Where do stars form?

1. Where do stars form? In H II regions along spiral arms HII regions M51-HST

2. NGC 3079 HST

3. Stars form in nebulae Orion A - NOAO

4. Star Formation: Main Steps 1. Gas cloud collapse 2. Main Sequence stage (H fusing or burning) 3. Red Giant or supergiant phase (He fusion) 4. Ejection: Planetary nebula or Supernova 5. Core remnant stage: • white dwarf • neutron star/pulsar or • black hole

5. Initial gas cloud collapse phase Giant Molecular Cloud 105~ 106s M cold gas & dust Protostar Conversion of GPE into KE & heat 2000~3000K IR & microwaves emitted T. Tauri stage star-like. Strong jets along rotational axes. Star starts fusion

6. Protostar & T Tauri track

7. Starbirth in nearby galaxies Large Magellanic Cloud 6 degrees, 160,000 ly Small Magellanic Cloud 5.6 degrees, 240,000 ly 30 Doradus (Tarantula nebula)

8. Tarantula nebula in the LMC (HST)

11. Barnard 86, a Bok globule NGC 6520, an open cluster

13. ORION

14. The Orion nebula - our nearest stellar nursery Visible light is absorbed by dust & gas IR light travels through the dust & gas, allowing us to view star birth

15. Proplyds (protoplanetary disks) these are possible precursors to solar systems

16. M16 - a stellar nursery

19. T Tauri type stars T Tauri (CFHT)

20. T Tauri type stars Material ejected along rotational axes

21. AFGL 2591: A Massive Star Acts Up Young star AFGL 2591 is putting on a show. The massive star is expelling outer layers of dust-laced gas as gravity pulls inner material toward the surface. AFGL 2591 is estimated to be about one million years old -- much younger than our own Sun's 5 billion-year age -- and has created a nebula over 500 times the diameter of our Solar System in just the past 10,000 years. The above image in infrared light is one of the first from the new NIRI instrument mounted on one of the largest ground-based optical telescopes in the world: Gemini North. Sharp details are discernable that are blocked by opaque dust in visible-light images. Close inspection of the image reveals at least four expanding rings, indicating an episodic origin to the mysterious activity. AFGL 2591 lies about 3000 light years away toward the constellation of Cygnus.

22. Stars form in clusters The Pleiades, an open cluster

23. The Jewel box open cluster M. Bessell (MSSSO)

24. Mass determines a star’s Main Sequence luminosity!!!

25. Lifetimes of main-sequence stars Heavier MS stars have shorter lives

26. Formation of stars & planets

27. Stellar Evolution and the HR Diagram • Our Sun as a star • Nuclear fusion and energy transport in the sun • Stages of stellar evolution for low and intermediate mass stars • The Hertzsprung Russell diagram

28. Light Travel from the Sun The speed of light is c = 3x108 ms-1. A photon leaving the surface of the sun reaches the earth after a time T = distance/c = 8 minutes. How does the Sun burn? The Sun must be at least as old as the earth (4.6 billion years). It has a luminosity of L = 3.9 x 1026 Joules s-1. Its mass composition is H: 74% He: 24% rest: 2% What produces the Sun’s energy?

29. SOHO image of the solar chromosphere in ultraviolet light.

30. Some Solar Values 1/2o

31. A star is a balancing act between: P: Pressure T: Temperature acting outwards P,T Gravity acting inwards Hydrostatic equilibrium The internal pressure gradients must counteract the gravitational force G. (What happens otherwise?) This is a fundamental requirement for all stars.

32. Solar Energy Source: Some early ideas: Normal chemical reactions - such as the combustion of coal Large numbers of meteorite impacts (10,000 years) Slow gravitational collapse (20 million years) In the 1930’s a major breakthrough in astronomy was the understanding that the energy source in stars is from : Nuclear fusion reactions at high temperatures and pressures.

33. Nuclear Fusion in the Sun Core temperature = 1.5 x 107 K Core radius = 0.25 Rsurface Core mass = 10% total stellar mass The sun’s energy is generated in the core by nuclear fusion reactions which convert Hydrogen to Helium: 4 1H 1 4He + energy (photons and neutrinos) Energy released = mc2 = 3.85 x 1026 J/s

34. Fusion: Hydrogen  Helium This Proton-Proton chain is the energy source of stars like our Sun

35. CNO cycle Uses C-12 as nuclear catalyst to convert 4 protons into He-4 Dominates in more massive (hence hotter) MS stars

36. Comparison of PP & CNO CNO contributes only ~1% Sun’s energy

37. What mass of hydrogen is converted to helium? Mass s-1 = luminosity / c2: 4 x 109 kg s-1 How long can the sun survive by burning hydrogen? Hydrogen burning lifetime = H mass available in core Rate of conversion This gives a timescale of approximately 1010 years, ie 10 billion years. Our Sun is roughly half-way through its hydrogen burning phase.

38. Energy transport from the core to the visible surface of the Sun 1. Core region: R < 0.25 Rsun Nuclear fusion zone 2 2. Radiative region: 0.25 < R < 0.75Rsun photons diffuse through hot gas. 3. Convective Region: 0.75 < R < Rsun Energy transported by bulk gas motions. 4. Photosphere - the visible surface of the sun. Thickness ~ 500 km. T = 6000K Energy from the sun’s interior is released as photons (‘particle of light’) and as neutrinos (zero or very low mass particles).

39. Stellar Evolution Stars form with masses between 0.1 and 100 times the mass of the sun. For most of their lifetimes they burn by the nuclear fusion of hydrogen to helium. These are the ‘Main Sequence’ stars. Low mass stars convert hydrogen more slowly and spend longer in this phase. They are also cooler and smaller in size. Main sequence lifetime ~ 1010 years for Mstar = Msun Main sequence lifetime ~ 106 yrs for Mstar = 30 Msun What happens when the core hydrogen runs out??

40. Later Stages As the core is used up the stellar core contracts under gravity. This raises the central gas density, pressure and temperature. At a temperature of ~ 2 x 108 K the stellar core ignites Helium in the triple-alpha reaction: 3 4He 12C +  (gamma ray). To balance the pressure gradients across the star the outer layers expand greatly and cool down. The star is now a luminous Red Giant.

41. Red Giant Stars Red Giant stars have dense compact cores and much lower density expanded atmospheres. Core helium burning Outer hydrogen atmosphere R By the time a star has become a Red Giant, its radius has become about 150 times larger than in the core-hydrogen burning stage. The Red Giant stars are very luminous: L = 4R2Teff 4 The surface temperatures are typically ~ 3000 K (reddish)

42. Triple alpha (helium) flash Fuses He into C, releasing energy. Red Giant phase

44. Asymptotic Giant Branch Stars For stars of one solar mass, the Red Giant phase lasts for approximately 107 years. After the helium core burning phase ends, the stellar energy is supplied by nuclear fusion in two layers around the core. In this ‘double-shell’ burning stage the star is known as an Asymptotic Giant Branch (AGB) star. The AGB stars have extremely strong STELLAR WINDS. The stellar winds remove most of the stellar atmospheres which are blown outwards into the interstellar medium. The mass-loss rates of AGB stars are typically 1018 kg s-1. This is a billion times higher than for the sun.

45. Nucleosynthesis in stars: Mass is the key factor! • Low mass stars; convert hydrogen into helium HHe • Stars like our Sun; hydrogen into helium, then helium to carbon and oxygen • High mass stars (>5xSun); HHe, He C,O, Ca, Fe, Ni, Cr, Cu & others! Then SUPERNOVA  heavier elements

46. Very high core temperature ~ 4 x 109 K Can fuse up to iron

48. Fusion products in MS & Red Giants

50. The death of stars Once all fusion has occurred and outer layers expelled the final remnant of the star depends on the mass of the remaining core: 1. If mass < 1.4 M white dwarf 2. For mass 1.4 M < M < 3.0 M neutron star (pulsar) 3. If M > 3.0 M black hole!