3 Kirchhoff's laws ,there are three types of spectra: continuum, emission line, and absorption line. High pressure, high temperature gasLow pressure, high temperature gasCool gas in front of continuous spectra source
8 Doppler effect similar in light and sound Waves compressed with source moving toward you Sound pitch is higher, light wavelength is compressed (bluer)Waves stretched with source moving away from youSound pitch is lower, light wavelength is longer (redder)
12 Stars are different colors, because they are different temperatures
13 Spectral Classification Annie Cannon classified stars according to the strength of the hydrogen absorption lines in the sequence A, B, C….PThese spectral classes were changed to a temperature-ordered sequence and some were discarded, finally leaving :SubclassesK7A5O B A F G K MSun(G2)35,000 K3,000 KOh, Be A Fine Girl (Guy) Kiss Me
14 O B A F G K M L The Spectral Sequence Bluest Reddest Hottest Coolest Spectral Sequence is a Temperature Sequence
15 O Stars B Stars A Stars F Stars Hottest Stars: T>30,000 K; Strong He+ lines; no H linesT = 11, ,000 K; Strong He lines; very weak H linesB StarsT = ,000 K; Strongest H lines, Weak Ca+ lines.A StarsT = K; H grows weaker Ca+ grows stronger, weak metals begin to emerge.F Stars
16 G Stars K Stars M Stars Solar Spectrum T = K; Strong Ca+, Fe+ and other metals dominate,G StarsT = K; Strong metal lines, molecular bands begin to appearK StarsT = K; strong molecular absorption bands particularly of TiOM Stars4000 A ASolar Spectrum
17 Quantum Mechanics Spectra Part II Electrons can only orbit the nucleus in certain orbits.n =1 First orbital: Ground State)Lowest energy orbit .
18 Down emission Up absorption Hydrogen Spectrum Hydrogen (1H) consists of:A single proton in the nucleus.A single electron orbiting the nucleus.
19 Emission Lines: Balmer Lines When an electron jumps from a higher to a lower energy orbital, a single photon is emitted with exactly the energy difference between orbitals. No more, no less.
20 Absorption Lines: Balmer Lines An electron absorbs a photon with exactly the energy needed to jump from a lower to a higher orbital. No more, no less.
21 Hydrogen lines absent in the hottest stars because, photons ionize electrons. They are also absent in the coolest stars because, photons don’t have enough energy to move the electrons from n=2 to higher energy levels.No electrons, no lines.
22 HR DiagramIn 1905, Danish astronomer Hertzsprung, and American astronomer Russell, noticed that the luminosity of stars decreased from spectral type O to M.To bring some order to the study of stars, they organize them in the HR diagram.
28 Giant Molecular Clouds (GMC) are mostly composed of molecular hydrogen.Properties:Radius ~50 pc (~160 ly)Mass ~105 MsunTemperature: KAlso, small amounts of He,and others
29 Size of cloud – large, Compression area - small GMC’s resist forming stars because of internal pressure (kinetic energy) so, a cooler gas is needed.A shockwave is needed to trigger formation, and to compress the material .
30 Sources of Shockwaves: 1.Supernova explosions: Massive stars die young .2. Previous star formation can trigger more formations3. Spiral arms in galaxies like our Milky Way:Spirals arms are probably rotating shock waves.
31 View all imagesAn expanding supernova explosion , occurring about 15,000 years ago.
32 Gravity ContractionAs the cloud is compressed, cool blobs contract into individual stars.The blobs glow faintly in radio or microwave light.As they heat up, blobs glow in the infrared, but they remain hidden .
33 As protostar compresses: Density increases Temperature rises. Photospheres (~3000K)Rotation increases as it shrinks in size.What types of stars form ?OB - FewAFG - MoreKM - Many, Many
34 Many of the cooler stars, spectral classes G,K,M, become heavy gas-ejecting stars called T-Tauri stars.Stars blows awaytheir cocoonLeave behind a T Tauri star with an accretion disk and a jet of hot gas.
35 A T-Tauri star can lose up to 50% of its mass before settling down as a main sequence star. False Color: Green = scattered starlight and red = emission from hot gas.
36 Motion of Herbig-Haro 34 in Orion You can actually see the knots, called Herbig-Haro objects, in the jet move with timeThey can have wind velocities of km/s. This phase lasts about 10 million years.
37 Collapse is slower for lower masses: Low-Mass ProtostarsCollapse is slower for lower masses:1 Msun (solar Mass) ~30 Myr0.2 Msun ~1 Billion yearsWhen core temperature ~ 10 Million K:Core ignites, P-P chain fusion beginsSettles slowly onto the Main SequenceHas a rotating disk, from which planetsmight form .
38 Actual Protoplanetary Disks The disks are 99% gas and 1% dust.The dust shows as a dark silhouette against the glowing gas of the nebula.
39 High-Mass ProtostarsCollapse is very rapid: 30 solar mass protostar collapses in ~30,000 yearsWhen core Temperature >10 Million K: Ignite first P-P Chain then CNO fusion in the core.
40 Clouds are blown away from the new stars near the stars
50 Stars Form in ClustersOur own Sun is part of an open cluster that includes Alpha Centauri and Barnard's star.Gravitational interactions will cause some stars to eventually leave over time
51 Resemble "Super Jupiters" Extreme :Minimum Mass: ~0.08 MsunBelow this mass, the core never gets hot enough to ignite H fusion.Star becomes a Brown DwarfResemble "Super Jupiters"Only about 100 are knownShine mostly in the infrared
52 Extreme :Maximum Mass: 60-100 Msun The core of a very massive star gets so hot:Radiation pressure overcomes gravity,star becomes unstable & disrupts.Upper mass limit is not well known.Such stars are very rare.
53 Star spends 90% of their life on the MS Main Sequence
54 Stars on the Main Sequence, are in Hydrostatic Equilibrium . Gravity pulling inward wants to contract the starPressure pushing outward wants to make the star expandThe star neitherexpands norcontracts.
55 Core Core-Envelope Structure Outer layers press down on the inner layers.The deeper you go, the greater the pressure.The star develops a :hot, dense, compact central COREsurrounded by a cooler, less dense, ENVELOPECoreCOREEnvelope
56 Energy is transferred inside stars by: Radiation (core) Energy is carried by photons from core.Photons hit atoms and get scattered.Slowly stagger to the surfaceTakes ~1 Million years to reach the surface.Convection (Envelope)Energy carried from hotterregions to cooler regions above bythe motions of the gas.Everyday examples of convection are boiling water.
57 Energy in a Main-Sequence star is generated by fusion of H into He This process is performed in two ways1. Proton-Proton (P-P) Chain: (Low mass stars)4 1H into 1 4He. + energy.Efficient at low core Temperatures (TC<18M K)2. CNO Cycle: (High mass stars)Carbon acts as a catalystEfficient at high core Temperatures(TC>18MK)
58 Main Sequence Lifetimes Main sequence lifetime (million years) More massive stars have the shorter life timeO & B stars burn fuel like an airplane!M stars burn fuel like a compact car!Every M dwarf ever created is still on the main sequence!!Main Sequence LifetimesSpectral TypeMass(Solar masses)Main sequence lifetime (million years)O5401B01610A03.3500F01.7BYG01.1BYK00.8BYM00.4BY
59 Death of Low Mass Star“It’s the end of the world as we know it” . REM
60 The End-States for Low and High Mass Stars Initial Stellar MassFinal Core MassFinal StateWhite dwarf8 - 30Neutron Star> 30> 3.0Black hole
61 Evolution of Low-Mass Stars Main Sequence PhaseEnergy Source: H core fusion (P-P cycle)Slowly builds up an inert He coreLifetime:~10 Byr for a 1 Msun star( Sun)~10 Tyr for a 0.1 Msun star (red dwarf)
62 Outer layer expands and cools When all H in core converted to HeHe core collapses and heats upHigh temperatures ignites H burning in a shellOuter layer expands and coolsStar becomes aRed Giant
63 Envelope ~ size of orbit of Venus Outside:Envelope ~ size of orbit of VenusThe star gets brighter and redder, climbs up the Giant Branch. (Takes 1 Byr)
64 *A secondary reaction forms Oxygen from Carbon & Helium: At the top of the Red Giant Branch:Tcore reaches 100 Million KHe fusion begins in coreFusion of three 4He nuclei into one 12C nucleus.*A secondary reaction forms Oxygen from Carbon & Helium:
65 Helium Flash in the core. Short period of fast burning, then.star contracts, gets a little dimmer, but hotter .Moves onto the horizontal branch.
66 Horizontal Branch Phase Structure:He-burning coreH-burning shellBuild up of a C-O core, still too cool to ignite Carbon
67 After 100 Myr, core runs out of He. C-O core collapses and heats up Inside:C-O core collapses and heats upHe burning shell outside the C-O coreH burning shell outside the He shellOutside:Star swells & cools
68 Climbs the Giant Branch again, slightly to the left of the original Giant Branch .
69 Core and Envelope separate. Helium shell flash produces a new powerful explosion, that pushes the outer envelope outward.Core and Envelope separate.With weight of envelope gone, core never reaches 600 million K, no Carbon fusionCore contraction is stopped by electron degeneracy.
70 A Planetary Nebula forms Hot C-O core is exposed, moves to the left Hot C-O core is exposed, moves to the leftBecomes a White Dwarf
71 Expanding envelope forms a ring nebula around the White Dwarf core. Ring is Ionized and heated by the hot central core of WD.Called planetary nebula because look like a tiny planet in a small telescope.The nebula expands at the ~ 35,000 to 70,000 miles/hour.Expands away in ~ 10,000 yrs
72 Planetary Nebulae Often asymmetric, possibly due to : Stellar rotation Magnetic fieldsThe Butterfly NebulaThe Hour Glass Nebula
73 White Dwarf Properties Radii ~ 1000-5000 km (~ size of Earth. ) Temp White Dwarf Properties Radii ~ km (~ size of Earth!) Temp. – from 100,000 to 2500 K. So small, that they can only be seen if close-by, or in a binary systems.White Dwarf’s mass < than the Chandrasekhar mass (1.4 Solar Masses).
74 White Dwarf Properties The core is tightly packed One teaspoon weighs about 5 tons.Shine by leftover heat, no fusion.Fade slowly, becoming a "Black Dwarf“.Takes ~10 Tyr to cool off , so none exists yet.
75 Size: 92% Earth's diameter Mass: 1.2 solar masses The most famous W.D. is Sirius’ companion .Sirius BTemp. 25,000 KSize: 92% Earth's diameterMass: 1.2 solar massesThe mass of a star, in the size of a planet.Sirius B
76 What about Binary Stars with one being a W.D. ! About half the stars in the sky are binaries.What about Binary Stars with one being a W.D. !But wait that’s not all!Mass could transferfrom the starto the W.D.
77 White Dwarf in a binary system….. Evolving (dying) starRoche LobesIIEvolving (dying) starWhite DwarfAccretion DiskIIIEvolving (dying) starRoche Lobe filled
78 A w. d. can take on material but , if the w. d. exceeds 1 A w.d. can take on material but , if the w.d. exceeds 1.4 solar masses, powerful explosions take place, and they can repeat.Type 1a super NOVA!!
79 Since the Type 1a supernova is always a white dwarf they can be used to judge very great distances (using the inverse square law).
80 Stellar GraveyardHigh Mass StarsStellar GraveyardHigh Mass Stars
81 The End-States for Low and High Mass Stars Initial Stellar Mass(Solar Mass)Final Core MassFinal State1 - 8White dwarf8 - 30Neutron Star> 30> 3.0Black hole
82 massive stars evolve more rapidly due to greater gravity. Evolution of High Mass StarsMassive stars go through about the same internal changes as low mass stars, except :massive stars evolve more rapidly due to greater gravity.massive stars can produce heavier elements
83 Evolution of High-Mass Stars O & B Stars (M > 8 Msun): (The James Dean of stars )Burn HotLive FastDie YoungMain Sequence Phase:Burn H to He in core using the CNO cycleBuild up a He core, like low-mass starsBut this lasts for only ~ 10 Myr
84 The Envelope expands and cools Envelope ~ size of orbit of Jupiter After H core exhausted:Inert He core contracts & heats upH burning in a shellThe Envelope expands and coolsEnvelope ~ size of orbit of Jupiter
85 Moves horizontally across the H-R diagram, becoming a Red Super giant star Takes about 1 Myr to cross the H-R diagram.
86 Core Temperature reaches 170 Million K Helium Flash : Helium Ignites producing C & OStar becomes a Blue Supergiant.
87 Inert C-O core collapses & heats up H & He burning shells expand He runs out in the core:Inert C-O core collapses & heats upH & He burning shells expandStar becomes a Red Supergiant again
88 C-O Core collapses until: Tcore > 600 Million K Ignites Carbon Burning in the Core.Carbon Burning:2- 12C fuse to form : Mg, Ne and OCarbon burning: 1000 years
89 End of the road ! Fusion now takes place rapidly Neon burning: ~10 yearsOxygen burning: ~1 yearSilicon burning: ~1 day.Finally builds up an inert Iron core.End of the road !
90 Core of a massive star at the end of Silicon Burning: Onion Skin
91 Collapse is final :Protons & electrons form neutrons & neutrinos. At the start of Iron Core collapse:Radius ~ 6000 km (~radius of earth)Density ~ 108 g/ccA second later!! , the properties are:Radius ~50 kmDensity ~1014 g/ccCollapse Speed ~0.25 c !
92 Material falling inwards is stopped by neutron degeneracy pressure . This material rebounds, causing the outer atmosphere, and shells, to be blown off in a violent explosion called a supernova.
93 Elements heavier than Lead are produced in the explosion. The supernova star will outshine all the other stars in the galaxy combined.The FamousSupernovaSN 1987Atype II Supernova
95 The Crab Nebula.This nebula is the result of a supernova that, exploded in 1054.The supernova was brighter than Venus for weeks before fading from view.The nebula is expanding at more than 3 million miles per hour.
96 Structure of a Neutron Star Diameter- 10 km in diameter 3> Mass > 1.4 times that of our Sun.One teaspoonful would weigh a billion tons!Rotation Rate:1 to 100 rotations/secInside a Neutron Star
97 Pulsar Magnetic axis is not aligned with the rotation axis. Lighthouse Model:Spinning magneticfield generates aa strong electric field.We will see regular, sharp pulses of light (optical, radio, X-ray) , if its pointed toward the earth.
98 The discovery of a pulsar in the crab nebula was the key connecting pulsars and neutron stars.
99 Black HolesWe know of no mechanism to halt the collapse of a compact object with mass > 3 Msun.
100 The effect of gravity on light Relativity implies nothing can go faster than light.As you travel faster, time slows down, you get more massive and your length appears to get shorter.The effect of gravity on light
101 SingularitiesIf the core of a star collapses with more than 3 solar masses, electron degeneracy and neutron degeneracy can’t stop the gravitational collapse.The star collapses to a radius of zero , with infinite density and gravity—called a Singularity.PositionParticle paths in acollapsing starsingularityEvent horizonTime
102 The Schwarzschild Black Hole The simplest of all black holes. A static, non-rotating mass.The Schwarzschild Radius defines the Event Horizon.We have no way of findingout what’s happeninginside the “Event horizon”
103 The Kerr Rotating Black Hole The singularity of a Kerr Black Hole is in infinitely thin ring around the center of the hole.The event horizon is surrounded by the ergosphere, where nothing can remain at rest. Here spacetime is being pulled around the rotating black hole.
104 It may be possible to avoid the singularity. An object is moving fast enough, can enter the ergosphere and fly out again. If the object stops in the ergosphere, it must fall into the Black Hole.General Relativity predicts Wormholes for Kerr Black Holes, but Astrophysicists are skeptical.It may be possible to avoid the singularity.
105 Primordial – can be any size (created with Big Bang). Various Black HolesPrimordial – can be any size (created with Big Bang).“Stellar mass” black holes – must be at least 3 Mo – many examples are knownIntermediate black holes – range from 100 to 1000 Mo - located in normal galaxies – many seenMassive black holes – about 106 Mo – such as in the center of the Milky Way – many seenSupermassive black holes – about Mo-located in Active Galactic Nuclei, have jets – many seen
106 Candidate For Black Hole Cygnus X-1 Binary Star w/ two objects: M=30 Msun primary ,M=7 Msun companionBright in X-rays.Far too massive to be a white dwarf or neutron star.The simplest interpretation is :A 30 M star and a 7 M black holeMeasured orbital motion of HDE
107 Evidence for BHA disk of dust fueling a massive black hole in the centre of a galaxy.The speed of the gas around the center indicates that the object at the centre is 1.2 billion times the mass of our Sun.800 light years