Presentation on theme: "Everything you always wanted to know about stars…"— Presentation transcript:
1 Everything you always wanted to know about stars… Material from Chapters 8 and 9 in Horizons by SeedsEverything you always wanted to know about stars…
2 The Spectra of StarsInner, 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
3 The Balmer Thermometer Balmer line strength is sensitive to temperature:Most hydrogen atoms are ionized => weak Balmer linesAlmost all hydrogen atoms in the ground state (electrons in the n = 1 orbit) => few transitions from n = 2 => weak Balmer lines
4 Measuring the Temperatures of Stars Comparing line strengths, we can measure a star’s surface temperature!
5 Spectral Classification of Stars (I) Different types of stars show different characteristic sets of absorption lines.Temperature
10 We have learned how to determine a star’s We have learned how to determine a star’ssurface temperaturechemical compositionNow we can determine itsdistanceluminosityradiusmassand how all the different types of stars make up the big family of stars.
11 Distances to Stars __ 1 d = p Trigonometric Parallax: 1 pc = 3.26 LY d in parsec (pc) p in arc seconds__1d =pTrigonometric Parallax:Star appears slightly shifted from different positions of Earth on its orbit1 pc = 3.26 LYThe farther away the star is (larger d), the smaller the parallax angle p.
12 The Trigonometric Parallax Example:Nearest star, Centauri, has a parallax of p = 0.76 arc secondsd = 1/p = 1.3 pc = 4.3 LYWith ground-based telescopes, we can measure parallaxes p ≥ 0.02 arc sec=> d ≤ 50 pcThis method does not work for stars farther away than about 50 pc (nearly 200 light-years).
13 The more distant a light source is, the fainter it appears. Intrinsic BrightnessThe 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
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):L__F ~d2Star AStar BEarthBoth stars may appear equally bright, although star A is intrinsically much brighter than star B.
15 The Size (Radius) of a Star We already know: flux increases with surface temperature (~ T4); hotter stars are brighter.But brightness also increases with size:Star B will be brighter than star A.ABAbsolute brightness is proportional to radius squared, L ~ R2.Quantitatively: L = 4 R2 T4Surface flux due to a blackbody spectrumSurface area of the star
16 Thus, Polaris is 100 times larger than the sun. 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 1002 = 10,000 times more than our sun’s.
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 ofLuminosityversusTemperature (or spectral type)Absolute mag.Hertzsprung-Russell DiagramLuminosityorTemperatureSpectral type: O B A F G K M
18 Most stars are found along the main sequence The Hertzsprung Russell DiagramMost stars are found along the main sequence
19 Stars spend most of their active life time on the Main Sequence. The Hertzsprung-Russell Diagram (II)Same temperature, but much brighter than MS stars Must be much largerStars spend most of their active life time on the Main Sequence. Giant StarsSame temp., but fainter → Dwarfs
20 Radii of Stars in the Hertzsprung-Russell Diagram RigelBetelgeuse10,000 times the sun’s radiusPolaris100 times the sun’s radiusSunAs large as the sun100 times smaller than the sun
21 Luminosity Classes Ia Bright Supergiants Ib Supergiants Ia Bright SupergiantsIaIbIb SupergiantsIIII Bright GiantsIIIIII GiantsIV SubgiantsIVVV Main-Sequence Stars
22 Luminosity effects on the width of spectral lines Same spectral type, but different luminosityLower gravity near the surfaces of giantssmaller pressuresmaller effect of pressure broadeningnarrower lines
23 Examples: Our Sun: G2 star on the main sequence: G2V Our Sun: G2 star on the main sequence: G2VPolaris: G2 star with supergiant luminosity: G2Ib
24 Binary StarsMore 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.
25 The Center of Mass center of mass = balance point of the system. center of mass = balance point of the system.Both masses equal => center of mass is in the middle, rA = rB.The more unequal the masses are, the more it shifts toward the more massive star.
26 “Placeholder” on Masses We can get masses of stars by measuring how they move in binary systems according to Newton’s Law of Gravitation.I’ll 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 massesThe higher a star’s mass, the more luminous (brighter) it is:High massesL ~ M3.5High-mass stars have much shorter lives than low-mass stars:Masstlife ~ M-2.5Low massesSun: ~ 10 billion yr.10 Msun: ~ 30 million yr.0.1 Msun: ~ 3 trillion yr.
28 The Mass-Luminosity Relation More massive stars are more luminous.L ~ M3.5
29 Determine properties of all stars within a certain volume. Surveys of StarsIdeal 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.
31 A Census of the StarsFaint, red dwarfs (low mass) are the most common stars.Bright, hot, blue main-sequence stars (high- mass) are very rare.Giants and supergiants are extremely rare.
32 The Interstellar Medium (ISM) 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 becausea) dense interstellar clouds are the birth place of starsb) dark clouds alter and absorb the light from stars behind them
34 Three kinds of nebulae 1) Emission Nebulae (HII Regions) 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 NebulaNGC 2246The Trifid Nebula
35 2) Reflection Nebulae Star illuminates gas and dust cloud; 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 it’s not cloudy).
37 3) Dark NebulaeDense clouds of gas and dust absorb the light from the stars behind;appear dark in front of the brighter background;Barnard 86Horsehead Nebula
38 Interstellar Reddening Blue light is strongly scattered and absorbed by interstellar cloudsRed light can more easily penetrate the cloud, but is still absorbed to some extentInfrared radiation is hardly absorbed at allBarnard 68Interstellar clouds make background stars appear redderInfraredVisible
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 statesNarrow absorption lines from Ca II: Too low ionization state and too narrow for the O star in the background; multiple componentsb) Small line width (too low temperature; too low density)c) Multiple components (several clouds of ISM with different radial velocities)
40 Structure of the ISM HI clouds: Hot intercloud medium: The ISM occurs in two main types of clouds:HI clouds:Cold (T ~ 100 K) clouds of neutral hydrogen (HI);moderate density (n ~ 10 – a few hundred atoms/cm3);size: ~ 100 pcHot intercloud medium:Hot (T ~ a few 1000 K), ionized hydrogen (HII);low density (n ~ 0.1 atom/cm3);gas can remain ionized because of very low density.
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.
42 Shocks Triggering Star Formation Henize 206 (infrared)
44 From Protostars to Stars Star emerges from the enshrouding dust cocoonIgnition of H He fusion processes
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.
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
47 Protostellar Disks and Jets – Herbig-Haro Objects (II) Herbig-Haro Object HH34
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 timeIn time jets, UV photons, supernova, will disrupt the stellar nurseryHubble Space Telescope Image
50 GlobulesEvaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars
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
52 The CNO Cycle 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.
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).
54 Hydrostatic Equilibrium Imagine a star’s interior composed of individual shellsWithin each shell, two forces have to be in equilibrium with each other:Gravity, i.e. the weight from all layers aboveOutward pressure from the interior
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.
56 The structure and evolution of a star is determined by the laws of Stellar ModelsThe structure and evolution of a star is determined by the laws ofHydrostatic equilibriumEnergy transportConservation of massConservation of energyA star’s mass (and chemical composition) completely determines its properties.That’s why stars initially all line up along the main sequence.
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.
58 Lifetime on Main Sequence L M3.5 T fuel / L = M/M3.5 = M-2.5Example: M=2 MSun L = 11.3 LSun T =1/5.7 TSunSpectral TypeMass (Sun = 1)Luminosity (Sun = 1)Years on Main SequenceO540405,0001 106B01513,00011 106A03.580440 106F01.76.13 109G01.11.48 109K00.80.4617 109M00.50.0856 109
59 The Deaths and End States of Stars Material from Seeds chapters 10-11The Deaths and End States of Stars
60 Less massive stars will die in less dramatic events. The End of a Star’s LifeWhen 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.
61 Evolution off the Main Sequence: Expansion into a Red Giant Hydrogen in the core completely converted into He: “Hydrogen burning” (i.e. fusion of H into He) ceases in the core.H burning continues in a shell around the core.He core + H-burning shell produce more energy than needed for pressure supportExpansion and cooling of the outer layers of the star red giant
62 Expansion onto the Giant Branch Expansion and surface cooling during the phase of an inactive He core and a H-burning shellSun will expand beyond Earth’s orbit!
63 Degenerate MatterMatter in the He core has no energy source left. Not enough thermal pressure to resist and balance gravity Matter assumes a new state, calleddegenerate matterElectron energyPressure in degenerate core is due to the fact that electrons can not be packed arbitrarily close together and have small energies.
64 Red Giant Evolution H-burning shell keeps dumping He onto the core. H-burning shell keeps dumping He onto the core.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”:4He + 4He 8Be + 8Be + 4He 12C + The onset of this process is termed thehelium flash
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!
66 HR Diagram of a Star Cluster High-mass stars evolved onto the giant branchTurn-off pointLow-mass stars still on the main sequence
67 Estimating the Age of a Cluster The lower on the MS the turn-off point, the older the cluster.
68 Red DwarfsRecall:Stars with less than ~ 0.4 solar masses are completely convective.Mass 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.
69 Sunlike Stars Expansion to red giant during H burning shell phase Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core.Mass Expansion to red giant during H burning shell phase Ignition of He burning in the He core Formation of a degenerate C,O core
70 White Dwarfs white dwarfs: Mass ~ Msun Temp. ~ 25,000 K Degenerate stellar remnant (C,O core)Extremely dense:1 teaspoon of white dwarf material: mass ≈ 16 tons!!!Chunk of white dwarf material the size of a beach ball would outweigh an ocean liner!white dwarfs:Mass ~ MsunTemp. ~ 25,000 KLuminosity ~ 0.01 Lsun
71 Low luminosity; high temperature => White dwarfs are found in the lower center/left of the H-R diagram.
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!
73 The Final Breaths of Sun-Like Stars: Planetary Nebulae Remnants of stars with ~ 1 – a few MsunRadii: R ~ light yearsExpanding at ~10 – 20 km/s ( Doppler shifts)Less than 10,000 years oldHave nothing to do with planets!The Helix Nebula
74 The Formation of Planetary Nebulae Two-stage process:Slow wind from a red giant blows away cool, outer layers of the starThe Ring Nebula in LyraFast wind from hot, inner layers of the star overtakes the slow wind and excites it => planetary nebula
75 Planetary Nebulae Often asymmetric, possibly due to Stellar rotation Often asymmetric, possibly due toStellar rotationMagnetic fieldsDust disks around the starsThe Butterfly Nebula
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.
78 Recycled Stellar Evolution Recycled Stellar EvolutionMass transfer in a binary system can significantly alter the stars’ masses and affect their stellar evolution.
79 White Dwarfs in Binary Systems Binary consisting of white dwarf + main-sequence or red giant star => WD accretes matter from the companionX ray emissionAngular momentum conservation => accreted matter forms a disk, called accretion disk.T ~ 106 KMatter in the accretion disk heats up to ~ 1 million K => X ray emission => “X ray binary”.
80 Explosive onset of H fusion Nova ExplosionsHydrogen accreted through the accretion disk accumulates on the surface of the white dwarfVery hot, dense layer of non-fusing hydrogen on the white dwarf surfaceNova Cygni 1975Explosive onset of H fusionNova explosion
81 Recurrent NovaeT PyxidisIn many cases, the mass transfer cycle resumes after a nova explosion. Cycle of repeating explosions every few years – decades.
82 The Fate of our Sun and the End of Earth Sun will expand to a red giant in ~ 5 billion yearsExpands to ~ Earth’s orbitEarth will then be incinerated!Sun may form a planetary nebula (but uncertain)Sun’s C,O core will become a white dwarf
84 The Deaths of Massive Stars: Supernovae Final stages of fusion in high-mass stars (> 8 Msun), 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
85 The Crab Nebula–Supernova from 1050 AD Can see expansion between 1973 and 2001Kitt Peak National Observatory Images
86 Remnant of a supernova observed in a.d. 1054 Supernova RemnantsX raysThe Crab Nebula:Remnant of a supernova observed in a.d. 1054The Veil NebulaOpticalCassiopeia AThe Cygnus Loop
87 The Famous Supernova of 1987: Supernova 1987A BeforeAt maximumUnusual type II supernova in the Large Magellanic Cloud in Feb. 1987
88 Observations of Supernovae Supernovae can easily be seen in distant galaxies.Supernova 1994D in NGC 4526
89 Type I and II Supernovae Core collapse of a massive star: type II supernovaIf an accreting white dwarf exceeds the Chandrasekhar mass limit, it collapses, triggering a type Ia supernova.Type I: No hydrogen lines in the spectrumType II: Hydrogen lines in the spectrum
90 The central core will collapse into a compact object of ~ a few Msun. Neutron StarsA supernova explosion of an M > 8 Msun star blows away its outer layers.Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object.The central core will collapse into a compact object of ~ a few Msun.Typical size: R ~ 10 kmMass: M ~ 1.4 – 3 MsunDensity: ~ 1014 g/cm3 Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!!
91 Discovery of Pulsars Angular momentum conservation Angular momentum conservation=> Collapsing stellar core spins up to periods of ~ a few milliseconds.Magnetic fields are amplified up to B ~ 109 – 1015 G.(up to 1012 times the average magnetic field of the sun)=> Rapidly pulsed (optical and radio) emission from some objects interpreted as spin period of neutron stars
92 The Crab Pulsar Remnant of a supernova observed in A.D. 1054 Pulsar wind + jetsRemnant of a supernova observed in A.D. 1054
95 The Lighthouse Model of Pulsars A pulsar’s magnetic field has a dipole structure, just like Earth.Radiation is emitted mostly along the magnetic poles.
96 Images of Pulsars and other Neutron Stars The Vela pulsar moving through interstellar spaceThe Crab Nebula and pulsar
97 Neutron Stars in Binary Systems: X-ray binaries Example: Her X-1Star eclipses neutron star and accretion disk periodically2 Msun (F-type) starNeutron starOrbital period = 1.7 daysAccretion disk material heats to several million K => X-ray emission
98 Some pulsars have planets orbiting around them. Pulsar PlanetsSome 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.
99 Neutron stars can not exist with masses > 3 Msun Black HolesJust like white dwarfs (Chandrasekhar limit: 1.4 Msun), there is a mass limit for neutron stars:Neutron stars can not exist with masses > 3 MsunWe know of no mechanism to halt the collapse of a compact object with > 3 Msun.It will collapse into a single point – a singularity:=> A black hole!
100 Escape VelocityVelocity needed to escape Earth’s gravity from the surface: vesc ≈ 11.6 km/s.vescNow, gravitational force decreases with distance (~ 1/d2) => Starting out high above the surface => lower escape velocity.vescIf you could compress Earth to a smaller radius => higher escape velocity from the surface.vesc
101 The Schwarzschild Radius => There is a limiting radius where the escape velocity reaches the speed of light, c:2GM____Vesc = cRs =c2G = gravitational constantM = massRs is called the Schwarzschild radius.
102 Schwarzschild Radius and Event Horizon No object can travel faster than the speed of light=> nothing (not even light) can escape from inside the Schwarzschild radiusWe have no way of finding out what’s happening inside the Schwarzschild radius.“Event horizon”
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:massangular momentum(electric charge)
105 The Gravitational Field of a Black Hole Gravitational PotentialDistance from central massThe gravitational potential (and gravitational attraction force) at the Schwarzschild radius of a black hole becomes infinite.
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.
107 General Relativity Effects Near Black Holes (II) Time dilationClocks 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.Event horizon
108 General Relativity Effects Near Black Holes (III) gravitational redshiftAll wavelengths of emissions from near the event horizon are stretched (redshifted). Frequencies are lowered.Event horizon
109 Observing Black Holes Mass > 3 Msun => Black hole! 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 Msun=> Black hole!
110 Compact object with > 3 Msun must be a black hole! Compact object with > 3 Msun must be a black hole!
111 Gamma-Ray Bursts (GRBs) Short (~ a few s), bright bursts of gamma-raysGRB of May 10, 1999: 1 day after the GRB2 days after the GRBLater discovered with X-ray and optical afterglows lasting several hours – a few daysMany have now been associated with host galaxies at large (cosmological) distances.Probably related to the deaths of very massive (> 25 Msun) stars.