The Milky Way

 

We live in a typical spiral galaxy.  The structure is:

 

q      The disk – Population I stars – young stars, dominated by the bright, hot stars, and thus bluish.  Open clusters and HII regions found here.

q      The nuclear bulge – Population II stars – older, red giant stars and thus reddish

q      Globular clusters – distributed in a spherical “halo” about the nuclear

 

The structure of the galaxy was not determined until the 20^th century.  Most astronomers in the early 1900’s assumed that the Sun was at the center of the galaxy.  Harlow Shapley found that the distribution of the globular clusters was asymmetrical.  If we assume that the globulars are symmetrically placed about the center of the galaxy, then the Sun is off-center by some 8500 pcs.

 

A spiral galaxy like the Milky Way has large amounts of gas and dust in between the stars.  This interstellar component has the effect of dimming the light and reddening the light from stars.  Because of the absorption by gas and dust, we cannot see the entire galaxy in visible light.  But radio light is not affected by the interstellar gas and dust, so most maps of the Milky Way are done at radio wavelengths.

 

We need to search for useful absorption lines that fall at radio wavelengths.  One of these occurs for the hydrogen atom.  Both the proton and electron have a property called spin.  Hydrogen can be made in two ways – one in which the two particles are spinning in the same sense and another in which their spins are opposed.  The electron can change its spin spontaneously (a “spin-flip” transition) with the emission of a photon of light at 21 cm wavelength.  Since there is so much hydrogen gas in between the stars, we can use the 21-cm line to map the location of hydrogen gas clouds between the stars.  In a particular direction we might observe the emission at slightly different wavelengths because the Doppler Effect has shifted the line for clouds moving with different radial velocity.

 

Once a map is complete we can compute the rotational velocity of the galaxy at different distances from the galactic center.  We expect on theoretical grounds that the inner portion of the galaxy should rotate like a rigid body, but further out, where we are, the stars should be orbiting according to Kepler’s laws.  We do not observe the expected Keplerian orbits, but rather the orbital speeds remain much higher than expected to great distances.  The obvious conclusion is that the Milky Way must contain substantial mass at great distance from the center.  The problem is that we don’t see the light from this mass.  We call this the problem of “Dark Matter.”  We know the mass is there (in fact, it may account for 90% of the mass of the galaxy) but it must be in an underluminous form.  There are several candidates that are being pursued:

 

q      Compact Halo objects – White Dwarfs, Neutron Stars, Black Holes.  Searches are being conducted by looking at many stars in neighboring galaxies (as many as one million stars in the Large Magellenic Cloud).  If there is a chance alignment between the distance star and a MAssive Compact Halo Object (MACHO), then the light should be bent and we should observe a very characteristic brightening of the star.  Several such events have been observed, but the studies are quite young.

q      Planets or brown dwarfs might be numerous in the halo of the galaxy, but it is difficult with these low mass objects to account for the necessary mass.

q      Weakly Interacting Massive Particles (WIMPS) – strange, as-yet undiscovered particles might account for some of the dark matter.

 

Other galaxies that have been well-studied also have a dark matter problem.