusters: many stars forming simultaneously

14. February 14, 2006Astronomy 201014 Stage 2: Protostars gas clump collapses and heats up as gas particles collide gravitational energy is converted to heat energy heated clump produces infrared and microwave radiation at this stage the warm clump is called a protostar Rotating gas clump forms a disk with the protostar in the center other material in the disk may coalesce to form another star or planets

15. February 14, 2006Astronomy 201015 Behind the visible part of the Orion Nebula is a much denser region of gas and dust that is cool enough for molecules to form. Many stars are now forming inside it.

16. February 14, 2006Astronomy 201016 trapesizium cluster: stars that provide much of the energy which makes the brilliant Orion Nebula visible other stars obscured by nebula

17. February 14, 2006Astronomy 201017 Observation of protostars Infrared detectors enable observation of protostars. Many stars forming in the Nebula above and to the right of the Trapezium stars. They can only be seen in the infrared image. Images are from the Hubble Space Telescope VisibleInfrared

18. February 14, 2006Astronomy 201018Protostar gravity pulls more matter into clump energy from falling matter creates heat protostar forms as hot matter begins to glow in infrared protostar surrounded by "cocoon" of dust matter falling into a rotating star tends to pile up in a disk

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21. February 14, 2006Astronomy 201021 Social Stars Young stars seem to be social Fragmentation of the giant molecular cloud produces protostars that form at about the same time. Stars are observed to be born in clusters. Other corroborating evidence for this is that there are no isolated young stars. This observation is important because a valuable test of the stellar evolution models is the comparison of the models with star clusters.

22. February 14, 2006Astronomy 201022 Stage 3: Wind Formation strong stellar winds winds eject much of the surrounding gas and dust Winds constrained to flow preferentially along the rotation axes With most of the cocoon gas blown away, the forming star finally becomes visible wind proto-planetary disk

23. Jets

24. February 14, 2006Astronomy 201024 Jets from Stellar Wind gravitational contraction continues eventually enough energy for stellar wind to form jets jets blow away cocoon fusion begins at end of this stage star reaches zero age main sequence when fusion starts

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27. February 14, 2006Astronomy 201027 Stage 4: Main Sequence We define the star’s arrival on the main sequence as the time when fusion begins. Eventually becomes stable because hydrostatic equilibrium is established. hydrostatic equilibriumhydrostatic equilibrium It settles down to spend about 90% of its life as a main sequence star. Fusing hydrogen to form helium in the core.

28. zero age main sequence point at which star begins fusing hydrogen into helium. moving to left – temperature is increasing evolution to main sequence

29. ages of forming stars in years as they grow towards main sequence mass determines position on main sequence evolution to main sequence

30. February 14, 2006 Astronomy 2010 30 Life Cycle of Stars: from Birth to Maturity (Recap) Stage 1: Giant Molecular Cloud – cold dust clouds in space Clumps (dust bunnies) accrete matter from cloud to form protostar Stage 2: Protostar – energy generated by gravitational collapse Stage 3: Wind formation – protostar produces strong solar winds winds eject much of the surrounding cocoon gas and dust winds blow mostly along the rotation axes Stage 4: Main Sequence -- the new star becomes stable Equilibrium: hydrogen fusion into helium in the core balances gravity. Fusion continues until most of the hydrogen in the core is used up.

31. February 14, 2006Astronomy 201031 Summary of Birth Process

32. February 14, 2006Astronomy 201032 ages of forming stars in years as they grow towards main sequence zero age main sequence – ZAMS point at which star begins generating energy by fusion Evolution to Main Sequence

33. time to reach main sequence stage short for big stars as low as 10000 years long for little stars up to 100,000,000 years for low mass

34. February 14, 2006Astronomy 201034 HR Diagram: Analogy to Weight vs Height for People

35. February 14, 2006Astronomy 201035 Weight and Height changes as Age increases (Marlin Brando)

36. February 14, 2006Astronomy 201036 Different paths for different body types (Woody Allen)

37. February 14, 2006Astronomy 201037 20.3 Evidence that Planets Form Around Other Stars It is hard to see a planet orbiting another star. Look for a disk of material before it clumps to form planets -- big disk is more visible than small planet. Look for evolution of disks -- evidence for clumping into planets.

38. February 14, 2006Astronomy 201038 proto-planetary disk

39. February 14, 2006Astronomy 201039 Proto-planetary Disks

40. February 14, 2006Astronomy 201040 Dust Ring Around a Young Star

41. February 14, 2006Astronomy 201041 Disk Around Epsilon Erdani

42. February 14, 2006Astronomy 201042 20.4 Planets Beyond the Solar System: Search and Discovery If we can’t directly observe planets, can we indirectly observe them? 3 methods have succeeded. First method: doppler shift A planet must orbit its star to be stable. Search for the effect of the planet’s orbit on the star. Both planet and star orbit around a common center of mass. The star “wobbles” a bit as the planet orbits it. The wobble has the same period as the planet’s orbit.

43. February 14, 2006Astronomy 201043 Search for Doppler Shift

44. February 14, 2006Astronomy 201044 Velocity from measured Doppler Shift vs. time - -shows the star’s orbit about unseen partner

45. February 14, 2006Astronomy 201045 Second Method to Find Planets Look for a small reduction of star light when an orbiting planet moves between us and the star. Works only when planet’s orbit is lined up properly. Will block all visible wavelengths -- this is a cross check.

46. February 14, 2006Astronomy 201046 Third Method to Find Planets Measure infrared (thermal) radiation of “hot” planet.

47. February 14, 2006Astronomy 201047 As of June 2005, more than 155 extrasolar planets found. Systems of 2, 3, and possibly more planets are seen. Masses are measured in Jupiter-masses. Discovered Planets

48. February 14, 2006Astronomy 201048 20.4.3 Explaining the Planets Seen Now that we have a large sample of planetary systems, astronomers can refine their models of planet formation. Almost all the planets are Jupiter-sized, and many have highly eccentric orbits close to their star. This is a surprise and is hard for the early models to explain. The formation of planetary systems is more complex and chaotic than we thought.

49. February 14, 2006Astronomy 201049 Planet Mass Distribution Not many brown-dwarf sized planets (M>10M Jup ). Jupiter-sized planets are common.

50. February 14, 2006Astronomy 201050 Eccentric Orbits Are Common

51. February 14, 2006Astronomy 201051 Planets and Star “Metallicity” Planets are more common around stars with more heavy elements (“metallicity”).