THE MILKY WAY GALAXY: CLUES TO FORMATION

Monolithic Collapse Model

In the 1960s, astronomers developed a model of galaxy formation called the "monolihic collapse" model. A galaxy would start out as a large collection of gas about 200,000 light years across. The steps are as follows: (1) As the gas contracts under its own gravity stars form throughout what will eventually become the halo and bulge. (2) As the gas contracts further, its rotation about the spin axis causes the gas in the plane to contract more slowly, forming what will eventually become the disk. (3) Stars start to form in the flattened region of of the future disk. At this stage, stars formed in the halo are now freely orbiting the gas cloud. (4) The processes continues until (5) the three main components of the galaxy are in place: the halo, the bulge, and the disk.

As for the stars: (1) The halo and most of the bulge stars formed first (2) no more halo stars formed, the bulge stars continued to form, and disks stars started to form. (3) Finally, the halo and bulge do not form stars anymore, while the disk continues to form stars. Thus, the halo stars are the oldest, the bulge stars are the decond oldest, and the disk is a mix of old and new stars.

Because stars form metals in their cores and the blast them back out into the ISM, and new stars from from this metal eniriched ISM, each generation of stars forms with a higher metal content (higher metallicity). Thus the oldest stars in the Milky Way have the lowest metallicities and the youngest stars have the highest metallicities.

Clues to the development of the Monolithic Collpase Model

The clues are (1) the motions or orbits of the stars, (2) the metallicity of the stars, (3) the ages of the stars, and (4) the locations of the stars. To simplify, astronomers divided stars into two "populations".

Population I - younger, higher metallicity stars found in the disk
Population II - older, lower metallicity stars found in the halo and bulge

From oldest to youngest (and therefore also from lowest metallicity to highest metallicit...

Extreme Pop II stars reside in the halo. They are the oldest and lowest metallicity stars. They orbit the galaxy in ellipses of any orientation around the galaxy center.

Moderate Pop II reside in the bulge. They are somewhat younger than halo stars and have somewhat higher metallicity than halo stars. They orbit the galaxy center in ellipses of any orientation, but the orbits are small, confining them to the bulge.

Moderate Pop I stars reside in the disk between the spiral arms. Their metallicity is higher than Population II stars. Their orbits are confined to the plane (disk) of the galaxy and are somewhat circular, but not exactly.

Extreme Pop I stars are the very youngest, highest metallicity stars. They reside in the spiral arms of the disk. Their orbits are confined to the plane (disk) of the galaxy and are very nearly to circular.

Pop I stars are second and third generation stars that formed from the metal enriched remains of the dead first generation of stars. The first generation stars are Pop II. The location of these stars in the galaxy provide a clear picture of the story of star formation in our galaxy (and presumably, other galaxies).

The above trends in the notions, metallicities, ages, and orbits of these populations of stars provide the data (information) from which the monolthic collaps model was developed. Halos stars formed first, followed by bulge stars, followed by disk stars. Thus, it would follow that the halo formed first, the bulge second, and the disk last. The reason sprial arms are so visible is that they contain the newest "O" and "B" stars. This is because the gas clouds in spiral arms are where stars form. "O" and "B" stars are young and bright (easy to see). As stars age, they migrate out of the spiral arms.

Star formation in spiral arms is described by the density wave theory. In this theory, the sprial are analogous to the build up of cars in traffic. The gas moves around the galaxy, but piles up near the congested sprial arms. Stars then form in the congested regions of the arms. The gas that did not partkae in star formation then continues through the spiral arm and eventually catched up to the next sprial arm. This sustains the appearance of the spiral arsm, which again, are highly visible because they contain the luminous O and B stars.