Formation and Development of the Milky Way


  • Present design of the Milky Way:

    Large nucleus at the center: lots of stars, gas and dust.

    It probably also has a bar through the nucleus.

    Spiral arms in a disk, gas, dust, younger stars, open clusters.
    Mostly circular, clockwise orbits.

    Spherical halo of globular clusters, with older, red stars.
    Highly elliptical, randomly oriented orbits.
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The abundance of heavier elements decreases throughout the disk, as we go from the nuclear bulge toward the outer edge.

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Probable cause of the differences in abundance of heavier elements:
    Original stuff of the universe was mostly hydrogen and helium.
    Heavier elements are produced by fusion in the cores of stars and in supernovas.
    Novas and supernovas explode their enriched innards into interstellar clouds.
    Therefore, older stars contain mostly the original hydrogen and helium,
    but more recent stars are made from clouds which are enriched by heavier elements.
    Usually only the outer layers of a star are blown away,
    so the enrichment of heavier elements is much less than 1%.

Imagine a "wave" is going on in a circular football stadium.
The wave travels around the stadium, but individuals hardly move at all.
The motion of the people is vertical, while the wave travels horizontally.
We call this a transverse wave, since the medium moves perpendicular to the wave.
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Now suppose that the fans move left-right instead of up-down.
From the blimp overhead, there would appear to be a "pinch" of people
travelling around the stadium, while individuals move only a foot or two
as the wave passes through their place in the stands.
This is a compression wave, also called a longitudinal wave,
since the motion of the medium is parallel to the motion of the wave.

Now suppose the fans are all running, equally spaced, around a circular track.
If Dolly Parton and/or Garth Brooks were to stroll around the inside edge of the track,
there would probably be an increased density of fans in their vicinity, trying to see them.
From the blimp overhead, there would be a "pinch" of people travelling at one speed,
while the individual people would be travelling at a higher speed.

In a similar fashion, stars may be orbitting the galaxy at one speed,
while a spiral compression wave is revolving at a slower pace.
As stars and clouds move into the leading edge of a spiral arm,
they are squeezed closer together, causing more stars to form.
Thus, we see a higher density of young stars in the spiral arms.
Our sun is passing through a spiral arm on its way around the galaxy.
A companion galaxy may trigger the formation of a spiral arm.
Similar spiral structures have been observed in computer simulations of galaxies.
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