05 Stellar Evolution Mc Neely

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05 Stellar Evolution Mc Neely

  1. 1. Astronomy Ch. 05: Stellar Evolution Sirius is a main sequence star with a small, white dwarf companion as displayed in this HST photo
  2. 2. Stellar Evolution The changes that take place in stars as they age Life cycle of stars Over millions-billions of years
  3. 3. Birthplaces Stars form out of gigantic interstellar clouds (nebulas) Famous Orion Nebula located 1500 light- years away, a region of intense star formation Orion Nebula, located in Orion’s Sword, appears as a greenish-cloud in telescopes
  4. 4. Orion’s Sword
  5. 5. A Star is Born Protostar: Star in its earliest phase of evolution; Baby star Proplyd: “Protoplanetary Disk”, another term for protostars and their nebular clumps
  6. 6. Protostars Protostar can be surrounded by rotating disk that will form a solar system Nuclear fusion when 10 million K internal temp Bipolar jets, material erupting into space along the axes of rotation
  7. 7. Hydrostatic Equilibrium Hydrostatic Equilibrium: Internal balance Gravity balances pressure of hot gases in star Holds star together Stars spend their lives fighting the inward crush of gravity
  8. 8. 3 Steps in Birth of a Star 1. Gravitational contraction within a cloud of gas and dust 2. Rise in interior temperature and pressure 3. Nuclear fusion begins once internal temperature reaches 10 million Kelvin
  9. 9. Protostar Diagram This artist’s view of a protostar displays bipolar jets
  10. 10. Beta Pictoris Circumstellar Disk; Orion Proplyd Star Beta Pictoris is surrounded by a disk of gas and dust, the nebula from which the star formed This HST image shows proplyds located in the Orion Nebula
  11. 11. Lifetimes A function of a star’s mass and chemical composition High mass stars evolve fastest, low mass stars evolve slowest Stars move throughout the HR Diagram as they age; i.e., their temperatures and luminosities change over time Main Sequence stars are “adults”
  12. 12. Life Cycles of Stars (HR Diagram)
  13. 13. Why Stars Shine Fusion: 4 hydrogen nuclei are converted into 1 helium nuclei, excess mass is given off as energy (heat, light) Energy released by fusion can be calculated using Einstein’s famous E=mc2 (E=energy, m=mass difference, c=speed of light)
  14. 14. Old Age of Stars Main sequence stars shine until all available hydrogen has been converted into helium Then the star begins to die The sun has been shining for about 5 billion years. It is middle-aged
  15. 15. Massive Stars Very massive, hot, bright stars die fastest because they use up their hydrogen rapidly; Massive stars spend only a few million years as main sequence stars. Ex: Rigel, hot, blue star in Orion Least massive, cool, dim stars such as red dwarfs can last billions of years
  16. 16. Red Giants Red giants are senior citizen stars After hydrogen fuel in core runs out, star swells into a giant Red giants are cooler and redder, they leave main sequence and enter upper right corner of HR Diagram Examples include Antares and Betelgeuse Our sun in the future
  17. 17. 5,000,000,000 AD Talk about global warming!
  18. 18. Red Giant Stars are HUGE! Ex: Betelgeuse
  19. 19. Nucleosynthesis The creation of elements in stars Main sequence hydrogen fusion Helium Fusion When red giant stars achieve 100 million K internally, helium is converted into carbon (helium flash)
  20. 20. Red Giant Nucleosynthesis Red giant stars form internal shells that produce progressively higher elements Large red giants can create heavier elements such as oxygen, aluminum, and calcium Stars can produce elements up to iron before exploding Elements higher than iron are produced in the brief explosions of stars
  21. 21. Red Giant Nucleosynthesis Each shell in the red giant produces progressively heavier elements with depth
  22. 22. Betelgeuse http://malyszp.tripod.com/stars/betelgeuse.jpg Beetle Juice (1989) was inspired by the star in Orion
  23. 23. Variable Stars Stars that change brightness in regular or irregular cycles Pulsating Variable Stars Move back and forth between the main sequence and red giant region of the HR diagram for unknown reasons Such stars vary in light output, expand and contract Ex: Cepheid variables
  24. 24. Cepheid Variables Luminous, yellow Brightness varies from 1-70 days Famous example, Delta Cephei Period-luminosity relationship, used to calculate distances
  25. 25. Cepheids: Distance Markers Period-Luminosity Relationship: For Cepheids, the longer the period of brightness change, the greater the luminosity This relationship enables the calculation of absolute magnitude. Compare absolute to apparent magnitude to estimate distance Good to about 10 million light-years (closest galaxies)
  26. 26. Delta Cephei Light Curve Delta Cephei has a roughly 5-day cycle of brightness
  27. 27. Delta Cephei Star Map Delta Cephei Delta is a naked eye star in Cepheus
  28. 28. RR Lyrae Variables Named for star RR in Lyra RR Lyrae stars are pulsating blue-white giants with periods less than 1 day Distance markers out to 600,000 ly
  29. 29. Long Period or Mira Variables Mira in Cetus, pulsating red giants Periods between 80-100 days from dim to bright Mira means the “Wonderful” star, proclaimed after its recognition in 1638 Mira first variable star discovered Mira brightest every 333 days
  30. 30. The Wonderful Star
  31. 31. Mira Light Curves •The diagram shows the changing brightness cycle of Mira •Each strip represents 15 years, and each dot represents a magnitude estimate •Most of these estimates were made by amateur astronomers who do this work as part of their hobby
  32. 32. Mira -Feb 2007 •In late winter, Cetus and Mira appear to be setting in the west after sunset •This photo was taken in Stuttgart, Germany
  33. 33. Death of Stars Depends on mass Small stars, up to 1.4 times the sun’s mass, go to planetary nebula stage, fade away into dwarf stars Larger stars (8 times the sun’s mass) explode
  34. 34. Planetary Nebulas Type of nebula ejected by dying stars Size 0.5-1 ly in diameter Leaves behind a white dwarf star in center Famous examples: M57, the Ring Nebula in Lyra; NGC6543, Cat’s Eye in Draco Ring Nebula
  35. 35. M57 Ring Nebula: HST Note the central star, a white dwarf
  36. 36. Cat’s Eye: Amateur & HST •The Cat’s Eye Nebula in Draco •Planetary nebulas can reveal bizarre and complex shapes
  37. 37. White Dwarfs Remains after planetary nebula stage Star can no longer resist inward pull of gravity, squeezes down into an object about the size of the earth Very dense, you would weigh 35,000 times greater if you could somehow stand on a white dwarf A teaspoon of white dwarf matter would weigh over a ton Can brighten suddenly as “novas”
  38. 38. Ziggy
  39. 39. Black Dwarfs Gradually, the white dwarf cools, turns dull red, and shines its last energy into space White dwarf becomes a black dwarf, corpse of a star Our sun’s ignominious end
  40. 40. Life Stages of a Sun-Like Star 1. Protostar, gravitational contraction of gas and dust 2. Stable, main sequence star shining by hydrogen fusion 3. Evolution to red giant when helium core forms 4. Red giant, shining by helium fusion 5. Variable star, formation of carbon core 6. Planetary nebula, outer atmosphere of star ejected into space 7. White dwarf, mass packed into a star about the size of the earth 8. Dead corpse, black dwarf in space
  41. 41. Exploding Stars Stars 8 or more times greater than our sun explode Supernova: A gigantic stellar explosion (exploding star) Core of star begins fusing elements up to iron Star collapses and explodes violently Supernovas can be seen in other galaxies, sometimes even in small telescopes
  42. 42. Supernovas 100 billion times the sun’s luminosity for a brief moment Brief instant fuse chemical elements higher than iron on the periodic table
  43. 43. M51 Supernova (SN2005cs) Where’s the supernova? A supernova appeared in M51, a bright galaxy in Canes Venatici, in 2005 This supernova was visible in large amateur telescopes
  44. 44. Historic Supernovae 1054, Crab Nebula 1597, Tycho’s Star 1604, Kepler’s Star Supernova 1987A Tycho (top) and Kepler
  45. 45. Supernova 1987A •SN1987a appeared in the Large Magellanic Cloud, a small satellite galaxy of our Milky Way that is visible from the Southern Hemisphere •The supernova was positioned near the Tarantula Nebula, the large red glow in left center of the image to the right Below: Large Magellanic Cloud; Right: March ’97 Time
  46. 46. 1054 Supernova, Chaco Canyon, Crab Nebula This rock art in New Mexico may depict the 1054 supernova The Crab Nebula (M1) is the remnant of the 1054 supernova
  47. 47. M1 StarMap (Taurus) The Crab Nebula is visible as a glowing patch of light in small telescopes, it is the first object in Messier’s list (M1) http://www.eurekalert.org/images/release_gra Ecliptic
  48. 48. Neutron Stars From explosions of massive stars Neutron star, a type of star more massive than the sun but squeezed into a ball 10 miles across Incredibly dense
  49. 49. Pulsars Pulsars are rotating neutron stars Pulsars can send sharp, strong signals towards earth Originally thought to be alien signals (LGM) when first discovered in the 1960’s Pulses range from milliseconds-4 second Pulsar found at center of the Crab Nebula
  50. 50. Black Holes Really massive stars can explode and collapse into black holes Black holes are denser than neutron stars Represent the mass of entire star shrunk into zero-radius object Gravity is so immense, even light can’t escape
  51. 51. Black Hole Terms Event Horizon: Boundary of no return where no light or matter will escape Singularity: Center of black hole, a point of infinite density where the pull of gravity is infinitely strong
  52. 52. Anatomy of a Black Hole Simulated black hole, the intense gravity distorts the light of stars in the background
  53. 53. Black Hole Candidates Cygnus X-1, intense X-ray source located 8000 ly away in Cygnus Believed to be an eclipsing binary star (two stars orbiting), period 5.6 days, with unseen companion Massive black holes may exist at the center of the Milky Way and other galaxies
  54. 54. Cygnus X-1 •Cygnus X-1 is located in Cygnus or the Northern Cross •It is not visible in a telescope, but you can identify its general area using a star map
  55. 55. Center of Milky Way: Sgr A Sagittarius A is a radio source at the center of the Milky Way and likely marks the location of a black hole Sgr A
  56. 56. Stellar Evolution Summary Sun-like stars Protostar Main sequence star (yellow star) Red giant Planetary nebula White dwarf Black dwarf Massive Stars Protostar Main sequence (blue star) Red supergiant Supernova Neutron star or black hole (depending on mass)
  57. 57. Summary

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