Messier: The “M” in M31

What does the “M” in M31 or M11 stand for? Messier [Me-Si-Eh]

Charles Messier

Charles Messier (1758-1772), a French astronomer, identified about 110 diffuse fuzzy objects that he named “Messier objects.” Messier then cataloged these objects in his Messier Catalog. He also discovered 13 comets; finding comets was a way to make a name astronomers of the 18th century).

Messier Catalog

Orion Nebula

M42: Orion Nebula

  • local region in the Milky Way (~1,300 light years away) with new stars
  • appears mostly red due to hydrogen gas abundance

M82: galaxy

  • ~12 million light years away
  • clouds of glowing hydrogen blown out, released by recent star formation

M31: Andromeda Galaxy


  • hundreds of nebulae (discovered 20th century)
  • with George E. Hale’s idea and Hooker’s money –> the Hooker Telescope (100-inch in diameter, 11 years to build, $100 million)

Today, the Sloan Digital Sky Map holds 15 Terabytes of data on the Universe.


Nebulae and Star Formation

Orion Nebula

Nebulae: a cloud of dust and gas that we see in light

  1. Emission Nebulae or Bright Nebulae: a glowing gas (hydrogen); e.g. Great Nebula in Orion, heated by the Trapezium
  2. Absorption Nebulae or Dark Nebulae: dark dust clouds; e.g. Horsehead Nebula
  3. Reflecting Nebulae: reflecting dust cloud; e.g. Pleiades in Taurus
  4. Planetary Nebulae: excited by central star; e.g. Dumbbell Nebula
  5. Cirrus



Stars form in Giant Molecular Clouds about 100,000 to 1 million solar masses. A few thousand in the Milky Way Galaxy, Giant Molecular Clouds break into denser bits, contract, and eventually form stars. The Orion Molecular Cloud has about 500 stars. The Trapezium and the Orion Nebula have solar masses of matter with young stars.

  1. Non-stellar galactic objects reside in HII regions with molecular clouds of pre-main sequence stars and dense clumps of dust.
  2. Protostars and newborn stars about 1/2 to 1 solar mass reside in Molecular Clouds.