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Chapter 18 The Bizarre Stellar Graveyard

Chapter 18 The Bizarre Stellar Graveyard. What is a white dwarf?. White Dwarfs. White dwarfs are the remaining cores of dead stars Electron degeneracy pressure supports them against gravity. White dwarfs cool off and grow dimmer with time. Size of a White Dwarf.

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Chapter 18 The Bizarre Stellar Graveyard

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  1. Chapter 18The Bizarre Stellar Graveyard

  2. What is a white dwarf?

  3. White Dwarfs • White dwarfs are the remaining cores of dead stars • Electron degeneracy pressure supports them against gravity

  4. White dwarfs cool off and grow dimmer with time

  5. Size of a White Dwarf • White dwarfs with same mass as Sun are about same size as Earth • Higher mass white dwarfs are smaller

  6. 0 The Chandrasekhar Limit The more massive a white dwarf, the smaller it is. Pressure becomes larger, until electron degeneracy pressure can no longer hold up against gravity. WDs with more than ~ 1.4 solar masses can not exist!

  7. The White Dwarf Limit • Quantum mechanics says that electrons must move faster as they are squeezed into a very small space • As a white dwarf’s mass approaches 1.4MSun, its electrons must move at nearly the speed of light • Because nothing can move faster than light, a white dwarf cannot be more massive than 1.4MSun, the white dwarf limit (or Chandrasekhar limit)

  8. What can happen to a white dwarf in a close binary system?

  9. Accretion Disk • White dwarf in a mass-transfer binary • Its gravity pulls matter from the companion star • The rotation results in the formation of an accretion disk • Temperature of the matter increases as it spirals in • Disk can radiate in visible UV and X-ray

  10. Accretion Disks • In addition to the heating due to the contraction, friction between orbiting rings of matter in the disk causes the disk to heat up and glow

  11. Nova • The temperature of accreted matter eventually becomes hot enough for hydrogen fusion • Fusion begins suddenly and explosively, causing a nova

  12. Nova Explosion • Exploding layer is only about 0.0001 solar masses • The spectrum reveals the details • Initially, blue-shifted gas with absorption lines – dense gas • Then, blue-shifted gas with emission lines – thin gas • It can be 100,000 times more luminous than the sun. • Fades over a period of weeks or months • Mass transfer process begins again

  13. 0 Nova Explosions Hydrogen accreted through the accretion disk accumulates on the surface of the white dwarf • Very hot, dense layer of non-fusing hydrogen on the white dwarf surface Nova Cygni 1975 • Explosive onset of H fusion • Nova explosion

  14. Nova • The nova star system temporarily appears much brighter • The explosion drives accreted matter out into space

  15. Novae • The star will “nova” again when the explosive layer accumulates. • Many novae take thousands of years to build an explosive layer, but some take only decades. • Mass Ejection from Novae • (a) Nova Persei • (b) Nova Cygni

  16. Our sun will not nova. It can’t. It’s not part of a binary system.

  17. Thought Question What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit? A. It explodes B. It collapses into a neutron star C. It gradually begins fusing carbon in its core

  18. Types of Supernova • Massive star supernova (Type II): • Core-collapse supernova • Iron core of a massive star reaches white dwarf limit and collapses into a neutron star, causing total explosion. • If the remaining core is large enough (>2-3 M ) the core will collapse into a black hole, if not it forms a neutron star. • They retain their outer hydrogen core prior to the explosion, so their spectra show hydrogen lines.

  19. Type I Supernovae • Type Ia supernovae • White dwarf in a binary undergoes a nova, but does not blow away all of the accumulated mass • Supported initially by electron degeneracy pressure • Mass slowly increases until star reaches the Chandrasekhar limit • Electron degeneracy pressure can no longer hold up the star against gravity • Core collapses and heats rapidly • Carbon fusion occurs everywhere in the star simultaneously • Carbon-detonation supernova • Massive explosion completely destroys the white dwarf. • About 6 times as luminous as a Type II • No H in the spectrum because there was none in the white dwarf.

  20. Type I Supernovae • Type Ib and Ic supernovae • Massive star in a binary looses its Hydrogen rich atmosphere to a companion (that is not a white dwarf) • Explodes due to core collapse • No hydrogen in spectrum because it lost it to the companion • Essentially a Type II but without the hydrogen • Ib and Ic differ by the presence of the 587.6nm helium line in the spectrum of Ib but not in Ic

  21. One way to tell supernova types apart is with a light curve showing how luminosity changes with time

  22. Supernovae Light Curves • Maximum luminosity can be more than a billion Suns. • Note the characteristic plateau of Type IIs

  23. Supernova Type: Massive Star or White Dwarf? • Spectra differ also differ (Type I don’t have hydrogen absorption lines) • Type Ia spectra become dominated by lines of iron. • Type Ib/Ic lack 635.5 nm silicon line and have oxygen, calcium and magnesium. • Type Ic lack lines of helium at 587.6 nm which are in Ib

  24. The Deaths of Massive Stars: Supernovae In the multiple shell burning stage of a high mass star: As each element is burned to depletion at the center, the core contracts, heats up, and starts to fuse the ash of the previous burning stage. A new inner core forms, contracts again, heats again, and so on. Through each period of stability and instability, the star’s central temperature increases, the nuclear reactions speed up, and the newly released energy supports the star for ever-shorter periods of time. For example, in round numbers, a star 20 times more massive than the Sun burns hydrogen for 10 million years, helium for 1 million years, carbon for 1000 years, oxygen for 1 year, and silicon for a week. Its iron core grows for less than a day.

  25. 0 Numerical Simulations of Supernova Explosions The details of supernova explosions are highly complex and not quite understood yet.

  26. Supernovae are rare. Only a few have been seen with the naked eye in recorded history. Arab astronomers saw one in AD 1006. The Chinese saw one in AD 1054. European astronomers observed two—one in AD 1572 (Tycho’s supernova) and one in AD 1604 (Kepler’s supernova). In addition, the “guest stars” of AD 185, 386, 393, and 1181 may have been supernovae. Observations of Supernovae

  27. 0 Observations of Supernovae They can sometimes be seen in distant galaxies.

  28. Supernova explosions fade in a year or two, but expanding shells of gas, supernova remnants, mark the sites of the explosion. The gas, originally expelled at 10,000 to 20,000 km/s, may carry away a fifth of the mass of the exploding star. They last a few tens of thousands of years before they mix with the interstellar medium and disappear Some can only be seen today in X-rays or radio wavelengths Some, like Cassiopeia-A, show evidence of jets of matter ejected in opposite directions. Observations of Supernovae

  29. Supernovae Remnants • Supernova Remnant: Expanding cloud of material from the explosion of a supernova • Crab Nebula (M1) • Supernova in 1054 A.D. • Angular diameter about one-fifth that of the full Moon. • Debris is scattered over a region of “only” 2 pc; Considered to be a youngremnant • The blue color is produced by synchrotron radiation

  30. Synchrotron radiation is produced by rapidly moving electrons spiraling through magnetic fields. Remember – accelerated charged particles emit radiation In most nebulae this radiation is in the radio part of the EM spectrum In the Crab Nebula it is in the visible spectrum. Synchrotron Radiation

  31. Supernovae Remnant Motion • By superimposing positive and negative images taken years apart, we can tell that the Crab Nebula is moving outward • We can extrapolate back to the origin of the explosion • Corresponds closely to the date of the 1054 A.D. supernova

  32. Vela Supernova Remnant Extrapolation shows it exploded about 9000 B.C.

  33. 0 Supernova Remnants X rays The Crab Nebula: Remnant of a supernova observed in a.d. 1054 The Veil Nebula Optical Cassiopeia A The Cygnus Loop

  34. Supernova 1987A • Stellar evolution theory predicts we should see about 1 supernova in our galaxy about every 100 years. • Hasn’t been one in over 400 years • Supernova 1987A occurred in the Large Magellanic Cloud • Its light curve is somewhat atypical

  35. SN1987A was produced by the explosion of a hot, blue supergiant rather than a cool, red supergiant. Evidently, the star was a red supergiant a few thousand years ago but had contracted and heated up slightly becoming smaller, hotter, and bluer before it exploded. SN1987A

  36. One observation of SN1987A confirmed the theory of core collapse. At 2:35 AM EST on February 23, 1987, hours before the supernova was first seen, instruments buried in a salt mine under Lake Erie and in a lead mine in Japan, recorded 19 neutrinos in less than 15 seconds. Trillions must have passed through our bodies About 1017 neutrinos must have passed through the detectors They came from the direction of the supernova SN1987A

  37. SN1987A • The glowing ring is gas expelled by SN1987A’s progenitor star while in its red-giant phase. • It glows from the UV light pulse from the supernova

  38. Neutron Stars and Black Holes • Gravity always wins. We believe stars end in one of three final states called compact objects. • white dwarf • neutron star • black hole • For Type Ia supernovae, the entire star is obliterated, leaving no remnant • Type II supernovae end up as a neutron star or a black hole

  39. A neutron star is the ball of neutrons left behind by a massive-star supernova. Degeneracy pressure of neutrons supports a neutron star against gravity.

  40. Theoretical Prediction of Neutron Stars • In 1934, two years after the neutron was discovered, Walter Baade and Fritz Zwicky predicted that massive stars would end in an explosion they called a supernova. • The core would form a small, tremendously dense sphere of neutrons. • Zwicky coined the term ‘neutron star.’ • The core is supported by degenerate neutron pressure

  41. Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos Neutrons collapse to the center, forming a neutron star

  42. A neutron star is about the same size as a small city ~15mi

  43. Neutron Stars

  44. Theoretical calculations suggest that stars that begin life on the main sequence with 8 to roughly 20 solar masses will leave behind neutron stars. They have a radius of about 10 km and a density of about 1014 g/cm3 Mass of about 1-3 M It should spin many times per second – conservation of angular momentum Surface many times hotter than the sun – energy from gravitational collapse Magnetic field a trillion times stronger than Earth’s – field squeezed into a small volume Theoretical Prediction of Neutron Stars

  45. Neutron stars are very hot Black-body spectrum peaks in X-rays Couldn’t detect them until x-ray telescopes where put in orbit Their surface area is very small Very faint Theoretical Prediction of Neutron Stars

  46. How were neutron stars discovered? “LGM”

  47. Discovery of Neutron Stars • Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky • The pulses were coming from a spinning neutron star—a pulsar

  48. The Discovery of Pulsars • Astronomers found other pulsars • Periods from 0.033 to 3.75 seconds and were nearly as exact as an atomic clock. • Periods were increasing by a few billionths of a second per day. • A star or white dwarf could not spin fast enough – it would fly apart • Pulses lasted only about 0.001 s. • Puts upper limit on the size – 300 km • An object cannot change its brightness appreciably in an interval shorter than the time light takes to cross its diameter. • The conclusion was that only a neutron star could be a pulsar

  49. The Discovery of Pulsars • In 1968, astronomers discovered a pulsar at the center of the Crab Nebula. • The Crab Nebula is a supernova remnant. • Pulses 30 times per second X-rays Visible

  50. Lighthouse Model

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