General GeoAstro II: Astronomy

1. General GeoAstro II: Astronomy • The name of the game: • slides will NOT be put on the web attend the lectures, take notes ! • suggested reading: “Universe” (Kaufmann & Freedman) • no laptops, no mobiles during class • classes are not complicated, but please repeat them regularly • only few formulae, but you have to know them

2. Stars Nature of stars Birth of stars Stellar evolution Endpoints: White Dwarfs Neutron Stars Black Holes Galaxies - Milky Way - Other galaxies - Supermassive black holes General GeoAstro II: Astronomy • Cosmology - Cosm. Expansion - Big Bang • Tests of our theories - New developments

3. Distance to the stars • From brightness? No! • Parallax-experiment … • Stellar parallax … d= 1/p

4. Distance to the stars • Define: “star has a distance of 1 parsec (pc) if its parallax is one arcsecond” • 1 pc = 3.26 light years • Brightest stars on the night sky: too far to measure parallax • Blurring of atmosphere: parallaxes < 0.01 arcsec extremely hard to measure reliable out to d= 1/p = 100 pc

5. Distance to the stars • Hipparcos: High Precision Parallax Collecting Satellite (Hipparchus: greek astronomer) • Parallaxes still important to gauge other distance indicators • Stellar motions ….

6. Brightness and Distance(“Inverse square law”) • Distance and brightness luminosity • Stars have different masses different luminosities • “luminosity= energy/time” [J/s] • “brightness= energy/(time surface area)” [J/s m2]

7. Brightness and distance • brightness …. • b= L/(4 p d2) • “double the distance • brightness reduced by a factor 4”

8. luminosities • huge variety of stellar luminosities: Lmax =1010 Lmin (1010 = number of all people that ever lived on earth)

9. The Magnitude system • System to classify stellar brightness • Very old: Hipparchus (200 B.C.): “ brightest stars: first magnitude half as bright: second magnitude …… sixth magnitude” “apparent magnitudes” • Attention: “scale backwards”

10. Magnitude system • 19th century astronomers: “first magnitude stars shall be 100 times brighter than sixth magnitude stars” • difference of 5 mag corresponds to a factor of 100 in brightness, i.e. x5 = 100 x= 2.52 “half as bright 1/2.52 as bright”

11. Magnitude system • Scales backwards: “the brighter the more negative” • Examples: • Venus: m= - 4 • Full moon: m= - 13 • Our sun: m= - 26.8 • Relation brightness – magnitudes... m2-m1= 2.5 log(b1/b2)

12. Absolute magnitudes • Definition: ”absolute mag.= relative mag. as seen from a distance of 10 pc” • Distance modulus (m-M)… • m - M= 5 log(dpc) – 5 dpc: distance in pc m : apparent magnitude M : absolute magnitude

13. Stellar colours • Stellar colours depend on the surface temperature ! • Wien’s law: lmax T = const …

14. Spectra of stars • How do we know the same laws of physics hold in the observable universe? • Sun: absorption line spectrum (=continuum + dark lines) • Spectral classification: O B A F G K M “Oh be a fine girl/guy kiss me…” “hot” Tsurf ~ 25 000 K Sun “cool” Tsurf ~ 3000 K

15. Spectra of Stars • Advent of quantum mechanics: Interpretation of absorption lines in terms of atomic energy levels

16. Stellar sizes • impossible to measure with telescopes • measure i) brightness ii) distance (parallax) iii) surface temperature(spectral type) . . luminosity Stefan Boltzmann law Radius

17. Hertzsprung-Russel diagram • Idea: plot luminosity vs. temperature (spectral type) information about radius classification of stars

18. Hertzpsrung-Russel diagram • not random, just a few classes • most stars on “Main Sequence” (hydrogen burning) • White dwarfs: same temperature, but lower luminosity small radius RWD ~ 10 000 km ~ Rearth • Giants: same temperature, but higher luminosity large radius Rgiant = 10 - 100 Rsun Tsurf = 3000 – 6000 K • Supergiants: up to 1000Rsun

19. Stellar Masses • need binary stars ! (~50% of all stars in binaries) • “double stars”either i) “optical double stars” ii) true binary star • How to get masses??? Kepler III: w2= G (M1+M2)/a3 M1: mass star 1 M2: mass star 2 a : separation between stars G : gravitational constant w= 2 p/T, T: orbital period measure a and Ttotal system mass

20. Stellar masses • individualmasses? i) find center of mass (CM) ii) distances from CM to stars, a1 & a2 a1= (M2/Mtot) a a2= (M1/Mtot) a

21. Mass-luminosity relation • Observation: L M3.5 ….. “proportional to” • Stellar lifetimet : t M-2.5 “fat blokes die young”

22. The Birth of Stars • “We see a region of space extending from the centre of the sun to unknown distances contained between two planes not far from each other…” (Immanuel Kant: “Allgemeine Naturgeschichte und Theorie des Himmels”) • Nuclear burning in the sun (“hydrogen to helium”): consumes 6 1011 kg/s of hydrogen no infinite fuel resources: finite life time stellar evolution (“birth, evolution, death”)

23. Birth ofStars • “snapshot problematic” stellar >> human lifetime • Derive evolutionary sequence from a set of “snapshots”

24. Stellar Birth • Stars are born in the gravitational collapse of giant molecular clouds

25. Stellar Birth • computer-simulationof the collapse of a giant molecular cloud by Mathew Bate • very dynamic process • stars form in groups • many binary/multiple star systems form • observation: ~ 50% of stars are in binary systems

26. Stellar birth • Where does star formation take place? …in the spiral arms of galaxies…

27. Interstellar Medium (ISM) • ISM provides matter of which stars are made • ISM consists of a combination of gas and dust • Interstellar clouds are (for historical reasons) called nebulae

28. Interstellar medium • Three kinds of nebulae: Emission N. Reflection N. Dark N.

29. Interstellar medium • Emission nebulae: • temperatures: ~ 10 000 K • masses: ~ 10 – 10 000 Msolar • density: n ~ few 1000 atoms/cm3 (compare with: “air” ~ 1019 atoms/cm3 ISM ~ 1 atom/cm3) • found near hot, young stars (O and B stars with Tsurf > 10 000K)

30. Interstellar medium: emission nebulae • Interstellar hydrogen found in two forms” • “HI-region”: neutral hydrogen • “HII-region”: ionized hydrogen (i.e. protons and electrons)

31. Interstellar medium: emission nebulae • Emission mechanism HII-region: recombination (proton captures electron, emits light as it cascades down) • most important transition from n=3 to n=2 (“Ha-photons”) reddish colour

32. Reflection nebulae • Lots of fine-grained dust, low density reflects short-wavelengths more efficiently than long ones blue colour

33. Dark Nebulae • High density of dust grains block view to the stars • Temperature: 10 – 100 K hydrogen molecules • Density: n ~ 104 – 109 atoms/cm3

34. Stellar Evolution • Protostars: • Gravity has to overcome gas pressure dense & cold regions preferred dark nebulae (“stellar nurseries”) • “standard cosmic abundances”: • 75 % Hydrogen • 24 % Helium • 1 % heavier elements

35. Protostars • young protostars more luminous than later on the main sequence (gravitational energy) • Decrease of luminosity at almost constant surface temperature, but central temperature rises • Evolutionary path in HR-diagram…

36. Protostars • At Tcentral ~ 106 K: thermonuclear reactions (H He) set in produce energy/pressure stop contraction hydrostatic equilibrium+nuclear burning = Main sequence (MS) reached • Exact position on MS determined by stellar mass…

37. Main sequence masses • Extreme cases: • Mass too small (<0.08 Msol) no ignition of hydrogen, no main sequence stage Brown Dwarf • Mass too big (>100 Msol) violent winds disruption of the star Main sequence: 0.08 < MMS < 100 Msol

38. Young stellar objects (YSOs): …youngsters in revolution… • Accretion disks: • Jets:

39. Young stellar objects • examples of accretion disk – jet connection • interaction of these outflows with surrounding matter: Herbig-Haro objects • Jets are usually short-lived: 104 years, but can eject large masses (~1 Msol) during this time • many young stars lose mass via strong winds: mass loss 10-7 Msol/year (our sun: 10-14 Msol/year)

40. Young stellar objects • Young stars like to hang around in groups (see previous movie) “open clusters” • fastest stars may leave “evaporation” of open clusters

41. Stellar evolution: overview • once formed, evolution of stars depends on their masses: • M < 0.08 Msol: no nuclear fusion “Brown dwarfs” • 0.08 < M < 8 Msol: i) Main sequence ii) Giant phase iii) White Dwarf + planetary nebula

42. Stellar evolution: overview • 8 < M < 25 Msol: i) Main Sequence ii) Giantphase iii) supernova explosion neutron star • M > 25 Msol: i) Main Sequence ii) Giantphase iii) supernova explosion black hole

43. Evolution of a M < 8 Msol star • “our sun”:-MS-star, H-burning in core - Red Giant: H in core exhausted, H-burning in shell - Red Giant:He ignites in stellar core, radius ~ 1 AU earth swallowed (~ 5 109 years from now) - final stages: hot, cooling Carbon-Oxygen core, eject envelope White dwarf + “planetary nebula”

44. 8 Msol -star • Planetary nebulae:

45. 8 Msol -star • Evolution in the HR-diagram:

46. Testing stellar evolution:globular clusters • Globular clusters • ~105stars • in halo of galaxy • Old: about same age as galaxy

47. Globular clusters and HR-diagrams • Basic idea: - heaviest stars have already evolved away from main sequence - lightest stars still on main sequence age of cluster

48. Evolution for M > 8 Msol • Stages: • Main sequence • Giant stage • Final stage:

49. Evolution for M > 8 Msol • No more nuclear fuel (beyond iron) “core”-collapse supernova explosion (type II)

50. Evolution for M > 8 Msol • Supernova explosion results in either i) a neutron star (M < 25 Msol) or ii) a black hole (M > 25 Msol)