Ch. 16 & 17: Stellar Evolution and Death

Transcription

1 Ch. 16 & 17: Stellar Evolution and Death Stars have lives: born, evolve, die Mass determines stellar evolution: Really Low Mass (0.08 to 0.4 M sun ) Low Mass: (0.4 to 4 M sun ) Long lives High Mass (4 to 8 M sun ): Short lives Very High Mass (> 8 M sun ): Very Short lives Star life is battle between two forces: Gravity vs. Pressure collapse expand Only in Hydrostatic Equilibrium while doing nuclear fusion in core. 1

2 Evolution of Lowest Mass Stars (Red Dwarfs) Mixed by convection => use up nearly all H, not just in core Never ignites hydrogen shell => cannot become giant Survive on MS for hundred billion years or more 2

3 Evolution After the Main Sequence What happens when H in core gone? 1) No more fusion in He core. Gravity => core contracts => heats up H around core => fusion starts in shell. 2) Shell fusion energy pours out => heats outer layers of star => expands => cools 3) Star moves toward upper right in the H-R diagram as Giant: x bigger than Sun Supergiants 1000 x bigger than Sun 3

4 4) He core reaches 10 8 K => He fusion begins 3 He nuclei 1 Carbon nucleus + Energy (+ little Oxygen) 5) Exhausts He fuel => Carbon core 6) Core collapses He-fusion shell ignites around it (+ H-fusion shell) Each step is shorter than the one before. 4

5 Evolution of Low Mass Stars like Our Sun Giant not hot enough to fuse C Core contracting Outer layers expanding Stellar wind blows some mass off Fusion shells unstable => outer layers gently ejected = Planetary Nebula 5

6 C core exposed = White Dwarf size of Earth hot, dense gas stabilized against gravity by electron degeneracy pressure generates no new energy radiates heat into space => get cooler and fainter until becomes Black Dwarf Chandrasekhar Mass Limit: if too much mass, electron degeneracy pressure cannot stop core collapse thus, all WD masses < 1.44 M sun 6

7 Ring Planetary Nebulae Eskimo Cat s Eye Hourglass White Dwarf core Twin Jet Helix 7

8 Fate of Us After 5 billion years, Sun runs out of H As red giant, 100 times larger, 1000 times more luminous Is that good for Earth? T E, Now = 300K T E, Then = 1000K Oceans boil away Mercury and Venus engulfed Sun shrinks again, then expands to come close to engulfing Earth, burnt & charred Planetary nebula floats past dead rock of Earth Where will we be? 8

9 Universe made Hydrogen and Helium All stars turn H(1) into He(2), and He into C(6) But High Mass stars also create Oxygen(8), Neon(10), Magnesium(12), Silicon(14), and Iron(26) in core fusion Periodic Table of the Elements 9

10 Deaths of Very Massive Stars When Supergiant ends He fusion in core: Gravity squeezes more => high temperature => new fusion H He C O Ne + Mg Si Fe Each step shorter than previous New fusion shell around core at each step Stops at Iron (Fe) as Fe fusion uses up energy => core collapses in only seconds! Neutrinos pour out of core removing even more energy. Core too massive to be WD (>1.44 M sun ) Collapses even more to Neutron Star or Black Hole. 10

11 Outer core falling inward recoils, creates outward shock wave => supergiant s outer layers blasted into space = Huge release of energy Type II Supernova explosion Brighter than 10 billion Suns for week Elements heavier than Fe instantly created & spewed out What happens to this gas?... Some of it made Us! SN 1987 neutrinos detected on Earth proved due to core collapse 11

12 Crab Nebula in Taurus supernova exploded in 1054 Supernova Remnants In last millennium, four supernovae observed in our part of Milky Way Galaxy: in 1006, 1054, 1572, & 1604 Veil Nebula Tycho s Supernova (X-rays) exploded in

13 Neutron Stars Very Massive Star s Final Battle: What happens to stellar core? Neutron Degeneracy Pressure: two neutrons cannot occupy same space with same momentum => Supports NS cores against gravity NS very dense : up to 3 M sun in diameter of 10 to 20 km Rotate very rapidly: spin times per sec Magnetic fields very strong No luminosity => how can we see them? The answer is pulsars Chandra X-ray image of the neutron star left behind by a supernova observed in A.D

14 Pulsars In 1967, graduate student Jocelyn Bell accidentally discovered radio source pulsing every 1.3 sec??? Solved when pulsar discovered in Crab Nebula supernova remnant only about 950 years old, 30 pulses/sec, emit IR, visible, X-ray, gamma ray light too Charged particles moving very fast around magnetic fields emit light Light focused into beam along magnetic poles (lighthouse model) 16

15 How do we know how stars die? Open Clusters: 100 s of stars, young, irregular shapes, gas Pleiades (8 x 10 7 yrs) Globular Clusters: 10 5 stars, old, spherical shape, NO gas Clusters for studying stellar evolution stars at same distance formed same time = same age M 80 (1.2 x yrs) 19

16 Position of hottest, brightest cluster star on MS = main sequence turnoff point => AGES of stars! Cluster as old as most luminous/massive star left on MS. All MS stars to left already used up their H fuel and are gone. 20

17 Which HRD shows the oldest cluster? 21