FA 4_Milky Way and Galaxies

• Galaxy

  
  

  • collection of stars, planets, hydrogen gas, and interstellar matter and debris
  • bound together by mutual gravitational attraction
• Immanuel Kant

  • argument: the galaxy is flattened with the Sun near the center
  • if the galaxy is spherical, we should be seeing the same number of stars in all directions
  • led to conclude that the galaxy is disk-like rather than a ball of stars
• William Herschel

  • supported Kant's idea
  • counted the number of stars in different directions
  • most of the stars lay in a flattened structure encircling the sky
  • found out that the disk of the galaxy is five times longer than its width
• H II Regions

  • characterize the spiral nature of a galaxy
  • made up of free electrons and protons ionized by young stars
• Jacobus Kapteyn & Harlow Shapley

  • used globular clusters to estimate the size of the galaxy
  • the galaxy is around 25,000 parsecs or 80,000 light-years
  • based on the galaxy's mass, there could be around 200 billion stars within it
• Disc/Disk

  • extends to about 30 kiloparsecs
  • made up of stars, open clusters, and nearly all our galaxy's dust and gas
  • has a subtle swoop
  • spiral arms
• Halo

  • globular clusters surrounding the disk
  • mostly old stars with little to no gases
  • is where majority of the galaxy's mass is in (dark mass comprising the larger portion)
• Bulge

  • dense cloud of stars surrounding the center
  • radius of about 2 kiloparsecs
  • contains little gas and dust
  • is where formation of stars is scarce
• Sun

  • located somewhere along the disk between the center and its edge
  • swirls around the center
  • takes 225 million years to complete its tour around the center of the galaxy
• Elliptical Galaxy

  • mostly reddish
  • lacks hot and bright stars
  • indexed with numbers 0-7 based on their roundness
  • E0: spherical
  • E7: elliptical
• Spiral Galaxy

  • has a disk with two or more arms swirling out of the center
  • all spirals have halos
  • contains young stars and sites of star formation
  • more luminous galaxy (easier to notice)

  CLASSIFIED INTO SEVERAL CATEGORIES

  
  

  • S: Spiral
  • SB: Barred Spiral

  
  

  • a: large nuclei and few hot stars
  • b: intermediate values
  • c: small nuclei and many hot and bright stars
• Irregular Galaxy

  • lacks visible patterns and uniform shapes
  • seen as splotches and patches of chaotic mixture of stars, gas, and dust
  • shape could be a result of gravitational interaction or collisions with other galaxies
• • The solar system is located near a small, partial arm called the Orion Arm (between the Sagittarius and Perseus arms)