White Dwarfs, Novae, andType 1a Supernovae:

The Vampire Stars

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White Dwarfs• ~Twice the size of the earth.• Typical surface temperatures of

20,000 to 100,000 Kelvin.• 200,000 times the density of the

earth.• Made of C and O, possibly crystalline.

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Earth

White Dwarf

Relative size of sun’s core;contains ½ the mass of the sun.

Credit: SOHO, NOAANews, NASA

• Contains about ½ the mass of the original star: from 0.4 to 1.4 solar masses.

• This upper limit (1.4 solar masses) is the Chandrasekhar Limit.(If the core of the star is heavier than 1.4 solar masses, it will turn into a neutron star instead of a white dwarf.)

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• Nuclear fusion has completely shut down – the star shines only from residual heat.

• If there’s no nuclear fusion to provide outward pressure…why doesn’t the white dwarf instead collapse further into a neutron star or black hole?

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• At the enormous density of a white dwarf, the empty space between atoms is squeezed out.

• Electrons of atoms repel electrons of other atoms, providing an outward pressure which the star’s gravity isn’t strong enough to overcome.

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• Degenerate electron pressure:Atoms are so close together that their atomic orbitals merge. Electrons simply flow around all the atoms (electrons are degenerate.)

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Sometimes white dwarfs explode!

• Since at least half of the stars occur in binary systems, we ought to find many white dwarfs in binary systems with other stars.

• If a white dwarf is in a close binary system with a red giant or supergiant, its gravity will pull hydrogen gas from the larger star.

The gas from the red giant spirals into thewhite dwarf – forming an accretion disk.

Ready to go BOOM!

• When enough hydrogen from the red giant accumulates on the surface of the white dwarf, the high temperatures cause the H to fuse into He.

• The star briefly flares hundreds of times brighter than normal.

A nova in Hercules. www.if.ufrgs.br/oei/stars/ novas/

novas.htm

• Although the explosion is violent, most of the energy and mass comes from the “stolen” hydrogen gas. The star itself isn’t destroyed.

• The star can go through the process dozens, even hundreds, of times.

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Nova light curvehttp://zebu.uoregon.edu/~js/ast122/images/nova_light_curve.gif

http://www.seed.slb.com/en/watch/cosmos/images/nova.jpg

Sometimes it goes too far…

• A Nova happens when the white dwarf is far below the Chandrasekhar Limit of 1.4 Msun.

• What happens if the white dwarf is just below or right at the Chandrasekhar limit?

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Type 1 Supernova

• As hydrogen from the red giant piles onto the surface of the white dwarf, it may cause the total mass of the white dwarf to surpass the Chandrasekhar Limit.

• The electron repulsion can no longer support the star, so the entire star collapses and explodes.

• Since a Type 1 Supernova always occurs at the same mass, (1.4 solar masses), these supernovas always explode with the same brightness.

• This makes them perfect “standard candles” or distance markers.

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http://antwrp.gsfc.nasa.gov/apod/ap061224.html

Supernova 1994, a Type 1 supernovain a distant galaxy

SN2006X in Galaxy M100 (near Coma Berenices)Credit: FORS Team, 8.2-meter VLT, ESO

Before & After SN 2005cs in M51 (Whirlpool Galaxy) in constellation Canes Venatici.

Credit: http://www.nasaimages.org, R. Jay GaBany

Type II or “Core Collapse” Supernovas

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Large stars begin like any other star

• Stars that eventually become Type II supernovae begin with 3 to >100 times the mass of the sun.

• A nebula collapses to become a protostar; nuclear fusion ignites - becomes a main sequence star (where it lives most of its life); begins to use up its fuel.

• As the H fuel in the core is used up, the core shrinks and heats up, while the outer layers swell and cool – the star becomes a red giant.

• As the core shrinks & heats it begins fusing HeHe (the ash from the previous reaction) into CC and OO.

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• The layer right next to the core begins fusing H into He – so now 2 fusion reactions are occurring.

• A small star would stop here, the core not being hot enough to do anything with the C and O.

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He fusing intoC and O.(Triple-alphaProcess)

H fusing into He.(Proton-Proton Chain)

No fusion in outer layers.

• A large star’s core can collapse and heat further. The core begins to fuse CC and OO into NeonNeon (Ne), MagnesiumMagnesium (Mg), and SiliconSilicon (Si).

• The next layer out fuses He into C and O. The layer outside that fuses H into He. The star starts to resemble the layers of an onion.

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Onionlayers

• The production of Ne, Mg, and Si continues for only a few hundred years.

• Eventually, when all the C and O is used up, the core shrinks once more, heats to over 600,000,000 K and starts fusing MgMg + SiSi into ironiron (Fe).

• Energy must be added to fuse iron into a heavier element, so fusion stops with iron.

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• The production of iron from Mg & Si happens very quickly, less than 1 day.

• When the iron core gets massive enough it implodes! (The needed mass is 1.4 x the mass of the sun1.4 x the mass of the sun – the Chandrasekhar limitChandrasekhar limit!)

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Inverse Beta Decay

• The gravity is so great in the core, that protons & electrons get squashed together into neutrons:

p+ + e- no

(inverse beta decay.)

• The core becomes a neutron star.

Neutron Stars

• Neutron stars are formed from stars that were originally 3 – 8 times the mass of the sun.

• They’re held up against gravity simply by the neutrons being jammed in tightly next to each other. This is called “neutron degeneracy”.

Black Holes

• What happens if the star was originally more than 8 solar masses?

• Even the pressure of the neutrons is overcome, and the neutron star collapses into a black hole.

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What about the rest of the layers?

• When the iron core collapses into a neutron star or black hole (at nearly the speed of light), the outer layers follow it in.

• The outer layers “bounce” or rebound off the immensely hot new neutron star and a gigantic explosiongigantic explosion occurs!

• The rebound is helped out by a blast of gamma rays & neutrinos.

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Credit: aether.lbl.gov/www/tour/elements/stellar/rebound.gif

How often do SN happen?

• On average, about every 100 years for any given galaxy.

• Our own galaxy has had several during recorded history:

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“Recent” Supernovas

• July 4th, 1054, in Taurus, 6500 light years away. This resulted in the Crab Nebula. It was recorded by Anasazi Indians in the American southwest.

The Crab Nebula in Taurus

• In 1572 Tycho Brahe saw a supernova in Cassiopeia – (16,000 light years away)

• The Chinese also saw and recorded the appearance of a “guest” star.

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From Astronomie Populaire, byCamille Flammarion, 1884.

TheremnantofCassio-peia A.

This isthebrightestradio-emittingobject inthe sky.

Credit: NASA/GSFC/U.Hwang et al.

Ancient SN’s

• When a supernova explodes, it leaves behind a cloud or supernova remnant.

• These remnants last for hundreds of thousands of years. New (small) stars can be formed from their gases and dust.

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This is howthe wholeCygnus LoopSN remnantlooks – remarkablylike the cloudfrom anordinaryexplosion.

Future Supernovas

• At present, astronomers are waiting for another star to go supernova in our galaxy: Eta CarinaeEta Carinae.

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Credit: N. Smith, J. A. Morse (U. Colorado) et al., NASA

A Very Special Supernova

• The only close supernova that astronomers have been able to study in detail is SN 1987ASN 1987A in the Large Magellanic Cloud, 170,000 light years away.

• The SN happened in the Tarantula Nebula.

March 23, 1987

The Large Magellanic Cloud

The Tarantula Nebula

SN 1987A

• This supernova was different than many – when it exploded, it was a blueblue supergiant.

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After the Explosion

• The brightness of SN 1987A has been monitored for the past 22 years. It didn’t follow the usual pattern (suddenly bright, with a quick fall-off).

• Rather, it got suddenly bright, grew brighter, then faded off gradually.

• A couple of years after the supernova faded, it suddenly brightened again.

• It wasn’t the supernova itself, but its light reflecting off a cloud of dust behind the SN. This reflected light is a “light echo”.

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http://apod.nasa.gov/apod/ap060125.html

Expanding light echos.

Credit: ESO (European Southern Obs.)

• This supernova has been observed extensively. Over the years, we’ve seen shock waves from the explosion slam into the clouds of gas that the star gave off just before it exploded.

• The shock waves heat the gas, producing rings.

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• The shock waves heat the shells of gas hot enough to give off X-rays.

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Credit: NASA/CXC/SAO/PSU/D.Burrows et al.

Nucleosynthesis

• Supernovas can make elements up to the mass of Fe (atomic number 26) before they explode. However, there are 80+ elements that are heavier than iron.

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• During the explosion, there are a lot of very fast, high energy neutrons flying around. Sometimes, one of these neutrons hits an iron nucleus:

56562626Fe + nFe + noo 5757

2626FeFe

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• The extra neutron inside the heavy iron nucleus can split into a proton and an electron:

57572626FeFe 5757

2727CoCo

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• This process of adding a neutron, then the neutron splitting into a proton and electron can happen over and over, producing elements heavier than iron.

57572727Co + nCo + noo 5858

2828Ni etc.Ni etc.

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• The next time you look at your ‘significant other’ – remember they truly are made of stars.

• So was your lunch today….that’s what that funny taste was!

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