Astronomy 100

Lectures

Table of Contents

Astro 100

Lecture 20
The Origin of the Milky Way

Terms to Know

top-down formation
bottom-up formation

The Copernican Revolution, Revisited

We know the Milky Way has spiral arms -- the Sun lies in one of them. We also observe "spiral nebulae" such as Andromeda (M31), the Whirlpool (M51), and the Pinwheel (M31). What are those? Are they small parts of the Milky Way, like planetary nebulae and star clusters and star-forming regions? Or are they "island Universes" like the Milky Way itself, each containing hundreds of billions of stars, completely outside of the Milky Way and millions of light years away?

This was one of the most important scientific debates of the 20th Century.

Shapley was right that the Sun does not lie at the center of the Milky Way Galaxy but rather many kpc away -- actually about 2/3 of the way from the center of the Milky Way to the "edge", i.e. where the stars appear to end. However, he got a bit carried away with his picture of the Milky Way, which he maintained was a huge system containing everything in the visible Universe. Meanwhile, Heber Curtis maintained that small points of light in some spiral nebulae that suddenly appeared and then faded were actually novae and supernovae -- meaning the nebulae must be millions of light years away.

The issue was settled in 1919 when Edwin Hubble turned the Mt. Wilson 100-inch telescope on M31, the Andromeda nebula, and discovered -- Cepheid variable stars! They appeared so faint that M31 had to be outside the Milky Way. The extragalactic age had dawned. Now we know that in fact all the spiral "nebulae" are external galaxies like the Milky Way, and that ours is but one of literally hundreds of billions of galaxies in the observable Universe. We live on an ordinary planet orbiting an ordinary star orbiting the center of an ordinary galaxy in an ordinary part of a very large, very old, and not very human-centered Universe.

Where did the Milky Way Galaxy Come from?

We don't know for sure!

Galaxy formation and evolution is one of the hot research topics of astrophysics today. Any good theory will need to be able to describe all of the characteristics of the Milky Way listed above, as well as several others, including:

The ages of globular clusters show a range from about 9 to 14 billion years, rather than a single age.
The metal abundances of bulge, disk, and halo stars all imply that those populations formed at different times out of different gas.
The dark matter seems to be distributed not just in the disk but in the halo, perhaps extending to 100 kpc (~300,000 light years) or more.

Two major theories of galaxy formation are top down and bottom up. Top-down theories propose that the Milky Way formed from a huge cloud of gas that collapsed all at once under its own gravity, similar to the way we now believe the Solar System formed. Bottom-up or "hierarchical" theories hold that small things formed first, then collected together into larger things, then larger, and so on until galaxies like the Milky Way were fully assembled. In 2006, bottom-up theories appear to be much more successful than top-down theories at explaining both the broad brush strokes and the complicated details of galaxies like the Milky Way.