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Astro 1050 Wed. Apr. 10, 2017 Today: Continue Ch. 16: Star Stuff

Reading in Bennett: For Monday: Finish Chapter 13 – Star Stuff

Reminders: Ch. 17, 18 HW now on Mastering Astronomy, due Monday.

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Chapter 17 Cont. – The Deaths of Stars

•  What happens when the hydrogen runs out?

–  With no internal heat source the center cools –  Star once again will contract and interior temperature

increases.

•  What is the result? –  “Unusual” fusion energy sources ⇒ giant stars

•  Hydrogen shell fusion •  Heavy element fusion

–  Degenerate electron pressure ⇒ white dwarfs –  Loss of outer gas envelope from the star

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Hydrogen Shell Burning •  Fusion stops in core when hydrogen runs out

–  Star has core of He, but T too low for fusion there •  Heat loss makes star contract, T goes up in interior •  Before T in core reaches He ignition point -- •  Hydrogen above He core begins rapid “shell

burning”

•  Shell burning changes the rules for structure –  Still need dense core to provide high T, P for fusion –  But high T on outside of core puts too much energy

into outer parts of star (more mass in dense core) –  Outer parts responds to hotter intermediate layer by

expanding and cooling •  Shell burning layer separates star into two parts

–  Hot dense core –  Extended envelope which shields and insulates H

shell-burning layer

From our text: Horizons, by Seeds

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Expansion into a Red Giant •  For 5 MSun “red giant”

–  Outer part swells to 75 RSun –  Inner core still contains most of

mass

•  Why is it red? Two equivalent ways to answer:

–  Thick envelope insulates outside from hot core

–  L ∝ R2 T4 and •  L is not much greater (so far) •  R2 is ≈(75/3)2 =252 = 625 times

larger •  Outer T must decrease to

compensate

From our text: Horizons, by Seeds

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HR Evolution for other Mass Stars

•  For higher mass (already luminous)

stars –  Evolution is more horizontal (to

red)

–  Hard to increase luminosity above already very high levels

From our text: Horizons, by Seeds

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Evolution in the HR Diagram •  During H core burning

(main sequence) –  L increases slowly as He

accumulates in the core (and TCore increases)

•  During H shell burning –  At first R increases, T

decreases –  L then increases slowly as

more He accumulates in core (and TCore increases)

–  Eventually He ignites in the core

From our text: Horizons, by Seeds

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Evolution in the HR Diagram •  During He core, H shell

burning –  Moving some of E generation

back into core shrinks size (and so raises TSurface)

–  Eventually He in core runs out leaving a C, O core inside the He region

–  Core contracts and heats up till He ignites in shell outside C, O core

•  During He shell, H shell burning –  At first R increases, TSurface

decreases –  L increases as more C,O

accumulate and TCore continues to increase

From our text: Horizons, by Seeds

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Expected Evolution of the HR Diagram for a Cluster

•  A cluster consists of stars of different mass which all

started to form at the same time (collapsing from the same fragmenting molecular cloud)

•  Higher mass stars contract to main sequence before low mass ones reach it

•  Higher mass stars are also the first to run out of H and leave the main sequence, becoming supergiants.

•  As time continues, lower and lower mass stars “leave” the main sequence (at the evolving “turn off point”) The original supergiants don’t live very long. The lower mass stars produce giants

From our text: Horizons, by Seeds

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Tests of Stellar Evolution using the HR Diagram

•  Stellar evolution too slow to see any changes in given cluster •  Can observe clusters and look for predicted patterns

From our text: Horizons, by Seeds

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Complications in Stellar Evolution •  Pressure forces other than thermal gas

pressure (radiation contributes) – Reminder: We’ve been assuming that when star

loses energy it contracts and actually heats up. Clearly not all objects do this (eg. Earth)

•  Convection bringing in fuel from outer regions

•  Mass loss from stellar wind, or mass gain from nearby star

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Pauli Exclusion Principle •  From quantum rules, electrons don’t like to be packed into a

small space, either in atoms or in ionized gas (causes shell structure)

•  At normal ionized gas densities, electrons are so spread out quantum rules don’t matter.

•  At very high gas densities, quantum rules need to be considered, just has they have been in atoms

•  Think of each “atom sized” region of space having a set of energy levels associated with it (although it is really more complicated)

• An energy level can only hold one electron (2 if we consider opposite electron “spins”)

– In multi-electron atoms, you can have at most two electrons in each energy level.

– In gas, to pack more electrons in same volume, new ones must be placed in higher energy levels (i.e. be going faster) than existing ones

From our text: Horizons, by Seeds

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Effect of Degenerate Electron Pressure •  Electrons begin to strongly interact with each other •  Loss of energy does not reduce pressure •  Star does not contract in response to loss of energy •  Gravity not available as energy source to heat up star

•  Electrons are already in lowest energy states allowed (equivalent to atoms in ground state) so no energy available there

•  If there is no other energy source, as energy is lost nuclei move slower and temperature drops.

From our text: Horizons, by Seeds

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Degenerate Pressure Can End Fusion •  Degenerate Electron Pressure can prevent contraction and

stabilize core temperature –  “Stars” with M < 0.08 MSun never burn H (brown dwarfs) –  Stars with M < 0.4 MSun never burn He (red dwarfs) –  Stars with M < 4 MSun never burn C (but do make red giants) –  Stars with M > 4 MSun do burn elements all the way to Fe

•  What happens to these objects? –  Brown dwarfs never become bright – sort of giant version of Jupiter –  Red dwarfs have such long lives none have yet exhausted H –  Red giants are related to white dwarfs –  Massive stars explode as supernova

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Effects of Convection

•  Energy can be transported by radiation or convection •  Convection in core brings in new fuel

–  Cooler material more opaque making radiation harder and convection more likely

–  Which dominates also depends on amount of energy from core

From our text: Horizons, by Seeds

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Mass Loss from Giant Stars •  Envelope of red giant very loosely held

–  Star is so big, gravity very weak at the surface •  Degenerate core makes nuclear “thermostat” sluggish

–  Core doesn’t quickly expand and cool when fusion is too fast and vice versa

–  Energy can be generated in “thermal pulses” •  Low temperature and more opaque envelope can also “oscillate”

–  Energy is transmitted in “pulses” as envelope expands and contracts –  Main cause of many “Variable Stars”

•  Some Red Giants have very strong stellar winds ejecting envelopes, forming “Planetary Nebulae” •  PN called this because some looked like planets in early

telescopes •  PN don’t have anything to do with formation of planets

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White Dwarfs •  The now exposed degenerate core is a “White

Dwarf”

•  For 1 MSun: R~REarth ρ ~ 3×106 gm/cm3

•  Faint because it is so small, despite high T

•  Can’t contract because P doesn’t drop with T (degenerate gas)

•  Slowly cools and fades

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Simple Planetary Nebula

•  IC 3568 from the Hubble Space Telescope

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Complicated P-N in a Binary System

•  M2-9 (from the Hubble Space Telescope)

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A Gallery of P-N from Hubble

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Complications in Binary Systems Mass can be transferred between stars •  1st (massive) star becomes red giant •  Its envelope expands and is transferred

to other star •  Hot (white dwarf) core exposed •  2nd star becomes red giant •  Its envelope transferred to white dwarf

–  Accretion disk around white dwarf •  Angular momentum doesn’t allow material to

fall directly to white dwarf surface –  Recurrent nova explosions

•  White dwarf hot enough for fusion, but no Hydrogen fuel

•  New fuel comes in from companion •  Occasionally ignites explosively,

blowing off partially burnt fuel

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Is a star stable against catastrophic collapse?

•  Imagine compressing a star slightly (without removing energy) –  Pressure goes up (trying to make star expand) –  Gravity also goes up (trying to make star collapse)

•  Does pressure go up faster than gravity? –  If Yes: star is stable – it bounces back to original size –  If No: star is unstable – gravity makes it collapses

•  Ordinary gas: P does go up fast – stable •  Non-relativistic degenerate gas: P does go up fast – stable •  Relativistic degenerate gas: P does not go up fast – unstable

–  Relativistic: Mean are the electrons moving at close to the speed of light

–  Non-relativistic degenerate gas: increasing ρ means not only more electrons, but faster electrons, which raises pressure a lot.

–  Relativistic degenerate gas: increasing ρ can’t increase electron velocity (they are already going close to speed of light) so pressure doesn’t go up as much

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Chandrasekhar Limit for White Dwarfs Add mass to an existing white dwarf •  Pressure (P) must increase to balance stronger

gravity

•  For degenerate matter, P depends only on density (ρ), not temperature, so must have higher density

•  P vs. ρ rule such that higher mass star must actually have smaller radius to provide enough P

•  As Mstar → 1.4 MSun velectron → c –  Requires much higher ρ to provide high

enough P, so star must be much smaller. –  Strong gravity which goes with higher ρ makes

this a losing game.

–  For M ≥ 1.4 MSun no increase in ρ can provide enough increase in P – star collapses

From our text: Horizons, by Seeds

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Implications for Stars •  Cores less massive than 1.4 MSun can end as white dwarfs

•  Cores more massive than 1.4 MSun can end as white dwarfs, if they lose enough of their mass (during PN stage) that they end up with less than 1.4 MSun

•  Stars whose degenerate cores grow more massive than 1.4 MSun will undergo a catastrophic core collapse:

• Neutron stars • Supernova

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Supernova •  When the degenerate core of a star exceeds 1.4 MSun it

collapses –  Type II: Massive star runs out of fuel after converting core to Fe (Iron

core cannot fuse, it photo-dissociates first actually absorbing energy!) –  Type I: White dwarf in binary, which receives mass from its

companion (collapses when M > 1.4 MSun ignites carbon burning).

•  Chronology of events: –  Star’s core begins to collapse –  Huge amounts of gravitational energy liberated –  Extreme densities allows weak force to convert matter to neutrons

p+ + e- → n + ν –  Neutrinos (ν) escape, carrying away much of energy, aiding collapse –  Collapsing outer part is heated, “bounces” off core, is ejected into space

•  Light emitted from very hot ejected matter makes supernova very bright •  Ejected matter contains heavy elements from fusion and neutron capture

–  Core collapses into either: •  Neutron stars or Black Holes (Chapter 11)

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Supernova in Another Galaxy

•  Supernova 1994D in NGC 4526

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Tycho’s Supernova of 1572

•  Now seen by the Chandra X-ray Observatory as an expanding cloud.

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The Crab Nebula – Supernova from 1050 AD

•  Can see expansion between 1973 and 2001 –  Kitt Peak National Observatory Images

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What happens to the collapsing core?

•  Neutron star (more in next chapter) –  Quantum rules also resist neutron “over-packing”

•  Densities much higher than white dwarfs allowed –  R ~ 5 km ρ ~ 1014 gm/cm3 (similar to nucleus)

•  M limit uncertain, ~2 or ~3 MSun before it collapses

–  Spins very fast (conservation of angular momentum)

–  Trapped spinning magnetic field makes it: •  Act like a “lighthouse” beaming out E-M radiation (radio,

light) •  Accelerates nearby charged particles Pulsar!

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Spinning pulsar powers

the Crab nebula

•  Red: Hα

•  Blue: “Synchrotron” emission from high speed electrons trapped in magnetic field