The Milky Way is an island of stars, of which the Sun is one member.
Because we live inside the Milky Way, it is very hard for us to determine its shape and
size. It is clearly planar, but estimating it's exact shape and our position inside it was
not done until the early 20th century.
Not only do we have a bad angle, but we also have to deal with dust in the plane of the
galaxy. This dust obscures our view of most of the Milky Way. A. Size and Shape
The Milky Way's shape was first determined through studying globular clusters.
Globular clusters are spherical clusters of up to 106 stars (usually Pop II),
typically outside the plane of the galaxy.
The distance to globular clusters is measured with Cepheid and RR Lyrae variable stars.
These variables change their brightness in a periodic way. Their outer layers are
unstable, so they pulsate. As they expand and cool, the opacity drops, so light can escape
more easily and the outer layers are no longer supported. As the star contracts, it heats
up, and the opacity goes up. Thus the outer layers become more strongly supported, so the
star begins expanding again.
The period of variation depends on the luminosity of the star. If we can measure the
period and the apparent brightness, we can determine the distance.
Cepheids are high luminosity stars, so they can be seen out to great distances.
In measuring Cepheid and RR Lyrae distances in globular clusters, we find that they are
centered on a point 8.5kpc (8500 pc) away, in the direction of Sagittarius. Shapley
reasoned that this must be the center of the galaxy.
Strong radio sources (H2 clouds and ionized hydrogen regions) show the
presence of spiral arms in our galaxy.
The Milky Way thus appears to be a spiral galaxy, and we are on one of the spiral arms.
From observations of our own and other spirals, we know that spirals have a central
bulge, a disk, and a halo.
The disk has mainly Pop I stars. In addition, about 10% of the disk mass is in the form
of gas and dust. Thus, there is active star formation in the disk.
Stars don't stay in spiral arms. The arms are waves of extra density in the gas of the
disk. The Sun has moved in and out of spiral arms dozens of times.
Since there is extra gas, most star formation occurs in the arms. Since high luminosity
O and B stars explode in a short time, they don't have time to leave the spiral arm. Thus,
spiral arms are where all of the very brightest stars are.
The disk also has open clusters: loosely bound groups of young stars, like the Pleiades.
These are probably stars that formed together and haven't yet dispersed.
Most of the 1011 stars in our galaxy are in either the disk or the bulge.
The halo has many fewer stars. Many halo stars are in globular clusters.
By looking at the main sequence cutoff (maximum age of the brightest main sequence
stars), we find that most globular clusters are 1010 years old.
Halo stars orbit radially, while disk stars orbit in roughly circular orbits within the
disk.
The bulge is a combination of young and old stars. Active star formation is
occurring,
but most of the stars are very old. Many old bulge stars (and globular clusters) have high
metallicity, implying that star formation was very early and very active. B. Mass of
the Milky Way
We estimate the mass of the Milky Way from the orbits of the Sun and other stars.
From the Sun's speed, we estimate its orbit period 2 x 108 years.
From Kepler's laws we find that the mass inside the Sun's orbit (solar circle) = 1011Mo.
We would expect this to be all of the mass of the galaxy, since there are far fewer
stars outside of the Sun's orbit.
When we estimate the galaxy's mass from objects beyond the solar circle, we find that
the galaxy has a flat rotation curve, i.e. objects beyond the Sun rotate at the same speed
as objects closer to the galactic center. This implies that there is much more material
outside the Sun's orbit. The galaxy's mass is closer to 1012Mo.
This ``dark matter'' could be anything from pebble-sized objects to brown-dwarf stars.
It could also be sub-atomic particles (e.g.., neutrinos).
The dark matter in the galaxy appears to follow the halo rather than the disk.
Any other galaxy for which we can measure the mass also shows strong evidence of the
presence of dark matter.
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