Lecture 17: The Galaxy, A Historical Overview

"Join me Luke, and together we will rule the Galaxy!"

Darth Vader, Star Wars

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    • Date: April 4, 1995
    Reading Assignment: pp. 520-532

    Description : historical overview of the Galaxy

    Objectives

  • be able to describe why pulsating variable stars change their brightness
  • be able to describe why variable stars are well suited for measuring the distance to distant clusters and galaxies
  • be able to describe Historical attempts to understand the size and shape of our Galaxy
  • be able to describe the structure of our Galaxy, including the spiral arms
  • be able to explain the observed distribution of red and blue stars

    • Lecture Outline

    Slide # 1: The Galaxy- A Historical Overview Slide # 2: Today's Lecture

  • variable stars
  • pulsating variables
  • the Curtis-Shapley debate
  • the size our Galaxy
  • the nature of spiral nebula
    • Slide # 3: Variable Stars
  • eclipsing binary
  • white dwarf's in binary systems
  • accretion disks, nova, supernova
  • supernova
  • pulsars
  • pulsating variables
    • Slide # 4: Eclipsing Binary
  • binary stars can eclipse each other
  • orbits aligned so one star passes in front of the other
  • the total amount of light from the stars varies because of the eclipses
    • Slide # 5: Binary Star Types (GRAPHICS)
  • Eclipsing Binary Stars
    • Slide # 6: Eclipsing Binary (GRAPHICS)
  • Light Curve
    • Slide # 7: Algol
  • binary star system
  • star 1 = 3.7 solar mass B8V (main sequence)
  • star 2 = 0.8 solar mass K8IV (subgiant)
  • eclipsing binary star system
  • short orbital period and very close orbits
    • Slide # 8: White Dwarfs in Binary Systems
  • accretion disks and cataclysmic variables
  • nova explosions
  • type I supernova
    • Slide # 9: Accretion Disks
  • material forms disk as it is transferred to a compact star
  • disk emits energy from collisions and compression
  • possibly more energy than the two stars
    • Slide # 10: Accretion Disks (GRAPHICS)
  • mass transferred to a compact star
    • Slide # 11: Cataclysmic Variables
  • the accretion disk can become unstable
  • the accretion disk can suddenly heat up and become very bright
  • the total light of the system increases
    • Slide # 12: Nova (GRAPHICS)
  • hydrogen rich material compresses and then ignites (nuclear fusion) on the sur
    • Slide # 13: Type I Supernova (GRAPHICS)
  • white dwarf mass exceeds Chandarsekhar limit
  • star collapses and carbon detonation occurs
  • MUST OCCUR IN A BINARY SYSTEM
  • does not produce a neutron star
    • Slide # 14: Type II Supernova
  • core collapse of SINGLE MASSIVE STAR
  • core made of degenerate iron
  • mass of core exceeds Chandrasekhar mass
  • electrons absorbed into nuclei
  • no pressure from electrons, so core collapses
  • very luminous
    • Slide # 15: Pulsars (GRAPHICS)
  • the lighthouse effect
    • Slide # 16: Pulsating Variable Stars (GRAPHICS)
  • some stars pulse in size and temperature
  • SIZE CHANGE IS ONLY ABOUT 15% OF RADIUS
  • they are NOT in hydrostatic equilibrium
    • Slide # 17: Pulsing Variable Stars (GRAPHICS)
  • internal pressure changes from an instability caused by ionization
  • ionization changes the stars OPACITY
  • these stars are always POST MAIN SEQUENCE
  • stars change luminosity regularly
    • Slide # 18: Light Curves of Pulsating Variables
  • Cepheid Variables
    • Slide # 19: The HR Diagram (GRAPHICS)
  • 10 Billion Years
    • Slide # 20: Cepheid Period-Luminosity Relationship
  • discovered by Henrietta Leavitt in 1908
  • period and average luminosity of Cepheids are related to each other
    • Slide # 21: Period-Luminosity Relationship (GRAPHICS)
  • the pulsation period of Cepheid variables is directly related to their average
    • Slide # 22: Constellation Corner (GRAPHICS)
  • Constellation De Jour
    • Slide # 23: Looking North (GRAPHICS)
  • March 6 - 8pm - NW - 4.0 - Fairfax
    • Slide # 24: Bootes (GRAPHICS)
  • Fairfax 11pm - April 4 - NE - 4.0
    • Slide # 25: Bootes (GRAPHICS)
  • Fairfax 11pm - April 4 - NE - 4.0
    • Slide # 26: Bootes (GRAPHICS)
  • Fairfax 11pm - April 4 - NE - 4.0
    • Slide # 27: The Curtis-Shapley Debate (GRAPHICS)
  • what is the size of our galaxy?
  • what is the nature of spiral nebula?
    • Slide # 28: Historical Notes
  • occurred in April 1920
  • did not resolve the issues
  • more data was needed
  • both scientists were partially correct
  • and partially wrong
    • Slide # 29: The Curtis View
  • spiral nebula are not a part of our galaxy
  • our galaxy is a small part of a huge universe
  • the galaxy is small
  • size about 10,000 parsecs
    • Slide # 30: The Shapley View
  • spiral nebula are part of the Galaxy
  • just a normal gas cloud with a strange shape
  • our galaxy was very large
  • 100,000 pc in size
    • Slide # 31: Our Galaxy (GRAPHICS)
  • measuring the shape of our galaxy is very difficult
    • Slide # 32: The View from Earth (GRAPHICS)
  • the Milky Way is a faint band of stars that circle sky
    • Slide # 33: The Milky Way
  • we've known these are stars since Galileo
  • easy to resolve even with a small telescope
  • what does this observation tell us about the structure of our Galaxy?
    • Slide # 34: The Galaxy is a Disk? (GRAPHICS)
  • most stars are in a narrow band
  • most stars seem to be in a disk-shaped arrangement
    • Slide # 35: Early Theories
  • Thomas Wright- mid-1700's
  • suggested first disk model
  • Immanuel Kant - late 1700's
  • suggested spiral nebula might be other galaxies
    • Slide # 36: Early Observations - William Hershel
  • built a 48 inch diameter telescope
  • assumed stars were uniform in brightness and had different distances
  • assumed no interstellar extinction
  • counted stars in different directions to determine the shape of our Galaxy
    • Slide # 37: Hershel's Result (GRAPHICS)
  • we are in the center of the Galaxy
  • spiral nebula are probably other galaxies
    • Slide # 38: The Kapteyn Universe
  • around 1900, data still indicated that we were at the center of the Galaxy
  • the nature of spiral nebula was still being debated
    • Slide # 39: Interstellar Extinction
  • interstellar dust absorbs, reddens, and polarizes light
  • we didn't know about interstellar dust before 1900
  • all observations were limited by the absorption of light
    • Slide # 40: Hershel's Observations (GRAPHICS)
  • Hershel only saw about 5% of our Galaxy
  • this view was distorted by variable amounts of dust
    • Slide # 41: Globular Clusters
  • Shapley noticed most of the globular clusters are located in one part of the s
  • perhaps globular clusters are orbiting around the center of our galaxy
  • how do you measure the distance to these clusters?
    • Slide # 42: Cepheid Variables
  • very high luminosity stars
  • found in globular clusters
  • by measuring the period, you can find the luminosity
  • from the luminosity and the brightness, you can calculate the distance
    • Slide # 43: Distribution of Globular Clusters (GRAPHICS)
  • Shapley's result
    • Slide # 44: What about Dust?
  • most interstellar absorption is inside the disk of our Galaxy
  • most globular clusters are outside the Galaxy's disk!
    • Slide # 45: Hershel's Observations
  • the real Galaxy
    • Slide # 46: Other Galaxies
  • variable stars were detected in the Spiral nebula
  • nova and supernova detected first
  • Cepheid variables were detected in M31, the Andromeda Galaxy
    • Slide # 47: Cepheids in M31 (GRAPHICS)
  • observations by Edwin Hubble
    • Slide # 48: The Current Picture
  • our Galaxy is 30 kpc in Diameter
  • it contains 100 billion stars
  • there are other Galaxies beyond our own
  • 100 billion galaxies
  • the Universe is really big
    • Slide # 49: A New Debate
  • what is the nature of Gamma-Ray bursts
  • are they inside or outside our Galaxy?
  • A Debate on April 22 at the Natural Hisotry Museum
  • Lamb vs Paczynski