Physics 321
Astrophysics II:  Lecture #18
Prof. Dale E. Gary

The Milky Way Galaxy - I

Counting Stars

It is a significant problem to determine the shape and extent of our galaxy from our point of view inside it.  On a clear night with a dark sky, the band of stars and dust that make up the Milky Way can be easily seen, but when mapped over the whole sky in optical light, it has a complicated appearance that does not represent its real shape.

The first attempts to determine the shape of our galaxy, called the Milky Way Galaxy were through simply counting stars.  Sir William Herschel made the first systematic star count in the 1780's, later expanded by Jacobus Kapteyn the early 1920's.  Both of these attempts ignored interstellar extinction (the dimming of starlight by dust), and concluded that the Sun resides near the center of a flat, pancake-shaped distribution of stars.

However, between 1915 and 1919, Harlow Shapley estimated the distances to globular clusters, using RR Lyrae and Pop II Cepheid variables as distance indicators.  He found that many more clusters where seen in the direction of Sagittarius, and concluded that globular clusters are distributed uniformly around our galaxy.  The concentration in the direction of Sagittarius was due to the fact that our Sun is off-center within the galaxy, and the center is in the direction of Sagittarius.

We can now obtain images in the infra-red region of the spectrum, which can penetrate dust easily.  When we image the entire sky in the infra-red, as was done with the COBE satellite, we obtain the much clearer picture of the Milky Way Galaxy shown below.

Image of the Milky Way Galaxy taken with the COBE satellite.
Note that now we can see through the dust to the central bulge, and the disk appears much more uniform.  Below are several views of the Milky Way seen in various wavelengths.
Morphology of the Galaxy
Below is a figure that gives an overview of the structure of our galaxy.  We will be discussing different aspects of this figure in the next several lectures, but for now let's consider only the three main regions.
Schematic view of the Milky Way Galaxy.  The top figure shows a "top view" showing the spiral structure as seen from above.  The bottom figure shows an "edge-on view" as seen from the side. (The top view image is of the spiral galaxy M100.  The side view shows the COBE image from the previous figure.)
In addition to these visible features, there is indirect evidence of another source of matter, called dark matter, which lies in a large halo extending far outside the visible confines of the galaxy.  This makes up the dark halo.  The evidence for this enigmatic matter comes from studies of the kinematics, or motions, of the constituents of the galaxy.
Kinematics of the Milky Way
As always in astronomy, the building blocks of great discoveries such as galactic rotation and the presence of dark matter come from very basic, simple measurements such as the motions of stars.  As we have seen, a star's speed is measured in two components: Star catalogs such as the Nearby Stars catalog give these measurements for each star.

Here, the proper motion (pm, or m") is given in arcseconds per year.  In these units, the tangential velocity of the star, in km/s, is

vq = 4.74 m"dpc = 4.74 m"/p"   (km/s)
where p" is the trigonometric parallax, in arcsec.

Example: For star Gl 4.2A, above, we have m"= 0.592 and p"= 0.0483, so vq = 58.1 km/s.  The radial velocity is shown in the table to be vr = 2.6 km/s, so the total velocity, or space velocity is v= (vr2+ vq2)1/2 = 58.2 km/s.

The quantities in the table, however, are not so simple to measure.  To convert proper motion to velocity, one must know the distance, which means either measuring a trigonometric parallax (which is impossible for all but the nearest stars) or using a spectroscopic parallax.  In addition, the proper motion must be measured.  For nearby stars, the proper motion can be quite large, but for distant stars it can be very small.  Luckily, we can monitor stars over many decades, after which the proper motion accumulates and can be measured.

Radial velocity is easier, since it is determined from spectral line shifts, which can be seen at any distance (as long as the star is bright enough).  However, for both radial velocity and proper motion one must subtract other motions such as the Earth around the Sun (~30 km/s), and the Sun's own motion through space (~19.5 km/s--our text gives 16.5 km/s).

Galactic Coordinates
We learned in the second lecture that stars are assigned right ascension (a) and declination (d) coordinates depending on their positions on the celestial sphere--the extension of the Earth's equator and poles into the sky.  However, it is often more convenient when discussing the kinematics of the galaxy to change to a new coordinate system based on the shape and orientation of the galaxy (the Milky Way) in the sky.
Local Standard of Rest Galactic Rotation Curves