Chapter 22
stars with mass when fusion stops of less than 1.4 solar masses trajectory
Sun-like star -> Red Giant ->Planetary nebula -> White Dwarf -> Black Dwarf
type II supernova SN1987A
1987 a type II supernova was observed in ht eLarge Magellanic cloud - first supernova visible to the naked ye since the invention of the telescop 10^55 neutrinos were emitted 20 hours before the supernova was visible - they passed through the Earth - 500 trillion through every square meter Very few interacted, but about 1 million people experienced a neutrino interaction in their bodies. Large water detectors in Japan and Cleveland saw 19 interactions
Black Hole trajectory
Giant star-> Red Supergiant -> Supernova ->Black hole
stars with mass when fusion stops of in between 1.4 nd 3 solar masses - trajectory
Huge star -> Red SuperGiant -> Supernova -> Neutron star
Nucleosynthesis
Neutrino from the type II supernova speed outward and strike the expanding layers of ejected material Can be absorbed nby nuclei liek iron where the neutrons turn into protons through beta decay Fe-->CO Light nuclei are thus transformed into nuclei heavier than iron All elements in the Sun other than H and HE were made in the ealier massive star or supernova. When star died, these heavy elements were thrown inot space, Elements like CA, N, O are older than the sun itself the heavy elemtns needed to form Earth and its life are only in massive stars many cycles of stellar birth and death of stars were required to produce these building blocks
Fusion Cycle - collapsing
When H in core is depleted - core collapses, increasing temp until He fuses. star no longer on the main sequence When He in core depleted - core collapses increasing temp until carbon fuses etc... Element at which fusion cycle stops depends on the mass of str Eveb the most massive stars (like 15 solar masses) stop at iron Our Sun would stop at carbon
Collapse of core of a star with mass less than 1.4 solar masses
When final fusion cycle is completed, the core begins to collapse for the last time gravitational energy of collapse heats the star's outer layers blowing them into space as a planetary nebula, revaling the hot contracting core exhausted core is so hot that all electrons are stripped from nuclei and gravity overwhelms the thermal pressure of nuclei, crushing the electrons and nuclei closer together collapse stops when the electrons reacha critical density - electrons at this new density exert new kind of pressure called "degenerate electron pressurE" collapsing core is made up of electrongs and completely ionized nuclei Because of their quantum nature, it is the electrons that fill the core volume, not the naked nuclei It is the electrons that will be crowded together before the nuclei though there are nuclei present, it is the electrons that stop the collapse of the core mass of the nuclei provide the inward gravity quantum nature of electrons provides th eoutward pressure Electrons can be crowded beyond the critical density inside the core, but it requires an enormous force gravity is too weak to force the electrons to beyond the critical density Gravity cannot overcome the degenerate electron pressure - so collapse is halted stellar core now size of Earth - nuclear fuel is exhausted It glows only from the gravitational heat of its contraction - it is a WHITE DWARF
Type I Supernova
White Dwarfs from stars like out sun - have carbon cores which never achieved fusion - -other stars have different core rapid collapse and heating og the core forces the carbon core to fuse almost instantaneously so much energy released in les sthan 1 second that it explosively destroys the white dwarf one was observed in 1994 AD on the edge of glaaxy NGC 4526 about 50 Million LY away Because of their extreme luminosities, typ 1 supernovae can be detected near the edge of the universe
Sirius A and B
White Dwarfs next to eacho other
Neutron Star
about 20km in diamter result from supernova
Lifetimes of stars
all stars spend about 90% of lifetime of their lives on the main sequence Lifetimes on main sequence depends on star's mass
stars with mass more than 1.4 but less than 3 solar masses
become neutron stars
Stars with mass when fusio stops of more than 3 solar masses
becomes Black Hole
collapse of Stars with mass when fusio stops of more than 3 solar masses
collapse does not stop at neutron star so much gravity that even the neutrons are crushed to a single point All th ematter of the star is now in a singularity at the center of a Black Hole
Star of about 15 solar masses
diagram pg 234 last phase of fusion collapse is silicon and lasts just days
"degenerate electron pressure"
during collpase when electrons reach a critical density - exert this kind of pressure unlike the thermal gas pressure in normal stars results from the quantum nature of electrons we have already seen this kind of quantum behavior when electrons are confined inside of atoms - Bohr model of H atom if proton is scaled up to the size of a tennis ball - the electron would be in upland diagram pg 236 the volume of the electron sphere is 100,000 times the volume of the proton - the atom is essentially completely empty space Ths quantum resistance to being crowded together beyond critical density
"Degenerate nuetron pressure"
exerted by neutrons in the formation of neutrino stars - preevnts further collapse of stars with mass in between 1.4 and 3 solar masses
WHITE DWARF
exposed hot core of the star stellar core now size of Earth - nuclear fuel is exhausted It glows only from the gravitational heat of its contraction masses of about 0.5-0.6 solar masses and densities of about 10 million g/c if mass added to the dwarf, additional gravity will crush the electrons further and decrease the radius more mass --> smaller radius extremely hot but very small so not very luminous fate of all white dwarfs is to cool over billions of years to form black dwarfs
Core collapsing for stars with mass more than 1.4 but less than 3 solar masses
fusion cycle stops - center of star in this mass range - core shrinks until it becomes white dwarf - however fusion continues in the shell surrounding core As fuel is depleted in shells - ash falls ontot he white dwarf core, increasing its mass Though the degenerate electrons resist, the increased mass causes a small contraction of the core As mass continues to fall onto the core, the compression becomes so great that some electrons are forced into the nucleus where they combien with protons The loss of electrons causes the electron pressure to decrease. The core can no longer resist collapse and this process continues In less than a second - core collapses at 1/4 light speed from an Earth sized sphere to about 20km in diameter core collapses so fast that the information doesnt have time to reach the outer layers outer layers are briefuly suspended over an empty space with nothing to support them Collapse is halted when all protons in hte core have been transformed into neutrons Neutrons exert "Degenerate nuetron pressure" preventing further collapse
White Dwarf companions
if white dwarf has a companion - when companion enters red giant phase - H can be pulled from the companion onto white dwarf as H layer accumulates over surface of whit edwarf - it is compressed nad heated When hot enouhg - H on surface can flash into intense fusion - causing a dramatic increase in luminosity - NOVA if H transfer is fats - white dwarf can be pused over Chandraeskhar limit White Dwarf again shrinks, compressing and heating the interior
3 Kinds of stars we consider and their fates
mass when fusion stops is less than 1.4 solar mass (turns into White dwarf) between 1.4 and 3 solar mass (turns into Neutron Star) greater than 3 solar mass (turns into Black hole)
Chandraeskhar limit
max mass of a white dwarf is 1.4 solar masses
Conservation of Angular Momentum
normal star - large radius/slow spin/weak magnetic field neutron star - small radius/ very fast spin/ verys trong agnetic field
Crab Nebula & pulsar
seen by Chinese in 1054 AD was visible during the day -6500 LY away and 6 LY across - radio pulsar 33 revs per sec - energy output 100,000 times of the Sun
Pulsars
spinning neutron star whose magnetic field accelerated electrons and protons near its surface These fast particles escape only at the magnetic poles, emitting radiation in a narrow beam bea sweeps through space like a light house if beam happens to sweep across the Earth, we see it as a pulse Most pulsars radiate primarily in radio waves but there are also visible, x-ray and gamma ray pulsars
Quantum H atom
the proton is 2000 times more massive than electron the elctron spends most of its time in a zone corresponding to the Bohr radius the quantum electron occupies almost the entire volume of the H atom represented by a probability cloud Though the electron is 2000 times less massive than proton - the quantum electron fills 100,000 times more space than the proton This is part of the strangeness of quantum nature of particles
The Ring Nebula (M57)
the remains of a star that died about 6000-8000 years ago the exposed hot core of the star - a white dwarf - is in the center
NOVA
when a white dwarf experiences a large increase in luminosity as a result of pulling H from a companion Red Giant which accumulates over surface and is compressed and heated and then which flashes on surface of Dwarf and causes intense fusion
Type II supernova
when collapse of star with mas in between 1.4 and 3 solar masses is abruptly halted, pressure shock wave rebound outward This shock combined the the enormous blast of neutrinos released when the nuetron core is formed explosively drive the outer layers of the star into space This explosion is called a type II supernova power carried by the neutrinos in the 1st second is 10^46 watts - greater than the total power output of all the stars in all the galaxies we can see emits more energy than 100 times the Sun's output during its 10 billion year life 99% of it in neutrinos 1% kinetic energy of ejected material 0.01% photons 1987 a type II supernova was observed in ht eLarge Magellanic cloud - first supernova visible to the naked ye since the invention of the telescope
Mass less than 1.4 solar masses
when fusio stops they become white dwarfs even stars with an initial mass from 7.5 to 10 solar masses can lose enough mass to end up in this category As they end each fusion cycle, the gravitational energy from the collapsing core heats the outer layers of the star With the end of each cycle the star expands resulting in a Red Giant
Black Dwarf
white dwarfs all will cool over billions of years and form Black Dwarfs Our sun will do this
