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Mathmatical expression of Hubble's Law

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1. The Nuclear bulge
2. The disk
3. The halo
4. The outer halo
   

• Contains the galactic nucleus (evidence seems to suggest that a black hole resides there); center of our Galaxy is located in the direction of the constellation Sagittarius

• At core of galactic center is a source of radio emissions, called Sagittarius A^* or Sgr A^*

•  Has densely packed old stars (Population II), interstellar gas and dust

• Possesses a flattened spherical shape and is about 16,000 light years across

• Outside the bulge and exhibits a pinwheel-like structure (spiral arms)

• Size of disk is approximately 100,000 light-years and it is about 1,000 light-years thick

• Contains a significant amount of gas and dust, and young stars (Population I) located in 4 spiral arms

• Young stars are usually found in the spiral arms because here is where star formation takes place; star formation is the result of spiral density waves. These density waves are what gives rise to the spiral arms.

• Large spherical region surrounding the bulge and disk, about 130,000 light years across

• Contains old stars and globular clusters (Population II), and small amount of interstellar gas

•  Probably contains non-luminous (dark) matter

• Diameter between 200,000 to 300,000 light years

• Gas and Dust
• Stars
• Clusters of Stars
• Nebulae
  

• Main sequence red giants
• White dwarfs
• Neutron stars
• Black holes
  

• Emission Line Nebulae
• Reflection Nebulae
• Dark Nebulae

70% hydrogen, 28% helium, and 2% heavy elements by mass.

Edwin Hubble

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1.  Spiral galaxies
2. Elliptical galaxies
3. Irregular galaxies