The Milky Way Galaxy External Links page.

What is a Galaxy? Some history: Galileo and stars in the Milky Way; Spiral nebulae or island universes? The issue was resolved only in the 1920's using Cepheid stars. The view from here: Like being inside a forest, seeing only the trees; 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.

Our position: We are about 28,000 light years from the center [and 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 Main idea: Similar to the Andromeda galaxy M31, contains about 100 billion stars. Disk: Diameter about 100,000 ly, thickness varies (about 1000 ly here), slightly warped; We get the shape from 21 cm radio waves emitted by interstellar matter, or 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 black hole with 2-3 million solar masses, 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 apparently random, 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. New structure: A giant ring of several hundred million stars outside the galactic disk was announced in 2003.

(* Website by Luca Bombelli <bombelli@olemiss.edu>; Content of this page last modified on 12 Jan 2003 *)