Presentation: Milky Way history

A computer simulation of a galaxy such as ours, the Milky Way, being built up from the accumulation of many small galaxies over time. This visualization is based on the best evidence and models we have so far to account for different types of stars in the Milky Way... (Video link: "Simulation of the formation of a galaxy similar to our Milky Way.")

...evidence that we will be looking into more detail, much like a paleontologist does in reconstructing the past based on present-day evidence.

This presentation concludes a series on characteristics of our Milky Way galaxy (shape/size/mass, and spiral arms).

First, metallicities of stars.

The early universe started out with basically just only hydrogen, and the first-generation massive stars then gathered this hydrogen and promptly got to work, fusing it into heavier elements in their cores, while their outer layers remained relatively pristine, free of metals.

When these first-generation massive stars then exploded as type II supernovae, they spread out the unused hydrogen from their outer layers out into space, along with the metals produced in their cores. The universe is progressively getting dirtier as each new generation of stars convert more and more hydrogen into metals. (Video link: "Supernova explosion (artist's impression).")

This gradual "dirtification" of the universe is a good thing--we're made up from this dirt--you, and I, this planet we live on, and our sun. "There are pieces of star within us all."

A young massive main-sequence star today will be metal-rich in its:
(A) core.
(B) outer layers.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)
(E) (Unsure/guessing/lost/help.)

A red dwarf in a very old star cluster today will be metal-rich in its:
(A) core.
(B) outer layers.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)
(E) (Unsure/guessing/lost/help.)

Second, metallicity clues to how the Milky Way was formed.

Since older stars born a long time ago will be metal-poor, and newer stars born more recently will be metal-rich, observing stars with few or many metal absorption lines from atoms in their outer layers then allows us to distinguish between older and newer generation stars, much like looking at how much pollution ("metals") there in a landscape.

In the Milky Way we can make a broad distinction between stars out in the halo, versus the stars in the disk. Halo population stars with tilted orbits are metal-poor, and must be the Milky Way's first-generation stars. Many disk population stars are metal-rich, and so the disk has a lot of newer stars. This suggests a simple "monolithic collapse" model of Milky Way evolution, where hydrogen for star formation gradually migrated from a spherical shape to its current flattened disk shape. This model is perhaps too simple.

Looking more carefully at the metallicity of stars shows that while halo stars are generally metal-poor, some are slightly less metal-poor than others. Also while disk stars are generally metal-rich, some are slightly more metal-rich than others. This suggests a more complex "bottom-up" model of Milky Way evolution, where it was built from the combination of smaller galaxies over time. (Video link: "Simulation of the formation of a galaxy similar to our Milky Way.")

Don't think of the monolithic collapse model and the bottom-up model of Milky Way evolution as being mutually exclusive of each other--neither model is correct or complete, but complement each other as they are both consistent with the overall and the detailed metallicity evidence that we see.

In the subsequent in-class activity you will compare the metallicities of the different population of stars in the Milky Way.