Lecture 21 - Galactic Dynamics (4/6/99)

Prev Our Galaxy --- | --- Galxies and Quasars Next

ASTR012 Reading:
Chapter 19, 20 (ZG4)

pages 86-89

M51, the Whirlpool galaxy. (Courtesy SEDS)
? Key Question: What is the shape and size of our galaxy?
! Key Principle: Differential Rotation
# Key Problem: Relate the velocities of stars to galactic properties


  1. Galactic Rotation and Kinematics
  2. Galactic Dynamics
  3. The Galactic Center

Our Galaxy in Outline - continued

  1. The Mass of the Galaxy
  2. Stellar Populations (see below)
  3. Evolution in Star Clusters (see below)
  4. Disk Kinematics
  5. Spiral Arms
  6. Halo Kinematics
  7. History of Our Galaxy
  8. The Center of Our Galaxy

Stellar Populations:

In the early 1940's, during wartime blackouts in Los Angeles, the astronomer Walter Baade at the Mount Wilson observatory was able to take many spectra of faint stars without the glare of city lights. What he discovered was that there were two populations of stars in our galaxy, distinguishable by the elements seen in their spectra.

The Population I stars are stars like our Sun, with similar amounts of heavy elements. The term metals is used to denote elements heavier than helium. Population I stars have solar metallicity, or at least a similar abundance of metals to the Sun. The Population I stars make up the disk of the galaxy, and thus have low velocities relative to the Sun, at least for those stars near the Sun.

The Population II stars are found in the halo of the galaxy. These stars are metal poor, since their spectra have few lines due to elements heavier than helium. The Pop II stars tend to have high velocities with respect to the Sun. This is because they are in the halo, not in the disk which is rotating along with the Sun. Halo stars have random orbits and thus cross the disk star orbits with relatively high velocities.

Pop II stars in the halo tend to be older than their Pop I disk counterparts. They appear to have formed early in the history of the galaxy out of almost pristine hydrogen and helium unenriched in metals by supernovae. This is why they are metal poor. There does appear, at least in the halo and disk, to be a direct correlation between when a star was formed and the abundance of metals in its photosphere. In fact, the most recently formed stars in the Orion Nebula are metal rich, and contain as much as twice the abundance of metals as the Sun!

An anomaly in this age - metallicity relation is the galactic bulge. It contains some of the oldest stars, and was probably one of the first things formed in the galaxy, but these stars are also metal rich! The probable explanation for this is that in the dense gas of the forming bulge, the large number of supernovae quickly enriched the clouds making the bulge stars, thus resulting with old stars (now) with solar or higher metallicities.

Evolution in Star Clusters:

We have said that Pop I stars are young, while Pop II and bulge stars are old. How do we know this? How do we find the age of a distant star? We need some sort of stellar clock. The H-R diagram and stellar evolution gives us this clock. Remember, the main sequence lifetime of a star increases with decreasing mass, since M/L is proportional to M^-2.5 measuring the fuel divided by the rate of burning. Thus, if we had a group of stars of a range of masses formed at the same time, we could find the point on the main sequence where the stars more massive than this had evolved off onto the giant branch - this is called the turn off point. Stellar models can give us the time it would take a star of this luminosity and temperature, and thus known mass, to evolve off the main sequence, and thus the age of the cluster.

There are two types of star clusters seen in the galaxy. Open clusters are loose groups of 100 to 1000 stars in a radius of about 3 to 30 parsecs. The open clusters are in the disk of the galaxy, and are Population I objects. The turn-off ages for open clusters are found to be less than 10 billion years.

The globular clusters are dense balls of 100,000 to a million stars contained in a radius of around 10 to 15 pc. Globular clusters belong to the Pop II halo The turn-off ages of globular clusters are 10 to 18 billion years, and thus gives the age of the halo as old.

Some metal rich clusters are found in the bulge and have ages of more than 10 billion years. This gives us the old age for the bulge. Note that since the most massive stars live less than 1 million years, you can have significant enrichment in much less than a billion years in a crowded environment like the bulge. The sparse halo region would take much longer to enrich since the metals from supernovae would need a long time to travel large distances to where new stars were being formed. This is a plausible explanation for the differences in metal content of halo and bulge stars.

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smyers@nrao.edu Steven T. Myers