1 / 22

After the Main Sequence

After the Main Sequence. Laws of Stellar Structure. Hydrostatic Equilibrium Energy Transport Conservation of Mass Conservation of Energy. Limits of the Main Sequence. Upper Limit: High mass stars Lower Limit: Low Mass Stars.

Faraday
Télécharger la présentation

After the Main Sequence

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. After the Main Sequence

  2. Laws of Stellar Structure • Hydrostatic Equilibrium • Energy Transport • Conservation of Mass • Conservation of Energy Limits of the Main Sequence Upper Limit: High mass stars Lower Limit: Low Mass Stars

  3. The electrostatic force that two charged particles exert on each other is called the Coulomb force and is given by the following equation k is a constant, q1 and q2 are the charges, and d is the distance between them. Why do heavier elements fuse at higher temperatures? For fusion to occur, the particles must get very close to each other. However, the closer they get, the more strongly they repel each other. They must collide at very high speeds in order to get close enough for the strong nuclear force to bind them. Since the average speed of a particle of an ideal gas is proportional to the square root of the temperature, fusion will only occur at high temperatures. A combination of high stellar core temperatures and quantum mechanical tunneling makes fusion possible in stellar cores. The heavier elements have more nuclear charge than the light elements. Since the Coulomb force is proportional to the product of the charges, the heavier elements undergo fusion at higher temperatures than the light elements.

  4. Energy Production in Stars • Proton-proton chain (M  1.1 MSun)This is the main source of a star’s energy for spectral classes cooler than F0It dominates at core temperatures between 3106 and about 2107 K • CNO cycle (M  1.5 MSun)Same result as the PP chain, but carbon acts as a catalyst; i.e., it facilitates the fusion of H to He but doesn’t get used up. This produces energy faster than the PP chain in spectral classes hotter than F0.It dominates at core temperatures greater than about 2 2107K • Triple a process 3 2He4 6C12 + energy Requires core temperatures  108 K • Carbon fusionTcore 6 108 K • Neon fusionTcore 1.5109 K • Oxygen fusionTcore 2109 K • Silicon fusionTcore 3109 KOnly stars with ZAMS masses greater than 20 solar masses will undergo silicon fusion. • The most tightly-bound element is 26Fe56. It can release energy by neither fusion nor fission.

  5. Inert He (not hot enough for the triple a process) H fusion He H and He envelope (not hot enough for fusion) What causes a star to become a red giant? As hydrogen fuses, the helium nuclei fall toward the center of the star and accumulate there to form a helium core. As more He rains down into the core, the conversion of gravitational energy into heat and light increases its temperature and pressure. The hot He core heats up the hydrogen in a shell outside the region that was the core of the main sequence star. This shell becomes hot enough for H fusion to begin. Eventually, the combination of radiation pressure and thermal pressure from the shell causes the star’s envelope to expand and cool. The star becomes a red giant. The temperature of the core is high enough to cause the H shell to fuse rapidly, resulting in a dramatic increase in the star’s luminosity.

  6. This scale implies that Betelgeuse is about 1000 times larger than the Sun! Hubble Space Telescope Image of a Red Supergiant

  7. The Pauli Exclusion Principle • Electrons, neutrons, protons, and neutrinos are examples of fermions. They are particles with an odd multiple of one-half the fundamental unit of angular momentum. • The Pauli exclusion principle is a physical law obeyed by all fermions.In a bound sample of fermions of a given type, no two particles can have both the same energy and the same spin orientation. • The condition in which all of the electrons in an object are in their lowest possible energy states is called electron degeneracy.

  8. If the electron density is low, as it is in a “normal” star, most energy levels are empty and an electron can easily acquire enough energy to jump to a higher energy level. Under these conditions, the electron gas is called non-degenerate. Its pressure is proportional to its temperature (PV = NkT) Degenerate Non-degenerate If the electron density is greater than about 109 kilograms per cubic meter, there will be electrons in all of the lowest levels. Under these conditions, the electron gas is called degenerate. Only the few electrons with the highest energies can easily acquire enough energy to jump to higher energy levels. For most electrons, the nearby levels are already filled with electrons. Energy Diagrams for Degenerate and Non-degenerate Electron Gases An electron bound to an atom can only have one of a set of discrete energies. This is also true for electrons bound inside a star.

  9. The Properties of Degenerate Matter • In order to compress it, we must change the energy of large numbers of electrons. However, only a few electrons (those in the highest occupied levels) can have their energies changed by small amounts. Therefore, the degenerate matter resists compression; it is extremely rigid. • It easily conducts both heat and electricity. • In contrast to an ordinary gas, its pressure depends only on its density – not on its temperature. • The “free” electrons in a metal form a degenerate electron gas; that’s why a metal is a good conductor of both electricity and heat.

  10. The Triple Alpha Process and the “Helium Flash” In stars with masses between 0.4 and 4 solar masses, the helium core becomes degenerate before the temperature is high enough to ignite helium. This results in an explosion called the helium flash. Helium ignition After a few minutes, the temperature is so high that the core becomes non-degenerate. Although the peak luminosity may be as high as 1014 times that of the Sun, all of the energy is absorbed by the red giant’s envelope. This, combined with the short duration of the event, makes the helium flash virtually unobservable.

  11. White Dwarfs • When gravity compresses a star so much that a mass comparable to the mass of the Sun is squeezed into a volume comparable to the mass of Earth, the density is about 109 kilograms per cubic meter. • This compact object, supported by electron degeneracy pressure, is called a white dwarf. Estimate of the density of a white dwarf A cubic inch of this material would weigh about 10 tons!

  12. White Dwarfs in Globular Cluster M4 • M4 is 7000 light years away. • The image on the right is the HST image of the small region indicated in the ground-based image on the left. Circles are drawn around the white dwarfs. • Globular clusters are old, so they are expected to contain many white dwarfs. Of M4’s 100,000 or so stars, about 40,000 are expected to be white dwarfs.

  13. WhiteDwarf BlackDwarf Low Mass Star Low mass stars are called red dwarfs Masses between 0.08 and 0.4 solar masses The lifetime of a 0.4 solar mass star is about 100109 years. A low mass star is convective throughout its entire volume. The helium created by fusion is mixed with the material in the rest of the star. Because of this, it never has a dense helium core surrounded by a shell in which hydrogen is undergoing fusion, which is the condition that results in a red giant. Consequently, these stars never become red giants. They will gradually cool to become black dwarfs composed mainly of a mixture of H and He.

  14. The white streak is a result of overexposure of GL 105A. Gliese 105C HST Near Infrared Image of a Red Dwarf • 27 light years away in constellation Cetus. • GL 105C is 25,000 times fainter than GL 105A. • The temperature of GL 105C may be as low as 2600 K. • Mass is about 0.08 to 0.09 times the mass of the sun. • Since this is an infrared image, the colors you see are false colors.

  15. Red Giant BlackDwarf Medium Mass Star Masses between 0.4 and 8 solar masses. The envelope of the red giant becomes unstable (thermal pulses)and is expelled. The nebula that results is called a planetary nebula. Fluorescence occurs only if the temperature of the star inside the expelled material is least 25,000 K. Eventually, a star will be incapable of generating energy by nuclear fusion. If its mass is then less than the Chandrasekhar limit (1.4 M) the star will be a hot white dwarf that slowly cools by emitting electromagnetic radiation. These white dwarfs are composed primarily of carbon and oxygen. WhiteDwarf+Planetary Nebula The carbon and oxygen are ionized and embedded in a degenerate gas of electrons. When the temperature is low enough, the carbon and oxygen crystallize.

  16. A Planetary Nebula • The hot relic of the dying star’s interior emits ultraviolet radiation, which causes fluorescence of the expelled envelope. • NGC 2392 is 5000 light years away in Gemini.

  17. L5 M1 M2 L1 L4 Gravitational Equipotential Surfaces – Accretion Disks in Binary Star Systems Inner Lagrangian Point 1= black (low potential) 2 = brown (Roche lobes) 3 = yellow 4 = green 5 = blue 6 = red (high potential)

  18. What is a Nova? • Always found in binary star systems with one of the stars being a white dwarf. • Material from the companion passes through the inner Lagrangian point, forming an accretion disk around the white dwarf. • Material from the accretion disk, rich in hydrogen, falls onto the surface of the white dwarf. • The hydrogen-rich surface layer gets thicker, hotter, and more dense, eventually becoming degenerate. • When the temperature reaches millions of degrees, the hydrogen detonates in a thermonuclear explosion that blasts away much of the surface layer. • The process is repeated, so these are called “recurrent novae”.

  19. Recurrent Nova T Pyxidis • ~ 6000 ly from Earth • Recurs with a period of about 20 years. • Consists of a couple of thousand bright knots • Diameter ~ 1 ly at the time of these pictures

More Related