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Degenerate Stars

Dr. Bill Pezzaglia

Stellar Evolution Part III 1

Updated Feb 24, 2012

Degenerate Stars

A. Planetary Nebula

B. White Dwarfs

C. Supernova & Neutron Stars

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An Overview

3A. Death of Low Mass Stars

1. Evolution of low mass star

2. Planetary Nebula

3. Measuring Planetary Nebs

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1. Low Mass Star Evolution

“Lightweight Stars” (0.08 < Mass < 4) like sun, probably main sequence types F, G, K, M

• Main sequence star for billions of years, build up inert helium core.

• Red Giant branch (shell burning of Hydrogen)

• Horizontal branch (triple alpha fusion)

• AGB star (shell burning of helium around inert carbon core)

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2. Thermal Pulses 8

The helium rains down onto the dormant helium shell, with increased mass it contracts, eventually it reaches critical temperature and we get a helium shell flash, creating the thermal pulse

•Late AGB stars have thermal pulses every 100,000 years, sheds matter (perhaps 10% of mass each time!)•Theory: Helium shell burning runs out, helium shell contracts, heats up outer hydrogen shell, causes increased hydrogen shell burning (creating more helium)

2c). Planetary Nebula 9

•Thermal pulses can blow off outer star, leaving the carbon-oxygen core

•M57 Ring Nebula (Lyra)

•Ejected material travels at 20 to 30 km/sec, and hence will disperse after about 50,000 years.

The Summer TriangleDeneb

CygnusThe Swan Lyra

The Harp

Vega

Altair AquilaThe Eagle

M27: July 12, 1764Dumbbell NebulaIn Vulpecula6’ in diameter

M57 (1779)Ring NebulaIn Lyra1’ in diameterMade of stars?

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PerseusThe Hero

Algol

Pleiades

CassiopeiaQueen of EthiopiaSchedar

M76 Barbell NebulaPerseus (1780)(Not recognized asPlanetary Nebulauntil 1918)

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Ursa MajorThe Big Bear

Alcor & Mizar

Ursa MinorThe Small Bear

Polaris

M97 Owl NebulaDiscovered 17816000 years old?

11B. 1786 Herschel discovers:

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Ursa Major

Ursa Minor

Polaris

Draco the Dragon

H IV 37 (NGC6543)Cats Eye Nebula(made of stars?)

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13Filtered for Hydrogen, Nitrogen & Oxygen

Filtered for Sulfur, added with H & O

NGC 6543 Cats Eye Nebula in Dracofrom http://heritage.stsci.edu/2004/27/supplemental.html)

GeminiThe Twins

Pollux

Castor

NGC2392 (1787)Eskimo Nebula(Herschel)

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3a. Age of Planetary Nebula 15

• Compare measurements of size of nebula over several centuries.

• In 100 years, M27 has expanded 2”

• Today it is 5’=300” in size

• How old is it?

• (300”)(100 yr/2”)= 15,000 years

3b. Size of Dumbbell

• (1970) Dumbbell has Doppler shift of 0.01%

• (0.0001)c=(0.0001)(300000km/s)=30 km/sec

• In 1 year expands:

• In 15,000 years has expanded to size:

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yearday

dayhr

hrkm 36524360030 secsec Billion km

yearkmbillionyr 1000,15 15 trillion km

(1.5 light year)

3c. Distance to Dumbbell

• Recall the “parallax triangle”

• Hence if the size is 100,000 AU and makes an angle of 300”, how far away is it?

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"300

000,100 AU

angle

sizeD 333 parsecs

Early Nebular Model 18

• Herschel (1786) thought that “Planetary Nebulae” might be a cloud of gas coalescing into a star to be surrounded by planets (hence the name)

• “Nebular Hypothesis” had been theoretically proposed earlier (1734 Swedberg, 1755 Kant)

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B. Central Star: White Dwarf! 19

• 1800 Friedrich von Hahndiscovers central star of Ring Nebula (very faint)

• Central Stars have Hydrogen, Helium and sometimes Carbon & Oxygen lines.

• 1918 Wright identifies as type “O” [VERY HOT 125,000 C], hence must be very very small to be so faint (magnitude m=15.3)! Not a normal star forming!

B.2 William & Margaret Huggins 20

• (1864) Spectra of Cat’s Eye Nebula shows single emission line in green

• Implies it’s a gas, NOT made of stars (which would have absorption lines)

• Unknown element “Nebulium” makes greenish color

• (1926) Bowen shows Nebulium is really Oxygen & Nitrogen under extraordinary conditions

• So its NOT a hydrogen gas cloud condensing into a “normal” star

White Dwarfs 21

• Doppler Velocity: shows the cloud of gas is EXPANDING, not contracting [who did this first, the Huggins?]

• (1956) Iosif Shklovsky proposes that (red giant) stars explode and form a planetary nebula + white dwarf star

• White dwarfs are then the end of the evolution of a star, the contracted core after star explodes.

• To measure properties, need to find one in a binary star system

Sirius B 22

• 1844 Bessel detects “wobble” in Sirius (A) with period of 50 years.

• Unable to detect companion Sirius B (aka “pup”)

• 1862 Alvan Clark resolves the double with 18 inch refractor.

• Orbital properties tell us Sirius B has mass of the sun (Sirius A is 2.3x bigger)

3c). Sirius B• Temperature can be measured

from Wien’s law (peak of black body curve): 24000 K

• Size of star determined from Stefan-Boltzmann law, get very small size (size of earth)

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• Density is hence VERY BIG. A handful of this electron degenerate matter would weigh several tons on the earth!

B. White Dwarf Evolution 24

• Carbon-Oxygen core gravitationally contracts

• It will heat up as it contracts (to type O !)

• Shrinks until Quantum Mechanics prevents further collapse (electron degeneracy)

• Small size, means very faint. Ends up in lower left corner of HR diagram

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3b). Chandraselckar Limit• The Pauli Exclusion

Principle of Quantum Mechanics says that electrons cannot share the same state

• So there is a limit how close they can be packed, giving an outward “electron degeneracy pressure”

• More mass, more gravity, causes size to be smaller.

• Chandraselcakar Limit (1931): at 1.44 Solar masses, star size becomes ZERO (evolves instead to a neutron star).

25 4a). Nova

• Phenomena: star brightens by 5 to 15 magnitudes in 1 or 2 days

• M=-7.5 (100,000 Solar Luminosities)

• Declines over a month

• Gas Ejected

• Might repeat! (so we can’t blame it on a star blowing up!)

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4b). Nova Theory• Low Mass White dwarf in binary system with giant which is

shedding matter• Mass falling onto white dwarf is heater by change in gravitational

energy• Eventually ignites hydrogen, get short term fusion on surface of

star, matter is ejected out into space.

27 C. Supernova and N stars

1. Type Ia Supernova

2. Type Ib, Ic, II Supernova

3. Neutron stars

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2a). Type Ia Supernova 29

• Phenomena: explosion, does not repeat, declines over months

• M=-19 (4 billion Solar Luminosities)

• No Hydrogen or Helium LinesHas Silicon Lines (no neutrinos?)

2b). Type Ia Supernova Theory 30

• Theory: High mass white dwarf (close to Chandraselckar limit) in binary system is accumulating mass fast from giant companion.

• Carbon (plus oxygen) core reaches critical temperature, carbon flash occurs

• Fusion: C + O = Silicon• No negative feedback due to degenerate

matter, runaway reaction explodes star.

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3a). Type Ibc & II Supernova 31

These have to do with a massive star (a supergiant) exploding

• Type II: Has Hydrogen (and other) lines, brightest (M=-17.5)

• Type Ib: no H lines (but has He), implies star lost outer envelope before exploding

• Type Ic: No Hydrogen or Helium lines, implies lost outer hydrogen envelope, and helium shell before exploding.

3b). Type Ibc & II Theory 32

1. Iron core is building up in supergiant star.

2. Eventually reactions stop, core contracts and heats.

3. Eventually begins to fuse Iron4. Iron is the most

stable nuclei, so Iron fusion LOSES energy, cools core

5. Causes runaway implosion (in ¼ second)

6. Superheats core

3c). Type Ibc & II Theory 33

7. Photodisintegration: Superheated core emits high energy photons, which break up nuclei into protons and neutrons (loses even more energy)

8. Electrons+protons pushed together to make Neutrons+Neutrinos

9. Neutrinos escape, drain energy out of core, causes core to implode faster

3d). Type Ibc & II Theory 34

10. Core collapses onto degenerate neutron core, which is “stiff”, causes rebound (again, Pauli Exclusion Principle, neutrons can’t be squashed)

11. Causes core to rebound, send shockwave through outer star

12. The shockwave and neutrino burst blow up star in 10 seconds!

C. Supernova Remnants

1. SNR Nebulae

2. Neutron Stars (Pulsars)

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1a). Mass Loss BeforeType Ib & Ic supernova tell us that

supergiant stars sometimes shed their outer envelope before exploding.

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Eta Carinae is perhaps the most massive star known (100 solar masses). This material was shed in 1843, moving away at 1000 km/secProbably it will go supernova soon.

1b). Mass Loss Before

NGC2359 (canis major): This 40 solar mass supergiant star is shedding matter from its solar wind.

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1c). Crab Supernova 1054 AD 39

Was visible in daylight!

Today there remains the Crab Nebula M1

The blue is synchrotron radiation

Note the “filaments”

1d). Tycho’s Supernova 1572 40

This is a recent picture of the SNR from that supernova in Cassiopeia. Chandra's image of the supernova remnant shows an expanding bubble of multimillion degree debris (green and red) inside a more rapidly moving shell of extremely high energy electrons (filamentary blue).

1e). Kepler’s Supernova 1604 41

This is a recent picture of the SNR from that supernova in Ophiuchus.

1f). Supernova 1987A 42

The first one that has gone off close by since we’ve had telescopes! And its not that close, its in the Large Magellenic Cloud

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1g). The Vela Nebula

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This is a very old (12000 years) SNR. Note the “filaments”.

1h). The Veil Nebula, Cygnus 44

This is a very old (15000 years) SNR. Note the “filaments”.

1i). SNR LMC N 63A 45

(in Large Magellanic Cloud in Dorado)

2). Neutron Stars

a) Pulsar Phenomena

b) Neutron Star Model

c) X-Ray Bursters (Black Widow Pulsars)

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a1). Pulsar Phenomena

• Discovered 1967

• Regular pulsation in Radio (UV, X-Ray)

• Short Periods: 0.001 sec to 10 sec

47 a2). Pulsars found in SNR

1968 found pulsar in Crab Nebula and Vela Nebula

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a3). Binary Pulsars• Centaurus X-3: an X-ray Pulsar (captured) in

an eclipsing binary star system. Tells us: • Mass is 2 (greater than Chandraselcakar limit)• Eclipse tells size smaller than white dwarf

49 b1). Neutron Star• Imploded core of supergiant

compressedelectrons+protons into neutrons.

• Shrinks untilneutron degeneracypreventsfurther collapse

• Small Size:1000 to 10,000 meters

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b2). Lighthouse Model• Current of superconducting protons

makes a BIG electromagnetic.

• Electronsspiralingaroundmagneticfield linesmakes abeam ofsynchrotronradiation

51 b3). Black Widow Pulsars

• Matter falling onto neutron star from binary companion will make it spin 1000x faster

• Beam will be more energeticx-rays

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c1). X-Ray Bursters

• Similar idea to novae and white dwarfs

• Matter falls on neutron star, causing a 1 meter shell of fusing hydrogen

• Helium builds up under layer

• Eventually Helium ignites,causing BIG BURST ofenergy in X-rays

• Phenomena repeatsevery few hours (or days).

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