Oort Limit

The purpose of this project is to determine the distribution of mass in the Galactic disk. We will measure a value for the local disk mass density, or the amount of mass per unit volume near the Sun. This quantity is known as the "Oort limit". The Oort Limit sets the calibration for the mass density of the entire Galaxy, and is essential to calibrating the mass density law shape we will determine through other means.

We will also measure the column density of the Milky Way in the vicinity of the Sun. Column density is defined as the number of stars contained in a column or cylinder of a given length.

A key step in this process will be to determine the dependence of the number density of stars called "K giants" upon distance above the plane of the Galaxy. The letter K refers to the spectral class of the star, which basically depends upon the temperature of the star (see diagram below). K giants will be used because they are very bright and more numerous than other spectral types.


The Hertzsprung-Russell diagram, showing the luminosity of stars versus surface temperature, from http://www.physics.hku.hk/~nature/CD/regular_e/lectures/chap12.html

In order to measure the dependence of K giants upon distance above the plane (disk) of the Galaxy, we need to know the distance to the K giants, and their motions perpendicular to the disk. This motion depends on how much gravity is in the disk; the more gravity there is, the faster stars move up and down in their epicyclic motion. This is basically a wobble in the star's orbit.

The current accuracy of local mass density and column density is about 30 - 50%. SIM PlanetQuest will measure the mass density with an error of 10%, slightly better for the column density.


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