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

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