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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 General GeoAstro II: Astronomy Stars - Nature of stars - Milky Way - Cosm. Expansion Birth of stars - Other galaxies - Big Bang Stellar evolution - Supermassive - Tests of our Endpoints: black holes theories White Dwarfs - New Neutron Stars Black Holes developments - Galaxies Cosmology Distance to the stars From brightness? No! Parallax-experiment … Stellar parallax … d= 1/p 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 Distance to the stars Hipparcos: High Precision Parallax Collecting Satellite (Hipparchus: greek astronomer) Parallaxes still important to gauge other distance indicators Stellar motions …. 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] Brightness and distance • brightness …. •b= L/(4 p d2) •“double the distance brightness reduced by a factor 4” luminosities huge variety of stellar luminosities: Lmax =1010 Lmin (1010 = number of all people that ever lived on earth) 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” 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” 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) 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 Stellar colours Stellar colours depend on the surface temperature ! Wien’s law: lmax T = const … 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 Spectra of Stars Advent of quantum mechanics: Interpretation of absorption lines in terms of atomic energy levels Stellar sizes impossible to measure with telescopes measure i) brightness luminosity ii) distance (parallax) iii) surface temperature (spectral type) . Stefan Boltzmann law . Radius Hertzsprung-Russel diagram Idea: plot luminosity vs. temperature (spectral type) information about radius classification of stars 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 1000 Rsun 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 T total system mass Stellar masses individual masses? i) find center of mass (CM) ii) distances from CM to stars, a1 & a2 a1= (M2/Mtot) a a2= (M1/Mtot) a Mass-luminosity relation Observation: L M3.5 ….. “proportional Stellar lifetime t : to” t M-2.5 “fat blokes die young” 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”) Birth of Stars “snapshot problematic” stellar >> human lifetime Derive evolutionary sequence from a set of “snapshots” Stellar Birth Stars are born in the gravitational collapse of giant molecular clouds Stellar Birth computer-simulation of 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 Stellar birth Where does star formation take place? …in the spiral arms of galaxies… 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 Interstellar medium Three kinds of nebulae: Emission N. Reflection N. Dark N. 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) Interstellar medium: emission nebulae Interstellar hydrogen found in two forms” “HI-region”: neutral hydrogen “HII-region”: ionized hydrogen (i.e. protons and electrons) 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 •Reflection nebulae Lots of fine-grained dust, low density reflects short-wavelengths more efficiently than long ones blue colour •Dark Nebulae High density of dust grains block view to the stars Temperature: 10 – 100 K Density: hydrogen molecules n ~ 104 – 109 atoms/cm3 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 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… 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… 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 Young stellar objects (YSOs): …youngsters in revolution… Accretion disks: Jets: 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) Young stellar objects Young stars like to hang around in groups (see previous movie) “open clusters” fastest stars may leave “evaporation” of open clusters Stellar evolution: overview once formed, evolution of stars depends on their masses: M < 0.08 Msol: 0.08 < M < 8 Msol: no nuclear fusion “Brown dwarfs” i) Main sequence ii) Giant phase iii) White Dwarf + planetary nebula Stellar evolution: overview 8 < M < 25 Msol: i) Main Sequence ii) Giant phase iii) supernova explosion neutron star M > 25 Msol: i) Main Sequence ii) Giant phase iii) supernova explosion black hole 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” 8 Msol -star Planetary nebulae: 8 Msol -star Evolution in the HR-diagram: Testing stellar evolution:globular clusters Globular clusters ~105 stars in halo of galaxy Old: about same age as galaxy 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 Evolution for M > 8 Msol Stages: • Main sequence • Giant stage • Final stage: Evolution for M > 8 Msol No more nuclear fuel (beyond iron) “core”-collapse supernova explosion (type II) 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) End stages of stellar evolution: White dwarfs: Left behind in center of planetary nebula No more nuclear burning away just cools until it fades Masses: 0.2 – 1.4 Msol above 1.4 Msol collapse to neutron star Densities: ~ 106 – 108 g/cm3 (earth: ~ 5 g/cm3) Equilibrium between gravity and degeneracy pressure White dwarfs Degeneracy pressure: purely quantum mechanical effect: Electrons are “Fermions” (spin= ½) don’t want to be in the same state (Pauli-exclusion principle) resist compression even at zero temperature • all mass from neutrons and protons •all pressure from electrons white dwarfs Mass-Radius relationship: “More massive WDs are smaller” R M -1/3 : Neutron Stars End Stages of stellar evolution Masses: ~1.4 Msol Radius: ~10 - 15 km Density: few 1014 g/cm3 observed neutron star mass distribution Magnetic field: 1012 - 1015 G (earth: ~ 0.5 G) Neutron stars hard to detect: new-born neutron star in Supernova remnant Neutron Stars Internal structure: mostly neutrons (~90% neutrons, ~10% protons) crust: iron-like nuclei center: “exotic” particles? End Stages of stellar evolution: Black holes neutron star has limiting mass, above that mass: collapse to a black hole not even light can escape from a black hole… How can a black hole be detected? Black holes Black hole “accretes” mass from companion star x-ray binary Galaxies: Our Galaxy: the Milky Way The Structure of the Milky Way Galactic Plane Galactic Center The actual structure of our Milky Way is very hard to determine because: 1) We are . inside 2) Distance measurements are difficult 3) Our view towards the center is obscured by gas and dust Structure of the Milky Way (MW) So what can we do to explore the MW ?? a) space craft? No b) select bright objects that can be seen throughout the MW c) observe in different wavelengths d) trace velocities of all visible objects Structure of Milky Way: a) space craft a) How long would it take to get good “outside view” of our Galaxy (travel at speed of light)? i) 2 months ii) 1 year iii) 500 years iv) 30 000 years v) 5 million years ?? Answer: iv) 30,000 years The Sun is about Sun 8.5 kpc = 8,500 pc ≈ 30,000 light years from the Galactic center. Galactic Center => No spacecraft will ever travel a significant distance through or even out of the Milky Way Structure of Milky Way: b) bright objects b) What are bright objects? A type stars ? Brown dwarfs ? White dwarfs ? O type stars ? Structure of Milky Way: b) bright objects Answer: O- and B-stars ! Remember: O and B stars are the most massive, most luminous stars Look for very young clusters or associations: O/B- Associations ! Structure of Milky Way: b) bright objects optically bright objects O/B Associations Sun O/B Associations trace out 3 spiral arms near the Sun. Distances to O/B Associations determined using Cepheid Variables Structure of Milky Way: b) bright objects Globular Clusters Globular Cluster M80 • Dense clusters of 50,000 – a million stars • Old (11 billion years), lower-main-sequence stars • Approx. 200 globular clusters in our Milky Way Structure of Milky Way: b) bright objects Globular cluster distribution: we are not in the centre of our Galaxy Structure of Milky Way: c) different wavelengths Galaxy (optical): absorption by gas and dust Galaxy (near-infrared): emission from warm dust Structure of Milky Way: c) different wavelengths Galaxy more transparent at longer (than optical) wavelengths…. most transitions in hydrogen atom at “short” wavelengths, but … coupling magnetic moments electron and proton in neutral hydrogen hydrogen: 21cm radio emission Structure of the Milky Way Neutral hydrogen creates radio emission (l= 21cm): coupling between magnetic moments of proton and electron… “21-cm radiation” can be used to trace the distribution of neutral hydrogen in the Galaxy Structure of the Milky Way 75,000 light years Disk Nuclear Bulge Sun Halo Open Clusters, O/B Associations Globular Clusters Animation Structure of Milky Way Stellar Populations: …heavier elements are formed in various burning stages of stars… Question: how does the metal content of young and old stars differ? 1) Old stars should be more metal-rich… 2) Young stars should be more metal-rich… 3) They should be the same… Structure of Milky Way Of course: young stars are “metal”-rich Stellar Populations Population I: Young stars: metal rich; located in spiral arms and disk Population II: Old stars: metal poor; located in the halo (globular clusters) and nuclear bulge Dynamics in the Milky Way (I) Population I (disk stars) Population II (halo stars) Dynamics in the Milky Way (II) Differential Rotation Sun orbits around Galactic center with 220 km/s 1 orbit takes approx. 240 million years. Dynamics in the Milky Way Question: What determines the velocity with which the sun is moving around the Galactic centre? Mass of the sun? Rotational period of the spiral arm pattern? Mass inside the orbit of the sun ? Angular momentum of the Milky Way ? Dynamics in the Milky Way Answer: Newton’s Laws tell us that it is the mass inside the radius of the sun that determines its velocity The more mass there is inside the orbit, the faster the sun has to orbit around the Galactic center (argument similar to Kepler’s III. law)… V= 220 km/s Minside ~ 1011Msol R= 8.5 kpc Dynamics in the Milky Way Forms of rotation rigid rotation differential rotation… Dynamics of the Milky Way 21-cm-radiation of neutral hydrogen to determine the rotation curve (“velocity as a function of radius”) of our Galaxy Use the observed expected (if mass concentrated in centre) Dynamics in the Milky Way explanation for the observed rotation curve: There must be more mass than is visible !!! “ DARK MATTER “ (DM) - 90 % of the matter in the Galaxy is “invisible” - only 10 % in stars Dynamics in the Galaxy What could dark matter be made of? i) dim stars, massive planets, black holes? (= massive compact halo objects= MACHOS) experiments: only small fraction of DM are MACHOS ii) A new kind of particle ? (=weakly interacting massive particle= WIMP) maybe, but none such particle has been detected yet… The centre of our Galaxy Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust: Only 1 out of 1012 optical photons makes its way from the GC towards Earth! Galactic center Wide-angle optical view of the GC region Radio View of the Galactic Center Many supernova remnants; shells and filaments Arc Sgr A Sgr A Sgr A*: The Center of our Galaxy Galactic Center contains a supermassive black hole of approx. 2.6 million solar masses. Centre of our Galaxy motion of stars close to Galactic centre (observed !) everything consistent with a black hole of 2.6 million solar masses Other Galaxies types galaxies spiral galaxies barred spiral galaxies Other Galaxies elliptical galaxies irregular galaxies Other galaxies ...some more beautiful galaxies… Sombrero galaxy Blackeye galaxy Andromeda galaxy Other galaxies Stars do not collide ! But galaxies do… (observations !) Other Galaxies galaxy mergers (computer simulation !) Other galaxies Do other galaxies also contain supermassive black holes ? YES ! Similar to accretion disk-jet connection in young stellar objects Cosmology Ancient Mythology and Modern Cosmology: Is there a Difference ? Creation Stories I: The Christian/Jewish View Genesis: In the beginning God created the heavens and the earth. And the earth was waste and void; and darkness was upon the face of the deep: … Creation Stories II: Greco-Roman Mythology Hesiod: In the beginning there was only “chaos” [the infinite emptiness]. Then out of the void appeared Erebus, the unknowable place where death dwells, and Night. All else was empty, silent, endless, darkness. Then somehow Love was born bringing a start of order. From Love came Light and Day. Once there was Light and Day, Gaea, the earth appeared. Then Erebus slept with Night, who gave birth to Ether, the heavenly light, ... Common Themes and Concepts: Anthropomorphism Action of a supreme craftsman Generation from a seedling/egg Imposition of order over “chaos” Life cycle dominates over eternal/unchanging: there is a beginning Hybrid schemes: act of creation, but supreme being/chaos existed forever Anthropocentrism Scientific “Creation” Story 2005: In the beginning there was neither space nor time aswe know them, but a shifting foam of strings and loops, as small as anything can be. Within the foam, all of space, time and energy mingled in a grand unification. But the foam expanded and cooled. And then there was gravity, and space and time, and a universe formed. … Is there a difference ? The Scientific Method general principle induction deduction observations prediction revision specific instances individual events History: Mythology vs the scientific method Cosmos = Earth solar system Milky Way Hubble sphere Edwin Hubble (1889-1953) Four major accomplishments in extragalactic astronomy The establishment of the Hubble classification scheme of galaxies The convincing proof that galaxies are island “universes” The distribution of galaxies in space The discovery that the universe is expanding Doppler effect (for light) The light of an approaching source is shifted to the blue, the light of a receding source is shifted to the red Doppler effect The light of an approaching source is shifted to the blue, the light of a receding source is shifted to the red. blue shift red shift Doppler effect redshift: 1 v / c 1 z = 1- v / c z=0: not moving z=2: v=0.8c z=: v=c The redshift-distance relation Key results Most galaxies are moving away from us The recession speed v is larger for more distant galaxies. The relation between recess velocity v and distance d fulfills a linear relation: v = H0 d Hubble’s measurement of the constant H0: H0 = 500 km/s/Mpc today’s best fit value of the constant: H0 = 71 km/s/Mpc (WMAP) Question: If all galaxies are moving away from us, does this imply that we are at the center? Answer: Not necessarily, it also can indicate that the universe is expanding and that we are at no special place. Einstein’s General Relativity + observation of expanding Universe: Universe started from a point: “Big Bang Model” Example: static universe R(t) t Example: expanding at a constant rate R(t) t i s s l Example: expansion slowing down t x p a n s i o n R(t) t l e : R(t) e x p a n s i Example: expansion accelerating Example: Collapsing Universe R(t) t Cosmological redshift While a photon travels from a distance source to an observer on Earth, the Universe expands in size from Rthen to Rnow. Not only the Universe itself expands, but also the wavelength of the photon l. lreceived Rnow = lemitted Rthen Cosmological redshift General definition of redshift: lreceived - lemitted z= lemitted for cosmological redshift: 1 z = lreceived lemitted Rnow = Rthen A large redshift z implies ... The spectrum is strongly shifted toward red or even infrared colors The object is very far away We see the object at an epoch when the universe was much younger than the present day universe most distant astrophysical object discovered so far: z=5.8 z>5.8: “dark ages” k>0 k=0 k<0 Are there any indications that this picture is correct? Yes ! Primordial Nucleosynthesis Cosmic Microwave background Primordial Nucleosynthesis Georgy Gamov (1904-1968) If the universe is expanding, then there has been a big bang Therefore, the early universe must have been very dense and hot Optimum environment to breed the elements by nuclear fusion (Alpher, Bethe & Gamow, 1948) success: predicted that helium abundance is 25% failure: could not reproduce elements more massive than lithium and beryllium ( formed in stars) The Cosmic Microwave Background (CMB) Last scattering surface transparent opaque Penzias and Wilson 1965 Working at Bell labs Used a satellite dish to measure radio emission of the Milky Way They found some extra noise in the receiver, but couldn’t explain it discovery of the background radiation Most significant cosmological observation since Hubble Nobel prize for physics 1978 More results from the CMB The Earth is moving with respect to the CMB Doppler shift The emission of the Galaxy Fluctuations in the CMB •Fluctuations in CMB responsible for structure formation in the universe