ASTROOOOOO

• Due to obscuring dust, we can only "see" about 10% of the Milky Way
• Infrared and Radio waves can penetrate the galactic dust clouds and yield information about the nature of our galaxy
• To determine the vast galactic distances, we need a new type of distance indicator
• Variable stars


  Giant stars go through a variable star phase, where the star pulsates in size, temperature and brightness
• Variable star brightness


  The brightness of the star varies with a period ranging from a few hours to several months
• The Large Magellanic Cloud is a satellite galaxy of the Milky Way
• Henrietta Leavitt (1912) studied Cepheid variable stars in the Large Magellanic Cloud and found a period-luminosity relationship
• The period of some variable stars are correlated with their intrinsic brightness
• 3 Types of variables with known period-luminosity relations


  • RR Lyrae stars
  • Type I Cepheids
  • Type II Cepheids
• We live in a great wheel-shaped star system: The Milky Way
• The Sun is one of over 200 billion stars in our galaxy
• Our galaxy is about 100,000 light years wide
• The Sun orbits once every 250 million years
• The Milky Way is only one of billions of galaxies in the universe
• The size and structure of our galaxy as well as the nature of other galaxies, was unknown 100 years ago
• Once a star's actual brightness (luminosity) is known


  Its distance can be estimated from the star's apparent brightness
• The shape of the Milky Way galaxy as deduced from star counts by William Herschel in 1785: The solar system was assumed to be near the center
• In the early 1920s, Harlow Shapley used variable stars to find the distance to star clusters
• 2 Types of star clusters in our Galaxy


  • Open clusters
  • Globular clusters
• Shapley noticed that open clusters all lie near the Milky Way in the sky
• Globular clusters were found far from the plane of the Milky Way, but concentrated on one side of the sky centered on the constellation of Sagittarius
• Shapley found the distances to globular clusters using RR Lyrae variables
• Shapley found that the globular clusters were centered on a point thousands of light years away in the direction of Sagittarius
• Shapley postulated that this point was the center of our galaxy
• Shapley quadrupled the measured width of our galaxy. We are about 30,000 light years away from the center
• Components of the Milky Way Galaxy


  • Spherical component
  • Disk component
• The spherical component formed first and then the rotating galaxy collapsed to a disk shape
• Disk component


  • Has gas and dust / young stars middle to lower sequence
  • Contains all the matter confined to the plane of rotation
  • Stars rotate in the plane of the disk around the center of our galaxy
• Stellar structures in the disk


  • Associations (10-100 stars)
  • Open clusters (100-1000 stars)
  • Emission nebulae (HII Regions)
  • Dust and Molecular clouds
• The disk is approximately 3000 light years thick, but most of the dust and young, bright stars are confined to a disk only 300 light years thick
• Spiral Arms


  • Long spiral patterns consisting of bright young stars, clusters, gas and dust clouds
  • Regions or "ripples" of compression that move through the disk
  • Bunch up material - stars and nebulae
  • Induce star formation
• Bright (O+B) stars are good tracers of the spiral arms since they are short lived and very luminous
• Spherical component


  • Little dust and gas, old stars
  • Halo: scattered stars in random, elliptical orbits about the nucleus
  • Most are old, cool, low luminosity stars
  • 100 globular clusters
  • About 75,000 light years in diameter
  • About 10-13 billion years old
• Nuclear Bulge


  A cross between the disk and halo in composition
• Stellar Populations


  • Population I stars (2nd, 3rd, and 4th generation stars)
  • Population 2 stars (early generation stars)
• Population I stars


  • Are 2nd, 3rd, and 4th generation stars (recycled stars)
  • Contain 1.5-3% "heavy elements" ("metals")
  • Primarily found in the disk of our galaxy
• Population 2 stars


  • Are "first" (early) generation stars
  • Almost entirely Hydrogen + Helium
  • Less than 1% heavy elements
  • Found in the Halo and Globular clusters
• Long wavelength radio waves can penetrate the galactic dust and trace the spiral arms
• Cold, neutral hydrogen clouds

  Emit 21 cm waves and are good tracers of spiral arms
• Hydrogen atom ground state


  • The energy is slightly lower if the electron spin is opposite to the proton spin
  • When an electron flips from parallel to opposite spin, it emits a 21 cm photon