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Everything you always wanted to know about stars… Material from Chapters 8 and 9 in Horizons by Seeds 0.

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Presentation on theme: "Everything you always wanted to know about stars… Material from Chapters 8 and 9 in Horizons by Seeds 0."— Presentation transcript:

1 Everything you always wanted to know about stars… Material from Chapters 8 and 9 in Horizons by Seeds 0

2 The Spectra of Stars Inner, dense layers of a star produce a continuous (black body) spectrum. Cooler surface layers absorb light at specific frequencies. Spectra of stars are absorption spectra. Spectrum provides temperature, chemical composition 0

3 The Balmer Thermometer Balmer line strength is sensitive to temperature: Almost all hydrogen atoms in the ground state (electrons in the n = 1 orbit) => few transitions from n = 2 => weak Balmer lines Most hydrogen atoms are ionized => weak Balmer lines 0

4 Measuring the Temperatures of Stars Comparing line strengths, we can measure a stars surface temperature! 0

5 Spectral Classification of Stars (I) Temperature Different types of stars show different characteristic sets of absorption lines. 0

6 Spectral Classification of Stars (II) 0

7 Oh Only BeBoy,Bad AAnAstronomers FineFForget Girl/GuyGradeGenerally KissKillsKnown Me Mnemonics Mnemonics to remember the spectral sequence: 0

8 Stellar spectra O B A F G K M Surface temperature 0

9 0

10 We have learned how to determine a stars surface temperature chemical composition Now we can determine its distance luminosity radius mass and how all the different types of stars make up the big family of stars. 0

11 Distances to Stars Trigonometric Parallax: Star appears slightly shifted from different positions of Earth on its orbit The farther away the star is (larger d), the smaller the parallax angle p. d = __ p 1 d in parsec (pc) p in arc seconds 1 pc = 3.26 LY 0

12 The Trigonometric Parallax Example: Nearest star, Centauri, has a parallax of p = 0.76 arc seconds d = 1/p = 1.3 pc = 4.3 LY With ground-based telescopes, we can measure parallaxes p 0.02 arc sec => d 50 pc This method does not work for stars farther away than about 50 pc (nearly 200 light-years). 0

13 Intrinsic Brightness The more distant a light source is, the fainter it appears. The same amount of light falls onto a smaller area at distance 1 than at distance 2 => smaller apparent brightness. Area increases as square of distance => apparent brightness decreases as inverse of distance squared 0

14 Intrinsic Brightness / Flux and Luminosity The flux received from the light is proportional to its intrinsic brightness or luminosity (L) and inversely proportional to the square of the distance (d): F ~ L __ d2d2 Star A Star B Earth Both stars may appear equally bright, although star A is intrinsically much brighter than star B. 0

15 The Size (Radius) of a Star We already know: flux increases with surface temperature (~ T 4 ); hotter stars are brighter. But brightness also increases with size: A B Star B will be brighter than star A. Absolute brightness is proportional to radius squared, L ~ R 2. Quantitatively: L = 4 R 2 T 4 Surface area of the star Surface flux due to a blackbody spectrum 0

16 Example: Polaris has just about the same spectral type (and thus surface temperature) as our sun, but it is 10,000 times brighter than our sun. Thus, Polaris is 100 times larger than the sun. This causes its luminosity to be = 10,000 times more than our suns. 0

17 Organizing the Family of Stars: The Hertzsprung-Russell Diagram We know: Stars have different temperatures, different luminosities, and different sizes. To bring some order into that zoo of different types of stars: organize them in a diagram of LuminosityversusTemperature (or spectral type) Luminosity Temperature Spectral type: O B A F G K M Hertzsprung-Russell Diagram or Absolute mag. 0

18 The Hertzsprung Russell Diagram Most stars are found along the main sequence 0

19 The Hertzsprung-Russell Diagram (II) Stars spend most of their active life time on the Main Sequence. Same temperature, but much brighter than MS stars Must be much larger Giant Stars Same temp., but fainter Dwarfs 0

20 Radii of Stars in the Hertzsprung-Russell Diagram 10,000 times the suns radius 100 times the suns radius As large as the sun 100 times smaller than the sun RigelBetelgeuse Sun Polaris 0

21 Luminosity Classes Ia Bright Supergiants Ib Supergiants II Bright Giants III Giants IV Subgiants V Main-Sequence Stars Ia Ib II III IV V 0

22 Luminosity effects on the width of spectral lines Same spectral type, but different luminosity Lower gravity near the surfaces of giants smaller pressure smaller effect of pressure broadening narrower lines 0

23 Examples: Our Sun: G2 star on the main sequence: G2V Polaris: G2 star with supergiant luminosity: G2Ib 0

24 Binary Stars More than 50% of all stars in our Milky Way are not single stars, but belong to binaries: Pairs or multiple systems of stars which orbit their common center of mass. If we can measure and understand their orbital motion, we can estimate the stellar masses. 0

25 The Center of Mass center of mass = balance point of the system. Both masses equal => center of mass is in the middle, r A = r B. The more unequal the masses are, the more it shifts toward the more massive star. 0

26 Placeholder on Masses We can get masses of stars by measuring how they move in binary systems according to Newtons Law of Gravitation. Ill save some of the details for exo-solar planets session. Plenty of other things to cover right now…

27 Masses of Stars in the Hertzsprung- Russell Diagram Masses in units of solar masses Low masses High masses Mass The higher a stars mass, the more luminous (brighter) it is: High-mass stars have much shorter lives than low-mass stars: Sun: ~ 10 billion yr. 10 M sun : ~ 30 million yr. 0.1 M sun : ~ 3 trillion yr. L ~ M 3.5 t life ~ M

28 The Mass-Luminosity Relation More massive stars are more luminous. L ~ M 3.5 0

29 Surveys of Stars Ideal situation: Determine properties of all stars within a certain volume. Problem: Fainter stars are hard to observe; we might be biased towards the more luminous stars. 0

30 0

31 A Census of the Stars Faint, red dwarfs (low mass) are the most common stars. Giants and supergiants are extremely rare. Bright, hot, blue main-sequence stars (high- mass) are very rare. 0

32 The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky. We are interested in the interstellar medium because a) dense interstellar clouds are the birth place of stars b) dark clouds alter and absorb the light from stars behind them The Interstellar Medium (ISM) 0

33 The Various Appearances of the ISM 0

34 Three kinds of nebulae 1) Emission Nebulae (HII Regions) Hot star illuminates a gas cloud; excites and/or ionizes the gas (electrons kicked into higher energy states); electrons recombining, falling back to ground state produce emission lines. The Fox Fur Nebula NGC 2246 The Trifid Nebula 0

35 2) Reflection Nebulae Star illuminates gas and dust cloud; star light is reflected by the dust; reflection nebula appears blue because blue light is scattered by larger angles than red light; Same phenomenon makes the day sky appear blue (if its not cloudy). 0

36 Emission and Reflection Nebulae 0

37 3) Dark Nebulae Barnard 86 Dense clouds of gas and dust absorb the light from the stars behind; appear dark in front of the brighter background; Horsehead Nebula 0

38 Interstellar Reddening Visible Infrared Barnard 68 Blue light is strongly scattered and absorbed by interstellar clouds Red light can more easily penetrate the cloud, but is still absorbed to some extent Infrared radiation is hardly absorbed at all Interstellar clouds make background stars appear redder 0

39 Interstellar Absorption Lines The interstellar medium produces absorption lines in the spectra of stars. These can be distinguished from stellar absorption lines through: a) Absorption from wrong ionization states Narrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple components b) Small line width (too low temperature; too low density) c) Multiple components (several clouds of ISM with different radial velocities) 0

40 Structure of the ISM HI clouds: Hot intercloud medium: The ISM occurs in two main types of clouds: Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 – a few hundred atoms/cm 3 ); size: ~ 100 pc Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm 3 ); gas can remain ionized because of very low density. 0

41 The Various Components of the Interstellar Medium Infrared observations reveal the presence of cool, dusty gas. X-ray observations reveal the presence of hot gas. 0

42 Shocks Triggering Star Formation 0 Henize 206 (infrared)

43 The Contraction of a Protostar 0

44 From Protostars to Stars Ignition of H He fusion processes Star emerges from the enshrouding dust cocoon 0

45 Evidence of Star Formation Nebula around S Monocerotis: Contains many massive, very young stars, including T Tauri Stars: strongly variable; bright in the infrared. 0

46 Protostellar Disks and Jets – Herbig-Haro Objects Disks of matter accreted onto the protostar (accretion disks) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig-Haro objects 0

47 Protostellar Disks and Jets – Herbig-Haro Objects (II) Herbig-Haro Object HH34 0

48 Herbig-Haro 34 in Orion Jet along the axis visible as red Lobes at each end where jets run into surrounding gas clouds

49 Motion of Herbig-Haro 34 in Orion Can actually see the knots in the jet move with time In time jets, UV photons, supernova, will disrupt the stellar nursery Hubble Space Telescope Image

50 Globules Evaporating gaseous globules (EGGs): Newly forming stars exposed by the ionizing radiation from nearby massive stars 0

51 The Source of Stellar Energy Stars produce energy by nuclear fusion of hydrogen into helium. In the sun, this happens primarily through the proton-proton (PP) chain 0

52 The CNO Cycle In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over: the CNO cycle. 0

53 Fusion into Heavier Elements Fusion into heavier elements than C, O: requires very high temperatures; occurs only in very massive stars (more than 8 solar masses). 0

54 Hydrostatic Equilibrium Imagine a stars interior composed of individual shells Within each shell, two forces have to be in equilibrium with each other: Outward pressure from the interior Gravity, i.e. the weight from all layers above 0

55 Hydrostatic Equilibrium (II) Outward pressure force must exactly balance the weight of all layers above everywhere in the star. This condition uniquely determines the interior structure of the star. This is why we find stable stars on such a narrow strip (main sequence) in the Hertzsprung-Russell diagram. 0

56 Stellar Models The structure and evolution of a star is determined by the laws of Hydrostatic equilibrium Energy transport Conservation of mass Conservation of energy A stars mass (and chemical composition) completely determines its properties. Thats why stars initially all line up along the main sequence. 0

57 The Life of Main-Sequence Stars Stars gradually exhaust their hydrogen fuel. In this process of aging, they are gradually becoming brighter, evolving off the zero-age main sequence. 0

58 Lifetime on Main Sequence L M 3.5 T fuel / L = M/M 3.5 = M -2.5 Example: M=2 M Sun L = 11.3 L Sun T =1/5.7 T Sun Spectral Type Mass (Sun = 1) Luminosity (Sun = 1) Years on Main Sequence O , B , A F G K M

59 The Deaths and End States of Stars Material from Seeds chapters

60 The End of a Stars Life When all the nuclear fuel in a star is used up, gravity will win over pressure and the star will die. High-mass stars will die first, in a gigantic explosion, called a supernova. Less massive stars will die in less dramatic events. 0

61 Evolution off the Main Sequence: Expansion into a Red Giant Hydrogen in the core completely converted into He: H burning continues in a shell around the core. He core + H-burning shell produce more energy than needed for pressure support Expansion and cooling of the outer layers of the star red giant Hydrogen burning (i.e. fusion of H into He) ceases in the core. 0

62 Expansion onto the Giant Branch Expansion and surface cooling during the phase of an inactive He core and a H-burning shell Sun will expand beyond Earths orbit! 0

63 Degenerate Matter Matter in the He core has no energy source left. Not enough thermal pressure to resist and balance gravity Matter assumes a new state, called degenerate matter Pressure in degenerate core is due to the fact that electrons can not be packed arbitrarily close together and have small energies. 0 Electron energy

64 Red Giant Evolution He core gets denser and hotter until the next stage of nuclear burning can begin in the core: He fusion through the triple-alpha process: 4 He + 4 He 8 Be + 8 Be + 4 He 12 C + H-burning shell keeps dumping He onto the core. The onset of this process is termed the helium flash 0

65 Evidence for Stellar Evolution: Star Clusters Stars in a star cluster all have approximately the same age! More massive stars evolve more quickly than less massive ones. If you put all the stars of a star cluster on a HR diagram, the most massive stars (upper left) will be missing! 0

66 High-mass stars evolved onto the giant branch Low-mass stars still on the main sequence Turn-off point HR Diagram of a Star Cluster 0

67 Estimating the Age of a Cluster The lower on the MS the turn-off point, the older the cluster. 0

68 Red Dwarfs Recall: Stars with less than ~ 0.4 solar masses are completely convective. Hydrogen and helium remain well mixed throughout the entire star. No phase of shell burning with expansion to giant. Star not hot enough to ignite He burning. Mass 0

69 Sunlike Stars Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core. Expansion to red giant during H burning shell phase Ignition of He burning in the He core Formation of a degenerate C,O core Mass 0

70 White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of white dwarf material: mass 16 tons!!! white dwarfs: Mass ~ M sun Temp. ~ 25,000 K Luminosity ~ 0.01 L sun Chunk of white dwarf material the size of a beach ball would outweigh an ocean liner! 0

71 Low luminosity; high temperature => White dwarfs are found in the lower center/left of the H-R diagram. 0

72 The Chandrasekhar Limit The more massive a white dwarf, the smaller it is. Pressure becomes larger, until electron degeneracy pressure can no longer hold up against gravity. WDs with more than ~ 1.4 solar masses can not exist! 0

73 The Final Breaths of Sun-Like Stars: Planetary Nebulae The Helix Nebula Remnants of stars with ~ 1 – a few M sun Radii: R ~ light years Expanding at ~10 – 20 km/s ( Doppler shifts) Less than 10,000 years old Have nothing to do with planets! 0

74 The Ring Nebula in Lyra The Formation of Planetary Nebulae Two-stage process: Slow wind from a red giant blows away cool, outer layers of the star Fast wind from hot, inner layers of the star overtakes the slow wind and excites it => planetary nebula 0

75 Planetary Nebulae Often asymmetric, possibly due to Stellar rotation Magnetic fields Dust disks around the stars The Butterfly Nebula 0

76 A Gallery of P-N from Hubble

77 Mass Transfer in Binary Stars In a binary system, each star controls a finite region of space, bounded by the Roche lobes (or Roche surfaces). Lagrangian points = points of stability, where matter can remain without being pulled toward one of the stars. Matter can flow over from one star to another through the inner lagrange point L1. 0

78 Recycled Stellar Evolution Mass transfer in a binary system can significantly alter the stars masses and affect their stellar evolution. 0

79 White Dwarfs in Binary Systems Binary consisting of white dwarf + main-sequence or red giant star => WD accretes matter from the companion Angular momentum conservation => accreted matter forms a disk, called accretion disk. Matter in the accretion disk heats up to ~ 1 million K => X ray emission => X ray binary. T ~ 10 6 K X ray emission 0

80 Nova Explosions Nova Cygni 1975 Hydrogen accreted through the accretion disk accumulates on the surface of the white dwarf Very hot, dense layer of non-fusing hydrogen on the white dwarf surface Explosive onset of H fusion Nova explosion 0

81 Recurrent Novae In many cases, the mass transfer cycle resumes after a nova explosion. Cycle of repeating explosions every few years – decades. T Pyxidis 0

82 The Fate of our Sun and the End of Earth Sun will expand to a red giant in ~ 5 billion years Expands to ~ Earths orbit Earth will then be incinerated! Sun may form a planetary nebula (but uncertain) Suns C,O core will become a white dwarf 0

83 0

84 The Deaths of Massive Stars: Supernovae Final stages of fusion in high-mass stars (> 8 M sun ), leading to the formation of an iron core, happen extremely rapidly: Si burning lasts only for ~ 1 day. Iron core ultimately collapses, triggering an explosion that destroys the star: Supernova 0

85 The Crab Nebula–Supernova from 1050 AD Can see expansion between 1973 and 2001 –Kitt Peak National Observatory Images

86 Supernova Remnants The Cygnus Loop The Veil Nebula The Crab Nebula: Remnant of a supernova observed in a.d Cassiopeia A Optical X rays 0

87 The Famous Supernova of 1987: Supernova 1987A BeforeAt maximum Unusual type II supernova in the Large Magellanic Cloud in Feb

88 Observations of Supernovae Supernovae can easily be seen in distant galaxies. 0 Supernova 1994D in NGC 4526

89 Type I and II Supernovae Core collapse of a massive star: type II supernova If an accreting white dwarf exceeds the Chandrasekhar mass limit, it collapses, triggering a type Ia supernova. Type I: No hydrogen lines in the spectrum Type II: Hydrogen lines in the spectrum 0

90 Neutron Stars Typical size: R ~ 10 km Mass: M ~ 1.4 – 3 M sun Density: ~ g/cm 3 Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!! A supernova explosion of an M > 8 M sun star blows away its outer layers. The central core will collapse into a compact object of ~ a few M sun. Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object. 0

91 Discovery of Pulsars => Collapsing stellar core spins up to periods of ~ a few milliseconds. Angular momentum conservation => Rapidly pulsed (optical and radio) emission from some objects interpreted as spin period of neutron stars Magnetic fields are amplified up to B ~ 10 9 – G. (up to times the average magnetic field of the sun) 0

92 The Crab Pulsar Remnant of a supernova observed in A.D Pulsar wind + jets 0

93 The Crab Pulsar Visual imageX-ray image 0

94 Light curves of the Crab Pulsar 0

95 The Lighthouse Model of Pulsars A pulsars magnetic field has a dipole structure, just like Earth. Radiation is emitted mostly along the magnetic poles. 0

96 Images of Pulsars and other Neutron Stars The Vela pulsar moving through interstellar space The Crab Nebula and pulsar 0

97 Neutron Stars in Binary Systems: X-ray binaries Example: Her X-1 2 M sun (F-type) star Neutron star Accretion disk material heats to several million K => X-ray emission Star eclipses neutron star and accretion disk periodically Orbital period = 1.7 days 0

98 Pulsar Planets Some pulsars have planets orbiting around them. Just like in binary pulsars, this can be discovered through variations of the pulsar period. As the planets orbit around the pulsar, they cause it to wobble around, resulting in slight changes of the observed pulsar period. 0

99 Black Holes Just like white dwarfs (Chandrasekhar limit: 1.4 M sun ), there is a mass limit for neutron stars: Neutron stars can not exist with masses > 3 M sun We know of no mechanism to halt the collapse of a compact object with > 3 M sun. It will collapse into a single point – a singularity: => A black hole! 0

100 Escape Velocity Velocity needed to escape Earths gravity from the surface: v esc 11.6 km/s. v esc Now, gravitational force decreases with distance (~ 1/d 2 ) => Starting out high above the surface => lower escape velocity. v esc If you could compress Earth to a smaller radius => higher escape velocity from the surface. 0

101 The Schwarzschild Radius => There is a limiting radius where the escape velocity reaches the speed of light, c: V esc = c R s = 2GM____ c2c2 R s is called the Schwarzschild radius. G = gravitational constant M = mass 0

102 Schwarzschild Radius and Event Horizon No object can travel faster than the speed of light We have no way of finding out whats happening inside the Schwarzschild radius. => nothing (not even light) can escape from inside the Schwarzschild radius Event horizon 0

103 0

104 Black Holes Have No Hair Matter forming a black hole is losing almost all of its properties. black holes are completely determined by 3 quantities: mass angular momentum (electric charge) 0

105 The Gravitational Field of a Black Hole Distance from central mass Gravitational Potential The gravitational potential (and gravitational attraction force) at the Schwarzschild radius of a black hole becomes infinite. 0

106 General Relativity Effects Near Black Holes An astronaut descending down towards the event horizon of the black hole will be stretched vertically (tidal effects) and squeezed laterally. 0

107 General Relativity Effects Near Black Holes (II) Time dilation Event horizon Clocks starting at 12:00 at each point. After 3 hours (for an observer far away from the black hole): Clocks closer to the black hole run more slowly. Time dilation becomes infinite at the event horizon. 0

108 General Relativity Effects Near Black Holes (III) gravitational redshift Event horizon All wavelengths of emissions from near the event horizon are stretched (redshifted). Frequencies are lowered. 0

109 Observing Black Holes No light can escape a black hole => Black holes can not be observed directly. If an invisible compact object is part of a binary, we can estimate its mass from the orbital period and radial velocity. Mass > 3 M sun => Black hole! 0

110 Compact object with > 3 M sun must be a black hole! 0

111 Gamma-Ray Bursts (GRBs) Short (~ a few s), bright bursts of gamma-rays Later discovered with X-ray and optical afterglows lasting several hours – a few days GRB of May 10, 1999: 1 day after the GRB 2 days after the GRB Many have now been associated with host galaxies at large (cosmological) distances. Probably related to the deaths of very massive (> 25 M sun ) stars. 0

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