The Milky Way
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What is the Milky Way?

  • Some history: Galileo and the stars in the Milky Way; Are the "spiral nebulae" clouds or island universes? The issue was resolved only in the 1920's using Cepheid stars.
  • The view from here: Like being inside a forest, and seeing only the trees; Because our view is obscured, it looks as if there are about as many stars in all directions; Need to use radio waves or other forms of radiation to see through interstellar matter.
  • What we now know: The Milky Way is a spiral galaxy containing about 100 billion stars; So are the "spiral nebulae" like M31, each one a distant "island".

Our location: About 28,000 ly from the center [we can only see out to 1/10 of that distance], moving along with the Sun at 220 km/s, almost 500,000 mph!

  Size and Structure of the Milky Way

  • Disk: Diameter about 100,000 ly, thickness varies (about 1000 ly here), slightly warped; We get the shape from 21-cm wavelength radio waves emitted by interstellar matter, or from open clusters, bright stars and nebulae if we see them; We can't yet map the spiral arms as well as those of other galaxies; Contains stars with more heavy elements [population I].
  • Halo: Radius of inner part 60,000 ly, outer part 200,000 ly or more; We get the shape from globular clusters; Contains redder stars, with fewer heavy elements [population II], but 90% or more of its composition is unknown (and may have some clumpy gas clouds).
  • Central bulge: Football shaped, dense, 6 by 4 kpc.
  • Core: Towards Sgr A, near M8; 10 ly across, difficulty to see, look with IR, radio and X-rays; A 3-million-solar-mass black hole, possibly surrounded by many thousands of smaller ones.

Evolution and Motion, Mass

  • Formation: The Milky Way is the result of the merger of previous fragments and smaller galaxies; The flattening developed from rotation.
  • Spiral arms: Just by rotation? Spiral density waves and self-propagating star formation.
  • Motion of nearby stars: Up to tens of pc the motions are apparently random, at about 40,000 mph with respect to us.
  • More distant stars: On average, disk stars revolve around center (bobbing up and down the disk) at 220 km/s, corresponding to a period P = 225 Myr; Halo stars move along huge ellipses in random directions.
  • Mass: Using Kepler's laws and rotation curves; inside the Sun's orbit, there are 1011 solar masses, and for the whole galaxy the value may be more than 1012 solar masses!

  Interesting and Puzzling Aspects

  • Rotation curve: The Milky Way has a differential rotation, but the rotation curve showing speeds at different distances from the center cannot be explained by the matter we see.
  • Anomalous motions: Some packs of stars move on paths unrelated to that of the rest of the galaxy's stars; They seem to be remnants of small galaxies absorbed by the Milky Way.
  • Immediate surroundings: There is a giant ring of several hundred million stars outside the galactic disk, high-velocity clouds in anomalous motions, and small satellite galaxies.

(* Website by Luca Bombelli <>; Content of this page last modified on 25 May 2003 *)