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

Star Clusters: An Overview

Star Cluster


  • Contain hundreds up to millions of stars
  • Held together by gravitational pull of the stars on one another
  • Stars formed nearly at the same time and the same age

Spiral GalaxiesAnatomy: bulge, disk, and halo

Open Cluster

Open Clusters

  • Contains typically hundreds of stars
  • Irregular shapes
  • Found in the disk region of our galaxy
  • Ages range few million years to few billion years
  • Some young clusters still contain diffuse gas and dust — the material from which the cluster formed

Globular Cluster

Globular Clusters

  • Very dense star clusters
  • Typically 10,000 to 1 million stars
  • Very old — up to about 12-13 billion years old
  • Have much lower abundances of heavy elements than the Sun
  • Found in the halo region of galaxies

*When plotted on the H-R Diagram, star clusters have different turnoff points, or the point where stars being to evolve and die; the turnoff point determines the age of the galaxy

  • Young clusters = turnoff point higher
  • Old clusters = turnoff point lower

Distance to Star Clusters

  • Apparent magnitudes and colors for many stars used to compare with a H-R Diagram that’s calibrated in terms of absolute magnitude

Variable Stars

  • Apparent brightness changes over time
  • Caused by eclipsing binaries or physical condition within a star itself
  • Certain kinds of stars pulsate, or regularly glow and go dark
  • In the “instability strip”: changes in temperature and luminosity, pulsating period ranges from hours to months
  • Light curves: used to plot a star’s luminosity
  • e.g. Mira: long period variable red giant – M3 to M9

Changes in Apparent Brightness of a Cepheid Variable

Cepheid Variables

  • Important class of variables
    • Very luminous super giants
    • Regular light curves with repetition periods of days or weeks
  • Henrietta Leavitt
    • Pulsation period is proportional to the mean absolute magnitude of the star
    • log P α absolute magnitude
      • More luminous Cepheids have larger pulsation periods

Useful in Determining Properties of Star Clusters

  • b = L/ (4∏d²) , where b = apparent brightness, L = intrinsic luminosity, d = distance
  • RR Lyrae Stars: metal-poor horizontal branch stars in the instability strip; common in globular clusters; average absolute magnitude = +0.6
  • Cepheid Variables: period-luminosity relationship; absolute magnitude = -2 to -8 magnitude
  • Type Ia Supernovae: peak luminosity related to the slope of the declining part  of the light curve; at peak of luminosity, absolute magnitude ranges from -17 to -19 magnitude