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General GeoAstro II: Astronomy

General GeoAstro II: Astronomy. The name of the game : Not all info on slides attend the lectures, take notes ! sugg. reading : - “Universe” (Kaufmann & Freedman): basic reference - Astronomy-fun, animations etc:

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General GeoAstro II: Astronomy

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  1. General GeoAstro II: Astronomy • The name of the game: • Not all info on slides attend the lectures, take notes ! • sugg. reading: - “Universe” (Kaufmann & Freedman): basic reference - Astronomy-fun, animations etc: http://www.opencourse.info/astronomy/introduction/ astronomy_links.html - more advanced: http://ocw.mit.edu/OcwWeb/Physics/8282JSpring2003/ StudyMaterials/index.htm: • no laptops, no mobiles during class • classes are not complicated, but please repeat them regularly • only few formulae, but you have to know/understand them • Statistics from previous years: “attendance= good grade” • Preparing the night before exam will not work ! • http://www.faculty.iu-bremen/course/spring06/GeneralGeoAstro2/astro

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

  3. Distance to the stars • From brightness? No! • Parallax-experiment … • full circle: 360 deg 1 deg = 60 arcmin = 60 ' = 60*60 arcsec = 3600 '' • Stellar parallax … d= 1/p

  4. Distance to the stars • Definition: “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 = 1/0.01= 100 pc

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

  6. Stellar velocities: • Important tool:Doppler shift • in words: • formula : (l-lo) / lo = vr/c vr : radial motion

  7. Stellar velocities • Proper motion m: “Which angle is travelled per time?” • Radial motion vr: measured via Doppler-shift • True velocity …

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

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

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

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

  12. 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= 2.512 “half as bright 1/2.512 as bright”

  13. 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)

  14. 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

  15. Stellar colours • Stellar coloursdepend on thesurface temperature ! • Wien’s law:max T = const …

  16. For your information: • Geo-Astro helpdesk: Tuesday 19:00 – 21:00, East Hall 5 1st session February 14th

  17. 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

  18. Spectra of Stars • Quantum mechanics: Interpretation of absorption lines in terms of atomic energy levels

  19. Stefan-Boltzmann law for black body radiation • F=s T4 - F:energy flux from star, “Joules per square meter per second” - s:a constant (Stefan-Boltzmann constant) - T:temperature • Luminosity of a star: L= 4pR*2s T4

  20. luminosity Stefan Boltzmann law Radius Stellar sizes • impossibleto measurewith telescopes • measurei) brightness ii) distance (parallax) iii) surface temperature(spectral type) . .

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

  22. Hertzpsrung-Russel diagram • not random, just afew 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

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

  24. 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

  25. Mass-luminosity relation • Observation: L M3.5 ….. “proportional to” • Stellar lifetime “fat blokes die young”

  26. 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”)

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

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

  29. 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

  30. Stellar birth • Where does star formation take place? …in thespiral armsof galaxies…

  31. Interstellar Medium (ISM) • ISM providesmatter of which stars aremade • ISM consists of acombination of gas and dust Interstellar gas • Very low density: ~ 1 H atom/ccm (“air” ~ 1019 atoms/cm3 ) but still ~20-30% of mass of galaxy • mainly Hydrogen& Helium, mixed with cosmic rays, magnetic fields and radiation

  32. Interstellar medium • Interstellar dust: -mainly H,C,O,Mg and Fe - size < 1/1000 mm ~ wavelength blue light blue scattered in all directions, intensity reduced “interstellar reddening” - also absorbs light, heats up, emits infrared radiation “insterstellar extinction”

  33. Interstellar medium • For historical reasons: interstellar clouds arecalled “Nebulae” • Three kinds of nebulae: Emission N.Reflection N. Dark N.

  34. Interstellar medium • Emission nebulae: - contain hot, young stars (O and B stars with Tsurf > 10 000K) - 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)

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

  36. Interstellar medium: emission nebulae • Emission mechanismHII-region: - Hydrogen ionized by UV-radiation from hot stars - recombination (proton captures electron, emits light as it cascades down) - most important transition from n=3 to n=2 (“Ha-photons”) reddish colour

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

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

  39. 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

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

  41. 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…

  42. 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

  43. Young stellar objects (YSOs): • Accretion disks: • Jets:

  44. Young stellar objects • examples ofaccretion disk – jet connection • interaction of these outflows with surrounding matter: Herbig-Haro objects • Jetsare usuallyshort-lived: 104 years,but caneject large masses (~1 Msol) during this time • many young stars lose mass viastrong winds:mass loss10-7 Msol/year (our sun: 10-14 Msol/year) Example: gas ejection from XZ-Tauri

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

  46. Stellar evolution: overview • once formed,evolutionof starsdepends ontheirmasses: • 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

  47. 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

  48. 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”

  49. 8 Msol -star • Planetary nebulae:

  50. 8 Msol -star • Evolution in the HR-diagram:

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