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Astronomy
A. Dayle Hancock
Office hours: MTWR 10-11am
http:// physics.wm.edu/~hancock/171/
Death of StarsNuclear reactions in small starsHow stars disperse carbonHow low mass stars dieThe nature of white dwarfsReactions in high mass starsHigh mass stars end in a supernovaSN 1987ANeutrinos in the death of high mass starsWhite dwarfs in close binary systemsRemains of a supernovaNeutron stars and pulsarsNovae and x-ray burster come from binary systems.
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Stars with 0.4-4.0 Mⵙ go through two Red Giant Stages
Stars with 0.4-4.0 Mⵙ eventually become red giants with a helium flash and core helium burning. In 108 years the helium is exhausted. The core contracts until it is stopped by degenerate electron pressure. The contraction heats the surrounding thin shell and shell helium fusion starts. The star enters a second red giant phase. 2
The shell helium fusion expands the outer layers. The star becomes a red giant for a second time with even greater luminosity than before. Stars in this second red giant phase are known as AGB (asymptotic giant branch) stars. An AGB star has a core of non-reacting C and O, and a helium and hydrogen fusion shell. The core is about the size of the earth while the AGB star is as large as the Earth's orbit. 3
The Second Red Giant Stage
AGB Stars
The greater the mass of the star, the faster it goes through its post main sequence phases. A 1 Mⵙ AGB star can have a luminosity of 104 Lⵙ compared to 103 Lⵙ for first helium flash red giant phase. This H-R diagram of an older cluster show the evolution of stars through the red giant phases.
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Dredge-ups and Carbon Stars
Convection in later stages of stellar evolution causes material (C, N and O) from the core to 'dredge up' (1st dredge up). Later as the helium core fusion nears its end, more carbon, nitrogen, and oxygen is 'dredged-up' for a 2nd time. Later in an 2 Mⵙ AGB star
large amounts of fresh carbon (3rd dredge up) are carried to the surface. Because of the carbon ( in the form C
2, CH and CN) in their spectrum these stars are
called carbon stars. AGB stars have strong solar winds that cause mass lose rates up to 10-4 Mⵙ per year. Because carbon is only formed by the triple α process, These stars are the source of carbon in the universe and for life.
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0.4 – 4.0 Mⵙ Stars Eject their Outer Layers
Because the mass loss rate (10-4 Mⵙ per year ) is so high, AGB stars in the later stages eject into space much of their mass leaving a hot exposed core. Models show the helium shell fusion flashes produce thermal pulses which eject the outer layers of the star into space. The ejected material forms a planetary nebula around the star. The nebula are illuminated by the dying star.
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Planetary Nebula
Planetary nebulae come in many shapes. The almost perfectly spherically nebula is about 1.5 pc across. Doppler measurement show the nebula are expanding at the rate of 10-30 km/s. The spectra of nebulae show H, O and N. After about 5 x104 years the nebula fades away as it mixes the the interstellar medium.
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White Dwarf Star
The burnt out core becomes a white dwarf. The core is not large enough to ignite any further nuclear reactions. The white dwarf does not collapse further because of electron degeneracy pressure. The density of the material in a white dwarf is very high 109 kg/m3
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The more massive a white dwarf star the smaller the radius! There is a limit to how much pressure degenerate electrons can produce. The limit is a star of 1.4 Mⵙ and is known as the Chandrasekhar limit. When a star exceeds this limit, the star will explode. As the white dwarf ages
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Mass Radius Relation for White Dwarfs
(after ~ 5 x 109 years), it cools down (400 K) and the luminosity drops to 10-4 Lⵙ. The ions become locked in a crystal lattice and the star becomes solid.
Evolution of a Red Giant to a White Dwarf
This H-R diagram show the evolution of 3 different mass red dwarfs into white dwarfs. The table shows the mass of the red giant, the mass ejected into the nebula and the final white dwarf mass.
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High Mass Stars
A high mass (> 4Mⵙ) star goes through the same initial stages of fusion as a lower mass star. High mass stars go through several other stages of fusion and end their lives in a very different fashion. For stars of mass ~4 Mⵙ, the pressure in the
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core is high enough to fuse carbon into O, Ne, Na and Mg. If the star has a mass of ~8 Mⵙ, the star will contract after the end of carbon fusion (t = 109 K) and neon will fusion will begin. This increases the amount of O and Mg in the star's core.
At the end of neon fusion, the core contracts and oxygen fusion begins. This produces primarily silicon (Si). After oxygen fusion ends and another contraction, silicon fusion produces nuclei
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from sulfur up to iron (Fe). The fusion process ends at iron. While all the chain of fusion reactions are going on in the interior of the star, the star is shedding material from its outer layers. Some of the fusion reactions release neutrons which creates different elements (via β decay) filling out the elements lighter than Fe.
High Mass Stars
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Iron (Fe) is the end of the road!
You can't get energy out by fusing iron (trying to squish more nucleons onto iron requires energy instead of releasing it)
The Violent End of a Large Star
A star < 8Mⵙ will shed most if its mass into a planetary nebula and eventually become a white dwarf. A star > 8Mⵙ will end in a a very different manner. As it ages, a large star will get hotter and contract as it burns through successive stages of nuclear fusion. As more and more iron accumulates in the inner most core, electron degeneracy pressure will briefly keep the non-fusioning iron core from collapsing. When enough iron has collected in the iron core (> 1.4Mⵙ the Chandrasekhar limit) gravity will overcome the electron degeneracy pressure and the core will suddenly collapse. The core during its collapse can reach speed of 0.2 c. The core temperature can reach 1010 K.
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Fusion in a 25 Mⵙ Star
This table shows the various fusion processes with their associated temperatures, densities and durations.Note the durations decrease rapidly until a violent end of the high mass star (supernova).
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The Violent End of a Large Star
The high temperature generates γ rays which break up iron nuclei into lighter nuclei in a process called photodisintegration. This blast apart the core.
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The Violent End of a Large Star
In another fraction of a second, the electrons are forced into the protons:
e- + p+ → n0 + ν
(reverse β decay). The neutrinos (ν) carry away significant energy. The core is about 20 km in diameter and has a density of 4 x 1017 kg/m3 at this stage.
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The neutron core is very solid and held together by the strong nuclear force. The in-falling matter forms a shock wave and the material bounces back sending a shock wave outward (core bounce). This provides some of the force to expel the outer layers in the explosion but not all. The huge number of high energy neutrinos are absorbed and heat the surrounding material. This produces hot inflating bubbles which produce shock waves which eject material completely away from the star. The resultIs a supernova explosion. A supernova is a gravity powered neutrino explosion. A 25 Mⵙ star can eject 96% of its mass into space which can eventually forms new stars.
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The Violent End of a Large Star
1987A Supernova 1987 a star in the Large Magellanic Cloud (a nearby galaxy) exploded as a supernova. It was an unusual supernova. Its luminosity was about 108 Lⵙ but only about 0.1 the luminosity of a typical supernova. The progenitor star was a blue supergiant (a 20Mⵙ ). The star was a population II star (low 'metal') star which made the star much smaller in diameter than a normal supernova. The outer layers were held more closely. This resulted in more energy being used to expel the surrounding material and less energy for light production.
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Several years after the initial supernova explosion, the Hubble telescope shows 'rings' around the remains of the explosion. The outer rings were material ionized by the initial UV flash. The inner ring is the debris colliding with the 'waist' of the hourglass.
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1987A Supernova
Two large neutrino detectors (IMB and Kamiokande) were in operation when the 1987A supernova happened. The detectors observed 11 (Kamiokande) and 8 (IMB) neutrons.The supernova was at a distance of 168,000 ly. Using the inverse square law, it was deducted that the supernova emitted 1058 neutrinos with an energy of 1046 joules in 10 seconds. The neutrinos arrived 3 hours before the light was observed. (No they don't travel faster than light!). The neutrinos moved passed through the outer layers of the star unimpeded while the light output only increased when the shock wave reached the star's outer layers.
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1987A Neutrinos
Types of Supernova
There are two ways for a star to explode as a supernova. We have seen the core collapse type.The second type of supernova involve a white dwarf in a binary system. However, the original definition of the types of supernova where defined by differing spectral lines so supernova are labeled by their spectral lines and not the underlying mechanism.
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A type 1a has no hydrogen and helium but does have strong absorption lines of ionized silicon. A 1a supernova is caused by rapid carbon fusion of a white dwarf.
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Type 1a Supernova
A type 1a is not the death of a massive super giant star. In a type 1a supernova, a white dwarf star accretes (sucks in) matter from its companion star. As the mass of the white dwarf nears 1.4Mⵙ (the Chandrasekhar limit) it causes carbon fusion to begin. Electron degeneracy stops the core from expanding and lowering its temperature. The increased temperature makes the reaction proceed quickly. The reaction proceeds rapidly outward and the star explodes dispersing 100% of it mass into space. 24
Type 1a Supernova
The light curve from a type 1a is a unique signature. Both the luminosity and the shape of the light curve over time are well know. Using the type 1a as a 'standard candle' and the inverse square law, the distant of the 1a supernova can be determined. A type 1a in a galaxy
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Type 1a Supernova
billions of light years away can be used to accurately determine the distance to the supernova. Studying type 1a supernova were used to discover 'dark energy'.
Type 1a – the Merger of two White Dwarfs
For a normal type 1a supernova, a white dwarf accretes matter from a red giant companion. In 2011 a type 1a supernova (SN2011fe) was observed which showed no evidence of having a red giant companion. It is though it was the merger of two white dwarfs. This type of supernova would look similar to a white dwarf with a red giant companion but it would have larger amounts of radioactive cobalt which powers the light curve. It will take several year of watching this supernova to see if it really was a merger of two white dwarfs.
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Type 1b Supernova
Type 1b supernova have no hydrogen spectra lines but do have un-ionized helium. Type 1b are core collapse supernova of a super giant star that has shed its out layer of hydrogen.
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A type 1c supernova does not have the spectra lines of hydrogen or helium. It is similar to the 1b in that it is a core collapse supernova of a giant star that has shed its outer hydrogen and helium.
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Type 1c Supernova
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Type II Supernova
A type II supernova have prominent hydrogen lines such as the H
α line. Type II are core collapse
supernova from a giant star which has not shed its outer layer of hydrogen.
Supernova Remnants
The remnants of a supernova explosion can cover large areas and remain visible for thousands of years. The expanding gas is nearly invisible but it expands so quickly that it collides with the interstellar medium and causes radiation from the radio wavelengths to X-rays.
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Stellar Evolution
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Neutron Stars and Pulsars The core collapse of a large star (8-25 Mⵙ ) leaves behind a neutron star as the result of reverse β decay ( e- + p → n + ν) when the electron degeneracy can no longer hold off gravity. The neutron star is made of almost pure neutrons supported against gravity by the strong nuclear force. A neutron stars with a mass of 1.4Mⵙ has a diameter of 20 km. They spin quickly because of angular momentum conservation. They were predicted in 1934 but only discovered 1967. The discovery of a neutron star was the result of observing extremely regular radio wave pulses. (At first it was thought they might be a SETI signal).
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The crab nebular is a supernova remnant with a rapidly rotating neutron star at its center. The neutron star rotates once every 33.3 ms. (30 times a second). Only something as small in diameter as a neutron star could rotate this quickly.
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Neutron Stars and Pulsars
Because they are so compact, the magnetic field of a neutron star is extremely strong. Charged particles are accelerated along the strong magnetic fields and emit radiation.
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Neutron Stars and Pulsars
This radiation is beamed from the opposite magnetic poles of the neutron star. We observe on Earth a 'light house effect' of the beam sweeping past us once every time the neutron star rotates.
Nova and White Dwarfs
A nova is when a faint star becomes much brighter and then dims over days. It can suddenly brightens by a factor of 104–108 and a luminosity of 105 Lⵙ. (This is not a supernova). A nova occurs when a white dwarf in a binary system accretes hydrogen
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from a companion star onto a dense layer on the hot surface of the white dwarf. As more more gas is deposited and compressed, the temperature can reach 107 K which ignites hydrogen fusion. The can occur repeated on the same white dwarf.
X-ray Bursters
A similar effect can occurs in a binary system containing a neutron star. The neutron star can accrete a layer of hydrogen on it surface with is converted to helium through hydrogen fusion.
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When the helium layer accumulates to about 1 m, helium fusion ignites explosively and heat the surface of the neutron star to ~ 3 x 107 K. At this temperature, the surface emits x-rays. The x-ray emissions stop after a few seconds as the surface cools.
