Issue 5.04

The Astrophysics Spectator

February 29, 2008

The measurement of mass in the universe has always been a difficult task, because the light we see from a galaxy does not trace matter.  The brightest, most massive stars radiate most of a galaxy's light, but they account for only a tiny fraction of the galaxy's mass.  Low-mass stars, white dwarfs, brown dwarfs, and interstellar gas, which are too dim to see, are responsible for much of this dark mass, but many physicists, particularly particle physicists, conjecture that part of the dark mass is in the form of fundamental particles or an energy associated with the vacuum.  The most reliable tracer of the mass within a galaxy has been the kinetic energy of the stars; from the velocities of the stars, one can derive the gravitational force field, which directly gives the distribution of matter in the galaxy.  This dynamic mass is often many times the mass implied by the starlight from a galaxy.  This problem of a dark mass is present within our own galaxy: the motions of the stars in the Milky Way implies that the bright Galactic disk is enshrouded by a dark, rather spherical halo of matter.

This discrepancy between luminous mass and dynamic mass had plagued studies of the local Galactic disk.  Two decades ago, the mass density of the Galactic disk derived from the kinematics of nearby stars was higher than the density derived from star counts and from measurements of interstellar gas; the discrepancy implied that dark matter accounted for half of the mass of the Galactic disk.  Over the past decade, improved distance measurements of stars and the ability to detect dim, nearby stars and brown dwarfs have brought the dynamical mass density in line with the star counts.  Whether a similar resolution solves the dark-mass problem of the Galactic halo is uncertain, but, in astronomy, discrepancies more often have an observational origin rather than a new-physics origin.

In this issue of the web site, a page is added that describes the two methods of deriving the mass density of the local Galactic disk.

Next Issue:  The next issue of The Astrophysics Spectator is scheduled for March 12.

Milky Way Galaxy

The Mass Density of the Local Galactic Disk.  The local mass density of the local Galactic disk has been estimated in two ways: from the dynamics of the stars in the disk, and from the number of stars seen near the Sun.  In the past, these two measures gave very different results, with the star counts giving a much lower density than the estimate from dynamics.  More recent estimates have closed this gap so that the two estimated are in agreement.  (continue)