The Life Histories of Stars I. Birth and Violent Lives

Transcription

1 The Life Histories of Stars I Birth and Violent Lives

2 Stellar evolution--first problem for new discipline of astrophysics What is a star? What is it made of? How does it produce and release energy? How is it born? How does it evolve and finally die? Might different types of stars evolve differently over their lifetimes?

3 Early measurements of stars Apparent magnitudes (photos) Distances for close stars (parallax) Masses of binaries (Kepler s third law) Temperature of surface (color) Wien s Law, 1897: the hotter an object, the bluer its radiation Chemical composition (spectral lines) Proper, i.e., transverse, motions (photos) Radial motions (Doppler shifts)

4 Doppler shifts & proper motion Barnard s Star at 22-yr interval-- transverse motion of 10 /yr transverse True space motion radial Astronomy Today 5th ed, Fig Fig. 17.3

5 Task of lecture How can stars be classified? Hertzsprung-Russell (H-R) diagram What powers stars? Nuclear fusion The battle of gravity versus pressure Stellar evolution, part I Formation Main Sequence life Red Giant life Star death (next lecture, part II)

6 Natural histories-stellar spectra William & Margaret Huggins (London), 1863 First spectral studies of stars (24), metals in atmospheres But how to classify the lines, before Bohr s quantum theory (position, number,width or flouting )? Angelo Secchi (Rome), 1870s 4000 stars, 5 spectral types by color Pickering s Harvard harem, ,300 stellar spectra on glass dry plates Pickering s 16 classes (A-Q), from H lines Annie Jump Cannon s 7 classes (OBAFGKM), derived from similarity to nebular spectra Payne-Gaposchkin, 1920s, shows spectral sequence is a temp. scale (30,000 K K)

7 Cannon s spectral sequence Star Color blue-violet >Essentially same chemical elements in all stars >Temperature determines which absorption lines appear blue white yellow white yellow Oh, Be A Fine Girl/Guy, Kiss Me orange red Astronomy Today, Fig

8 Apparent magnitudes and luminosity Recall Hipparchus s 1-6 scale of apparent magnitudes (m) Magnitude change of 1 means brightness change of 2.5 Define: Absolute magnitude (M) = m at 10 pc M Sun = 4.8 M Rigel = -6.8 (over 40,000 times more luminous than Sun!) Luminosity = total energy output/sec M provides good approximation for L If know distance to star and measure m, can compute M (or L)

9 Hertzsprung-Russell diagram, , for prominent stars Well-known stars of known distances (stellar parallax), can compute M or L Red Giant arm The Main Sequence when plot L versus T Astronomy Today, Fig Enables natural history of stars by position on H-R diagram

10 H-R diagram for nearby stars Stars within 5 pc of Sun --mostly on Main Sequence --few White Dwarfs Astronomy Today, Fig

11 H-R diagram for brightest stars Mostly high Luminosity stars --OB types on Main Sequence --Red Giants --Blue Giants Astronomy Today, Fig

12 H-R diagram: 3 main regions Main Sequence (90% of all stars) Radius = 1 (solar radius) Luminosity from 0.01 to 100 (solar L) Spectral types O to M Red giants (1%) Radius = 100 Luminosity from 100 to 10,000 Spectral types K to M White dwarfs (9%) Radius =.01 Luminosity about.01 Spectral types B to A

13 What fuels the stars? Solar luminosity (L) = 4 x watts or J/sec Solar constant = 1400 w/m 2 (on earth) Solar mass = 2 x kg Solar lifetime = Total energy available/l Theories of stellar fuel Anthracite coal? ( 5000 yrs!) Gravitational collapse? (several million yrs) Kelvin and Helmholtz, 1860s Proton-electron annihilation? (10 trillion yrs) Russell, 1913 (wrong atomic physics--protons-electrons do not annihilate each other at temperatures of Sun) Nuclear fusion! (10 billion yrs) Bethe, von Weiszacker, 1938

14 Nuclear fusion in stars Making heavier out of lighter elements (Hbomb, 1952) Mass difference between constituents and products yields energy (E=mc 2 ) Atomic weight (p+n) atomic # (p) Stars convert H into He He 4 2 Sun converts 6 x kg H to He each second! Requires high temperature (10 million K) to overcome electromagnetic repulsion of H nuclei (i.e., of bare protons)

15 Proton-proton chain reaction Step 1 Step 2 Step 3 Astronomy Today 4, Fig Net result: H He + Energy 1 2

16 What fuels a star? Enormous sphere of hot gas Equilibrium between Gravity to contraction the gas Heat to expand the gas Stellar evolution Gravity wins at initial formation Gravity wins at end as star collapses Battle of gravity-heat during life Stellar mass determines stellar fate (KEY to all stars lives!!)

17 Stellar formation Interstellar matter, stuff for making stars Gas (H, He) and dust, very cold ( K) Unstable gravitational clumping Shock waves compress the ISM Supernova explosion starts wind New bright stars turn on, start pressure wave Spiral-arm waves of Milky Way galaxy Interstellar cloud breaks to fragments, each collapsing under gravity Fragments 100 x volume of solar system after several tens of thousands of yrs

18 Stellar formation, continued Collapsing cloud fragment Heat of collapse radiated away until density becomes too great and stops photons from escaping Temperature rises, pressure increases, contraction slows over thousands of years Protostar forms Contraction continues, decreasing L (less surface area to release energy) T at core rises to 10 million K and fusion begins M-type (lighter than Sun) require 1 billion years for fusion to begin G-type stars (Sun) require 50 million years O-type (heavier than Sun) require only 1 million years

19 Stellar formation, continued The more massive the star, the higher it lands on the Main Sequence but then it spends most of its life at that place on H-R diagram Luminosity drops as protostar contracts Astronomy Today, Fig. 19.8

20 Life on the Main Sequence H burning produces energy Stable gravity-heat balance Rate of energy production in core matches rate of energy radiated Size, temperature, L (i.e. location on Main Sequence) stay constant Stars DO NOT move on the Main Sequence during their life there!! Main-Sequence lifetimes very by mass G-type: remain 10 billion years M-type: remain 1 trillion years O-type: remain several million years

21 Leaving the Main Sequence (for stars < 8 solar masses) No fusion in He core, continues in H-shell around core Core contracts and heats, increasing fusion rate in H-shell Increased T expands star to Red Giant He fusion to C begins at 100 million K Helium flash, runaway explosion for hrs Higher density in core (electron degeneracy) enables flash Stable He burning for 10s of million years C core forms, with He and H shells C fusion requires 600 million K (not reached with G-type stars like Sun) 3He C+Energy

22 Leaving the Main Sequence (G-type stars) C core contracting He core expands, L decreases He core contracting Astronomy Today 4, Fig Tracks off Main Sequence depend of mass Fate of star after Supergiant? NEXT LECTURE!

23 Stars cluster evolution Thousands of stars All same age, same distance from us Snapshots of cluster s H-R diagram reveals stellar evolution Observed H-R diagram enables cluster to be dated!

24 H-R for a globular cluster (M3) Astronomy Today, Fig. 20.9