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.


The Milky Way – Timeline

The Milky Way Galaxy


1750: Immanuel Kant: advocated the “lens-shaped” distribution of stars, or an “island universe” with galaxies like the Milky Way

1785: William Herschel + Caroline (wife): made the first attempt to determine the shape of the galaxy; found few stars near the edge  and many stars toward the center; determined the galaxy to be an irregular “grindstone” or hockey puck

1900: If the Sun is at the center of the Universe, why is it not brighter at the center? Gas and gas prevent seeing far toward the center and light absorbed and refracted by Earth’s atmosphere  only allows us to see a small portion of the galaxy

Harlow Shapley and Herbert Curtis

1920: The Curtis- Shapley Debate

Harlow Shapley: rising star and “golden boy” of astronomy

  • Since globular clusters are not uniformly distributed uniformly around the Sun, the center of the Milky Way must be centered 30,000 light years away
  • Concluded that the Milky Way is much larger than previously believed (>100,000 light years in diameter)
  • The “nebulae” seen are not island universes but contained in the Milky Way

Herbert Curtis: established astronomer and respected

  • Spiral nebulae are galaxies out side the Milky Way, with high recessional velocities
  • Predicted that these spiral nebulae are the right size to be galaxies –> “huge” galaxy idea

While Shapley advanced that the Sun is not at the center of the galaxy and the galaxy is much larger than believed, Curtis argued that since spiral galaxies are external, there must be more big galaxies.

Who was right? BOTH. Who was wrong? BOTH.

Shapley was right the Sun is not at the center of the Universe. Curtis was right the Universe is composed of many galaxies. However, the size of the Milky Way was in-between their estimates.

1920s-1930s: Edwin Hubble: With the Hooker Telescope on Mt. Wilson, Hubble observed Cepheid Variable stars in the Andromeda Galaxy (M31); Cepheid Variable stars are 500-10,000 brighter than the Sun (in absolute magnitudes)

  • 1920s: Discovered that M31’s distance is too large to be within the Milky Way; M31 is a galaxy like the Milky Way
  • 1930s: Further understanding of the distances and distribution of globular clusters; the scientific community accepted that they underestimated the size of the Milky Way and the Sun is not at its center

The Milky Way – Structure and Origin

The Milky Way and Its Magellanic Clouds

The Milky Way and Its Magellanic Clouds

In the Southern Hemisphere, the Magellanic Clouds, or the galaxy’s satellite galaxies (revolves around Milky Way), are visible. The Magellanic Clouds are named for the Portuguese explorer Ferdinand Magellan, the first circumnavigator of the world. Because of interstellar dust (rocky planets and other material), we can only see 6,000 stars, but the Milky Way has 100 billion stars total. The farthest are 4,000 light years away. Earth’s atmosphere smears the sky, so stars appear to twinkle. About 10^6 stars— old as the universe— inhabit In globular clusters (~200 in Milky Way’s halo).

All pictures of the Milky Way are artists’ conceptions because no telescope can travel high enough (billions of light years) to capture the entire galaxy.

Milky Way – Structure

Shapley’s Subdivision of the Milky Way

  1. Nuclear Bulge: (10^6 solar masses) nucleus in the center, old stars (red)
  2. The Disk: (10^11 solar masses) thin, diffuse layer of material revolving around the bulge; the Sun is half-way on the disk; all young stars
  3. The Halo: hot gas about 100,000 K
  4. Galactic Corona: mass exists but unseen; 5-10 times as much mass as the nucleus, disk, and halo together, 95% of galaxy mass unknown matter
  • Visible Matter: 96% stars, 4% interstellar gas


  1. (13.6 billion years ago) A gas cloud of 75% hydrogen and 25% helium with mass ~ 1 trillion solar masses
  2. Contraction and rotation form spherical shape
  3. Inner part flattens to form disk of younger stars
  4. Galactic rotation forms spiral arms
  5. Supernovae gives off more heavy elements that eventually become the Sun

Black Holes – Formation

Black Hole

Black holes form after supermassive stars (> 10 solar masses) explode as Type II supernovae. The remaining cores of these stars range from 2- 103 solar masses. Nothing is strong enough to hold the remaining mass against the force of gravity and the dying supermassive stars collapse into black holes. Nothing escapes from black holes, not even light; thus, the escape velocity of black holes is the speed of light. Since nothing can travel faster than the speed of light, nothing can escape. Matter that disappears from black holes loses contact with the rest of the Universe. Black holes are a consequence of Einstein’s theory of gravity, or General Relativity.

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.

Interstellar Medium – The Material Between Stars


– Interstellar Medium

Interstellar Medium

Interstellar Medium is gas and dust between stars, nebulae, and giant molecular clouds (basic building blocks of galaxies in star formation). The four types of matter in interstellar medium are: interstellar dust, interstellar atoms, interstellar molecules, and interstellar snowballs.

Interstellar Dust

  • Interstellar Reddening: dust that scatters blue light and causes stars to look redder
  • Extinction of Obscuration: high dust content that diminishes the brightness of stars, by as much as 25 magnitudes
  • Can be smaller than smoke particles
  • Consists of graphite, silicates, or ices
  • In core of heavy elements (e.g. iron, magnesium), mantle of organic compounds (oxygen, carbon, nitrogen), and outer mantle of ice


  • Radio waves = longest wavelength of electromagnetic waves
  • Brightest optical objects not necessarily the brightest radio objects
  • e.g. Taurus A (Crab Nebula) and Sagittarius A (center of the Milky Way Galaxy)
  • Radio Spectral Line: the frequency or wavelength at which radio noise is slightly more or less intense
    • Hydrogen: 21 centimeter line
    • Radio spectra lines of molecules
      • OH (hydroxide): 1963
      • H20 (water): 1968
      • NH3 (ammonia): 1968
    • Over 50 molecules in interstellar space
    • Gives information on temperature, density, and motion
    • Molecular absorption line in UV

Interstellar Molecules

  • Molecules: two or more atoms bound together (e.g. H2O, CO, CH4, OH, H2, NH3)
  • Give absorption or emission bands
  • Observable in very cold, low density interstellar environments

Interstellar Snowballs

  • Between the sizes of  grains and comets
  • Composed of water, carbon, silicates, and other molecules

Interstellar Regions

  1. HI region: 200 K
  2. HII region: 10,000 K
  3. Molecular clouds: 50% gas in our galaxy
  4. Hot interstellar medium: 1 million K, super-heated gas from expanding supernova blasts (up to 90% of total volume)
  • HI Region
    • High density of neutral hydrogen atoms about a million atoms per cubic centimeter (e.g. Orion Nebula)
    • ~ 200 K
  • HII Region
    • Hydrogen with electron removed; e.g. ionized hydrogen gas (in emission nebulae)
      • Average density of hydrogen elsewhere is 1 atom per cubic centimeter
    • ~ 10,000 K