chapter11+12+13

chapter11+12+13 - Stars and their Evolution Stars Stars are...

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Unformatted text preview: Stars and their Evolution Stars Stars are the major luminous part of galaxies Many of them are more luminous than the Sun. Then why don’t they appear as bright? Why is it difficult to see them in detail? Stars and their Evolution Stars Stars are the major luminous part of galaxies Next star out is 260,000 times as far away as the sun (4.2 light years away.) If sun = basketball, earth = apple seed about 30 m away, and nearest star ~7,000 km away and So we difficult to see structural details (Need interferometry to resolve big stars also e.g., 0.05” for Betelgeuse) But we can still study many properties of stars using measurements of stellar positions, total light emitted by stars, and their spectra T he N e a r by S t a r s Proxima Centauri: The Nearest Star (4.2 light years away) Polaris: The North Star (390 ly away) Are all stars loners like the Sun? Pleiades Star Cluster (“The Seven Sisters”) “Open Cluster” About 400 ly away About 13 ly across About 3000 stars The Largest “Ball of Stars” in our Galaxy Omega Centauri globular cluster 15,000 ly away 150 ly across 10 million stars! Measuring Stellar Properties Measuring Stars come in many varieties Massive vs. less massive, blue vs. red, young vs. old, quiescent vs. exploding, etc. But they all have nuclear reactions going on in their centers Unlike planets, comets, asteroids, etc. Star Names: α, β, etc. followed by constellation in decreasing order of apparent brightness E.g. α Orionis = Betelgeuse (brightest star in Orion) E.g. Fainter stars by catalog numbers Orion constellation Measuring Stellar Properties Measuring Distance: 1 light year = 3 x 108 m/s x 3.1 x 107 s = 9.3 x 1015 m 1 pc = 3.26 ly Parallax Up to ~ 0.01” → 100 pc Use other more indirect methods beyond that Whole cosmic distance ladder starts here Luminosity: Energy radiated per second Can also estimate L from spectra Effective Temperature: From colors, using Wien’s Law From λmax Teff = 0.0029 m-K T also indicated from relative line strengths Visual portion of stellar spectrum Spectra of stars Stellar spectral types: Classification based on spectral features To search for patterns among stellar spectra As T decreases, ionization decreases λmax = 0.0029 mk / T (Wien’s Law) Measuring stellar Mass: Kepler’s Third Law + Newton’s Law of Gravitation: For 2 objects (eg. Stars A, B) moving around one another, P2 = a3 / (M1+M2) In eclipsing binary stars, we can measure a, P → mA + mB The stars orbit around the center of mass rB/rA gives mA/mB Knowing (mA+ mB), mA/mB, derive mA, mB R a n g e o f M a s s e s s e e n in s t a r s : A b o u t 0 .0 8 t o 1 5 0 t im e s t h e Sun’s mass Motions of stars: ~few x 10-100 km/s relative to Sun Components of motions can be in 2 directions Radial & Tangential Radial Velocity: Measure with Doppler Effect Just shift, not broadening Blue shift if approaching Red shift if receding Tangential Velocity: Measure directly as change in star’s angular position with time: “proper motion” : few arcsec/yr for nearby stars The Life of A Star How can we study life cycles of stars? Can we watch any particular star from birth to death? What could an extraterrestrial spacecraft looking Wh down on the Earth infer about the life cycle of human beings? Studying Stellar Evolution Take a “census”--“snapshots” of a large number of stars and infer how they evolve The Hertzsprung-Russell (H-R) diagram Not H vs R! But luminosity vs. spectral type, or luminosity vs. temperature Bright → → Faint Hot (blue) Cool (red) → → → → → Gives many clues to stellar evolution! Most stars occupy a zone “Main sequence” Recall flux f = L/4π r2 Flux at the star’s surface = L /4πR2 (R= radius of star) Also, flux from surface of a black body= σ T4 (StefanAlso, Boltzmann Law) Boltzmann --> L = 4πR2 σ T4 --> A t a g iv e n T Higher L → Larger R → Bigger stars (Giants above MS) Higher Even higher L → Supergiants Even Giants, supergiants have R ~ 102 – 103 R๏ Lower L → Smaller R → Smaller stars (Dwarfs below MS) Lower Would our extraterrestrial observer see most human beings to be (a) “young” (< 16 yrs) (b) “middle-aged” (say 16-64 yrs) (c) “old” (> 64 yrs)? W hy? Why do most stars lie on MS? Why do most stars lie on MS? because that’s the longest phase of their life! A stable MS star has managed to balance the gravitational force (that tends to collapse it) by the thermal and radiation pressure generated because of nuclear reactions in the center (which tend to blow the star apart) Star remains on MS for a long time, since it has a lot of H and can remain stable Giants, supergiants, and white dwarfs have a different energy production process, so lie off MS Why is there a spread of L and T for MS stars? A more massive star needs to produce more energy to oppose the higher force of gravity, so it’s more luminous. Less massive → less luminous Less Mass – Luminosity Relation for MS stars Mass It also compresses and heats gas more. MS = group of stars of different masses that have a stable configuration and produce energy by burning H What decides how fast a star evolves-What e.g. how long it shines on MS? Rate of Evolution depends on Mass Massive stars evolve much more rapidly tMS (tH-burning) ~ M-3 1 M๏ star: ~ 11 x 109 yrs from birth to white dwarf ~ 24 x 106 yrs 10 M๏ star: In each case, MS still takes most time. Other phases much more short lived How do we know how old stars are? As a star cluster ages, the H-R diagram for the cluster becomes progressively deficient in early type stars First O, then B, etc. use MS turn-off to infer the cluster’s age (All stars in a cluster form at ~ same time. So evolutions governed by only mass differences.) Simplified picture of Star Formation A ) co res fo rm B ) c o lla p s e , d is k f la tte n s C ) o u tf lo w a lo n g p o le s D ) B a r e s ta r ( s ) le f t b e h in d Initial collapse may be triggered by a nearby event eg: supernova shock wave HST Images of Proto-planetary HST Disks in Orion Bipolar Jets of Matter from Young Stellar Objects Solar Composition Change Solar After H in the core is exhausted… After Core starts shrinking, gradually heating H shell outside the core which becomes hot enough to start burning Core “eats its way outward” The energy released causes outer layers to expand Star becomes a red giant Inner core still contracting, heating. outer layers expanding, cooling. When Tcore ~ 2 x 108 K, He ignites When Triple-Alpha Process: He4 + He4 --> Be8 + photon Be8 + He4 --> C12 + photon Be8 unstable so next stable species requires 3 He4 nuclei to combine Needs high temp to overcome electric repulsion Outer layers of star are thrown out (lost gradually in a “stellar wind”) Helium-Shell Burning Helium-Shell Planetary Nebulae Planetary Mass ejected around central star Gas atoms excited & ionized by UV light from the star Called planetary nebulae because rings looked like planetary rings Core stars often white dwarfs Heavy Element Fusion Heavy Ultimate Fate of Various Stars Ultimate Initial Mass < 0.01 M๏ 0.01 – 0.08 M๏ 0.01 0.08 – 0.25 M๏ 0.08 0.25 – 8-10 M๏ 0.25 8-10 – 12 M๏ 8-10 12 – 40 M๏ 12 > 40 M๏ Final state at End of Life Planet Brown dwarf White dwarf, mostly of He White dwarf, mostly of C, O White dwarf, made of O, Ne, White Mg Supernova, neutron star left behind Supernova, black hole left behind White Dwarfs The CO core keeps collapsing, gets denser, attains a new state of matter Star becomes a compact white dwarf R ~ Rearth! (Largest mass of a stable WD=1.4 M) Pressure of electrons in the new state of matter stops further collapse White dwarf gradually cools off by radiating away energy of motions of nuclei in its interior N e ut r o n S t a r s If Mcore > 1.4 M๏, electron degeneracy pressure not sufficient to prevent collapse ~ For 1.4 < Mcore < 2.5 M๏, electrons squeezed in to combine with protons & form neutrons Degeneracy pressure of neutrons halts collapse Neutrons resist being in same position, same motion Typical Neutron Star Size N e ut r o n S t a r s a r e S T RAN G E places! Rotate at ~ 1/10 speed of light Collapse causes rapid spins Compare: strongest lab magnets ~ 106 Gauss Earth ~ Gauss Sun ~ 103 Gauss Magnetic fields on surface ~ 1012 Gauss White Dwarfs Mass Radius Density < 1 .4 M ๏ ~ 5 ,0 0 0 k m (~ Earth) Neutron Stars > 1.4, < ~ .5 M๏ 2 ~ 10 km (~ small town) ~ 5 x 105 gm/cc ~ 1 0 1 4 gm /cc Black Holes Any mass distorts space & time nearby Gravity ↔ Curvature in space time Gravity Stronger gravity → Higher curvature Stronger Event Horizon Even light can’t escape No photons escape “vesc = c” 2 GM R =c Schwarzschild Radius 3 km for M = M๏ GM at R = 2 S c2 Event Horizon size depends on mass Sun 3 km 105 Globular cluster 300,000 km Entire Galaxy 0.1 ly ly Earth 1 cm (~ a grape) Asteroid ~ atomic nucleus Black holes don’t go on gobbling things with their gravity They only gobble up things close to event horizon. Far away from event horizon the gravitational field is about the same as that of the original pre-collapse star. Black holes can make time machines! Time passes much more slowly near the event horizon of a black hole A person visiting region near event horizon & then returning back would arrive “in the future ” T w o t y pe s o f S upe r no v a e ...
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