|Formation and Development of the Milky Way|
Copyright by snow, linked from
|Abundance of elements is
about the same as our sun.
(about .13%, > helium.)
|Located in spiral arms, open clusters,
near clouds of gas and dust.
Tend to be younger (< 5 B years).
|Abundance of elements
heavier than He is very low.
(about .001%, > helium.)
|Located in globular clusters, nuclear bulge,
where star formation has ceased.
Also in "high velocity" stars
passing through the disk.
Mostly old, red stars.
|Abundance of elements is
between population I and II.
|Located in the disk, outside the spiral arms.|
The abundance of heavier elements decreases throughout the disk, as we go from the nuclear bulge toward the outer edge.
(Image copyright by Snow, linked from
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%.
|Many galaxies have spiral arms, with enhanced densities
of gas, dust and young, hot stars along their leading edges.
These might be due to streamers ejected from the galaxy at formation,
which wind around the galaxy as it revolves.
However, most spiral galaxies have revolved 20-40 times since forming,
and spiral arms are only wound 1-3 times or less.
It appears that the stars, gas and dust are not lodged in a spiral arm.
A spiral arm is a compression wave which stars, etc., are passing through.
Image copied from
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.
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.
Formation of the Milky Way
About 10-16 billion years ago: a huge cloud of mostly hydrogen and helium.
Throughout the cloud, sub-clouds began to form into stars and clusters of stars.
Gravity pulled the remaining gas and dust inward. Rotation
flattened it into a disk.
The stars and clusters previously formed have remained in large, random, elliptical orbits.
The disk remained dense enough to keep on producing stars.
Large, early stars died and exploded, "recycling" their material into interstellar clouds.
So gas and dust were kept dense enough to keep up star production for billions of years.
A nearby galaxy (or two or three??) may have triggered a spiral
In the enhanced density of the spiral arms, new star formation is still going on today.
The variations in the abundance of heavier elements is due to the
"recycling" of material through stars, novas and supernovas into the interstellar clouds.
If more recycling has occurred in a region, stars formed there have more heavy elements.
Since globular clusters formed very early, they contain almost all hydrogen and helium.
Disk population stars formed later, in the flattened disk, and are slightly enriched.
Much more recycling has occurred in the spiral arms, where stars are greatly enriched.
Near the nuclear bulge, the density was greater, so recycling was at a faster pace.
This brought about the gradual decrease in abundance of heavier elements
for stars in the spiral arms, as we look farther from the nuclear bulge.
The above scenario for the formation of the Milky Way seems
what is observed throughout the Milky Way today.
Notice the similarities between the developments of the Milky Way and the solar system:
Each started as a cloud, collapsed into a disk containing objects in circular orbits,
and left behind a spherical cloud of condensed objects in random, elliptical orbits.
In the distant future, it may be that star production and recycling
will grind to a halt.
The Milky Way may eventually have no spiral arms and become an elliptical galaxy.