Astronomy 100


Lectures Table of Contents Astro 100

The Milky Way: Our Galactic Home

    General Properties 


  1. Where do Stars Live? The Concept of Galaxies
  2. The Size and Shape of the Milky Way

Terms to Know

period-luminosity relation
open star cluster
globular star cluster
island Universe
spiral arms
rotation curve
dark matter

1. Where do Stars Live? The Concept of Galaxies

What is the band of light we know as the Milky Way? What is its relation to the Sun and the constellations? Is it the only one of its kind?

A galaxy is made up of stars the way a body is made up of cells, or a beehive of bees. All the stars you can see with your naked eye are in the Milky Way Galaxy. So are all the star clusters, both globular and open . So are all the supernova remnants, the planetary nebulae, and the gaseous nebulae that are forming stars now, such as the Orion nebula.

Until Copernicus, the Earth was assumed to be the center of the Solar System; similarly, the Solar System was for millenia thought to be the center of the Milky Way Galaxy.

Two kinds of objects together helped shake that assumption, which we now know to be false:

  1. Globular Clusters, and
  2. Cepheid variable stars

Henrietta Leavitt, astronomer at Harvard, recognized in 1912 that Cepheid variable stars in the Large Magellanic Cloud (all at roughly same distance from Earth) obeyed a Period-Luminosity Relation: the brighter they are, they slower they blink. So you can use the period of their "blink" to determine their luminosity, and thus to infer their distance!

This is a fundamental tool astronomers use to measure distances to all kinds of star clusters and galaxies in the Universe

Harlow Shapley applied the P-L Relation to globular clusters of stars and determined that the clusters were not centered around the Sun, but rather around a point thousands of parsecs away, in the direction of the constellation Sagittarius! The Sun wasn't at the center of the Milky Way Galaxy at all!

2. The Size and Shape of the Milky Way

The true shape of the Milky Way Galaxy is hard to discern from the inside. It's like trying to figure out what your house looks like without going outside of it!

It's easier if we look in the infrared and radio, because optical light is absorbed by the enormous clouds of dust that lie between the stars in the Galaxy, but IR and radio radiation passes right through. Combining information from all different wavelengths and all different kinds of objects -- stars of all types, atomic gas, moleclar gas dust, supernova remnants, planetary nebulae, open clusters, globular clusters, etc. -- we can map out the shape, size, and contents of the two major components of the Galaxy:

Disk Component Spherical Component
flat pancake/frisbee "Dandelion puff"
Diameter = 25 kpc Diameter = 25 kpc (halo)
Thickness = 1 kpc Diameter = 4 kpc (bulge)
blue red
gas and dust no gas or dust
open star clusters globular star clusters
bright young stars old dim stars
star formation no star formation
spiral arms little or no structure
smooth rotation (whirlpool) chaotic orbits, like swarm of bees
metal-rich stars = Population 1 metal-poor stars = Population 2

The Sun lies in one of the spiral arms in the disk component, about 8.5 kpc (25,000 ly) from the Galactic center.

The orbital speeds of stars at different radii from the center of the Milky Way imply the mass of the Milky Way: about 1012 MSun. But it may be even higher: rather than orbit slower and slower at greater distances from the Galactic center, like Pluto compared to Mercury going around the Sun, stars seem to orbit just as fast at 10 kpc from the Galactic Center as they do at 2 kpc, and just as fast at 15 kpc -- beyond the visible edge of the Milky Way's disk -- as they do at 10. These rotation curves imply that there is more than meets the eye: much or even most (90%?) of the Milky Way is made of something OTHER than stars or gas or dust, something that has mass and therefore gravity but that doesn't shine. This dark matter is one of the great mysteries of contemporary astrophysics.

How the Milky Way Was "Discovered"

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.

The Origin of the Milky Way

Terms to Know

top-down formation
bottom-up formation

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 1998, 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.
Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100