Evolutionary Path of a SolarMass Star Planetary nebula
Evolutionary Path of a Solar-Mass Star Planetary nebula Helium flash Asymptotic giant branch nt Horizontal branch Re ia dg Main sequence W hi te dw ar f Nov 5, 2003 Astronomy 100 Fall 2003 r Protosta
The Life of a 1 Solar Mass Star: 0. 4 MSun < M < 4 MSun Example of how low mass stars will evolve on the HR Diagram– http: //rainman. astro. uiuc. edu/ddr/stellar/archive/sunt rackson. mpg Nov 5, 2003 Astronomy 100 Fall 2003
A Low Mass Stellar Demise Solar-mass mainsequence star Nov 5, 2003 Helium-burning red giant Astronomy 100 Fall 2003 White dwarf and planetary nebula
Evolution of a Solar-Mass Star Red giant Shell hydrogen burning 1010 yr Main sequence Core hydrogen burning Tcore ~ 16 million K 109 yr Helium flash Our Sun has about 5 billion more years left on the main sequence. Shell helium burning Nov 5, 2003 Astronomy 100 Fall 2003 Planetary Nebula and White Dwarf
White Dwarfs and Planetary Nebulae • Outer layers of the red giant star are blown away by radiation from the hot new white dwarf – loses from 20 to more than 50% of its mass T > 200, 000 K • As they expand, they are lit from within by the white dwarf Nov 5, 2003 Astronomy 100 Fall 2003 NGC 2440
Electron Degeneracy e e p p e e Matter in the core of a normal star Nov 5, 2003 p p p e e e p p p Electron-degenerate matter in a white dwarf 1 ton per cubic cm Astronomy 100 Fall 2003
Degeneracy Pressure Electrons are forced into higher energy levels than normal – all of the lower levels are taken Effect manifests itself as pressure Nov 5, 2003 Astronomy 100 Fall 2003 NASA
Relative Size of White Dwarf 12, 000 km White dwarf– but will weigh about 0. 7 Solar Masses Nov 5, 2003 Astronomy 100 Fall 2003
Binary Systems? • In a close binary pair of stars with slightly different mass, the first higher mass low-mass stars evolves into a white dwarf. • Then later on the other stars evolves into a red giant. • What happens? Nov 5, 2003 Astronomy 100 Fall 2003
What Happens in Binary Systems? Nov 5, 2003 Astronomy 100 Fall 2003
Novae Accreted hydrogen envelope If enough material piles up onto the surface of a white dwarf, can undergo explosive nuclear fusion 100 m White dwarf (carbon-oxygen) White dwarf blows off this envelope and brightens by 100 x – 1000 x over a period of days – weeks Nov 5, 2003 Astronomy 100 Fall 2003 Nova Cygni 1992
Novae Process often repeats Novae are very common, about 20 in our galaxy a year. BUT, it is possible that the whole star can explode– causing a Type Ia Supernova– too much material exceeds the electron degeneracy (1. 4 solar masses) Nov 5, 2003 Astronomy 100 Fall 2003
Stellar Evolution for Intermediate Stars: 4 MSun < M < 8 MSun. Example of how 8 stars 1 through 8 solar masses will evolve on the HR Diagram– http: //rainman. astro. uiuc. edu/ddr/stellar/archive/onet oeighttrackson. mpg Nov 5, 2003
Evolutionary Path for Intermediate Stars Carbon ignition Mass loss Blue supergiant Helium flash t n ia g r e ed sup R Protostar Main sequence M e- -N O g ite wh f ar dw Nov 5, 2003 Astronomy 100 Fall 2003
And when the Hydrogen Runs out? • The more massive stars have convective cores and radiative envelopes, but still very similar to low-mass in the first few stages. • First the hydrogen is burned in the core– still not hot enough to burn helium • Then the core starts to shrink a little– hydrogen shell burning (around the inert helium core) starts. • This stops the collapse, and actually the outer envelope expands quickly becoming a Red Supergiant. …but then… http: //www-astronomy. mps. ohiostate. edu/~pogge/Ast 162/Unit 2/Lower. MS. gif
Evolution of an Intermediate-Mass (> 4 MSun) Star 5 x 106 yr Main sequence Core hydrogen burning Tcore ~ 40 million K Heli um Red supergiant Shell hydrogen burning 106 yr flash C Burning Core 105 yr Blue supergiant Core helium burning Tcore ~ 200 million K 103 yr Red supergiant Core carbon burning Tcore > 600 million K Red supergiant Shell helium burning White Dwarf
Stellar Demise of a Massive Star 10 MSun mainsequence star Nov 5, 2003 Helium-burning red supergiant Other supergiant phases Astronomy 100 Fall 2003 Core-collapse supernova
Stellar Evolution for Massive Stars: M > 8 MSun. Example of how a 15 solar mass star will evolve on the HR Diagram– http: //rainman. astro. uiuc. edu/ddr/stellar/archive/high massdeath. mpg
Evolutionary Path of High-Mass Stars Supernova Carbon ignition Blue supergiant Helium nt flash ia g r e p u ed s R Protostar Main sequence
High Mass Stars • These are very similar to the intermediate mass stars, but as they have more mass, they can “burn” heavier and heavier atoms in the fusion process. • Until they create Iron– after that it takes energy to produce heavier atoms • Nothing left! Stage Temperature (million K) Duration H fusion 40 7 million yr He fusion 200 500, 000 yr C fusion 600 yr Ne fusion 1, 200 1 yr O fusion 1, 500 6 months Si fusion 2, 700 1 day
Game Over!
Supernova Explosions in Recorded History 1054 AD • Europe: no record • China: “guest star” • Anasazi people Chaco Canyon, NM: painting Modern view of this region of the sky: Crab Nebula—remains of a supernova explosion
Supernova Explosions in Recorded History November 11, 1572 Tycho Brahe A “new star” (“nova stella”) Modern view (X-rays): remains of a supernova explosion
November 11, 1572 Tycho Brahe On the 11 th day of November in the evening after sunset. . . I noticed that a new and unusual star, surpassing the other stars in brilliancy, was shining. . . and since I had, from boyhood, known all the stars of the heavens perfectly, it was quite evident to me that there had never been any star in that place of the sky. . . I was so astonished of this sight. . . A miracle indeed, one that has never been previously seen before our time, in any age since the beginning of the world.
Types of Supernovae • Type III, IV, V…. . sorta
Types of Supernova
Type I • Observationally, astronomers originally classed supernovae into two “types”, I and II. • Type I had no Hydrogen emission lines in their spectra whereas Type II exhibited Hydrogen emission lines. Later it was realised that there were in fact three quite distinct Type I supernovae, now labelled Type Ia, Type Ib and Type Ic.
Type Ia • Type Ia supernovae (SNIa) are thought to be the result of the explosion of a carbonoxygen white dwarf in a binary system
Nov 5, 2003 Astronomy 100 Fall 2003
Type Ib and Ic • Types Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed (or been stripped of) their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon.
Type Ib + Ic • Compared to Type Ib, Type Ic supernovae are hypothesized to have lost more of their initial envelope, including most of their helium. The two types are usually referred to as stripped core-collapse supernovae.
Type II supernovae • A Type II supernova (plural: supernovae or supernovas) results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, and no more than 40– 50 times, the mass of the Sun (M☉) to undergo this type of explosion.
Type II • Unlike the Sun, massive stars possess the mass needed to fuse elements that have an atomic mass greater than hydrogen and helium, albeit at increasingly higher temperatures and pressures, causing increasingly shorter stellar life spans. • The degeneracy pressure of electrons and the energy generated by these fusion reactions are sufficient to counter the force of gravity and prevent the star from collapsing, maintaining stellar equilibrium.
Type II • The star fuses increasingly higher mass elements, starting with hydrogen and then helium, progressing up through the periodic table until a core of iron and nickel is produced. • Fusion of iron or nickel produces no net energy output, so no further fusion can take place, leaving the nickel-iron core inert. Due to the lack of energy output creating outward pressure, equilibrium is broken and the core is compressed by the overlying mass of the star.
Type II Supernova • When the compacted mass of the inert core exceeds the Chandrasekhar limit of about 1. 4 M☉, electron degeneracy is no longer sufficient to counter the gravitational compression. A cataclysmic implosion of the core takes place within seconds. Without the support of the nowimploded inner core, the outer core collapses inwards under gravity and reaches a velocity of up to 23% of the speed of light and the sudden compression increases the temperature of the inner core to up to 100 billion kelvin.
Supernovae and nucleosynthesis of elements > Fe
Death of low-mass star: White Dwarf • White dwarfs are the remaining cores once fusion stops • Electron degeneracy pressure supports them against gravity • Cool and grow dimmer over time
A white dwarf can accrete mass from its companion
Tycho’s supernova of 1572
Expanding at 6 million mph
Kepler’s supernova of 1609
Two kinds of supernovae Type I: White dwarf supernova White dwarf near 1. 4 Msun accretes matter from red giant companion, causing supernova explosion Type II: Massive star supernova Massive star builds up 1. 4 Msun core and collapses into a neutron star, gravitational PE released in explosion
light curve shows how luminosity changes with time
A neutron star: A few km in diameter, supported against gravity by degeneracy pressure of neutrons
Discovery of Neutron Stars • Using a radio telescope in 1967, Jocelyn Bell discovered very rapid pulses of radio emission coming from a single point on the sky • The pulses were coming from a spinning neutron star—a pulsar
Pulsar at center of Crab Nebula pulses 30 times per second
Pulsars
Thought Question Could there be neutron stars that appear as pulsars to other civilizations but not to us? A. Yes B. No
Thought Question Could there be neutron stars that appear as pulsars to other civilizations but not to us? A. Yes B. No
What happens if the neutron star has more mass than can be supported by neutron degeneracy pressure? 1. It will collapse further and become a black hole 2. It will spin even faster, and fling material out into space 3. Neutron degeneracy pressure can never be overcome by gravity
• Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass is > about 3 Msun
18. 3 Black Holes: Gravity’s Ultimate Victory A black hole is an object whose gravity is so powerful that not even light can escape it.
Escape Velocity Initial Kinetic Energy = Final Gravitational Potential Energy Where m is your mass, the mass of the object that you are trying to escape from, and r is your distance from that object
Magic Numbers • • 1. 4 M☉ 8 M☉ 25 M☉ 40+ M☉ Nov 5, 2003 Astronomy 100 Fall 2003
- Slides: 55