Here's the basic plan view. You are looking down on the face of the Milky Way
disk from somewhere over the North Galactic Pole. The Milky Way contains
roughly 100 billion stars in a large disk 100,000 light years across. Most gas,
dust, and newly forming stars occur in a set of spiral-shaped arms. The
position of our Sun is marked with a small white circle, and the Galactic
center is marked with a white cross. Near the center is a slowly-rotating
stellar bar within a large ring of ionized and molecular gas. Present thinking
favors four major spiral arms, with the Outer and Norma labels applying to
different parts of the same arm. The Local arm we inhabit is not a major arm,
and may in fact branch off either the Perseus or Sagittarius arm somewhere.
Similar small arm fragments yet to be discovered may exist elsewhere in the
Galaxy.
The two large white circles mark radii of 15 and 25 kiloparsecs (49,000 and 82,000 light years) from the Galactic Center. The 15 kpc circle indicates the approximate edge of the Milky Way's stellar population. However, atomic hydrogen gas (abbreviated "H I" by astronomers) extends beyond this to at least 25 kpc, and dark matter (mysterious stuff which we can't see, but which reveals itself through gravitational attraction) goes out to 50-100 kpc. For reference, the Large Magellanic Cloud, a dwarf galaxy visible from the southern hemisphere, is 50 kpc away, and M31, a large nearby spiral galaxy in Andromeda, is 700 kpc away. Our Local Group of galaxies is roughly 2000 kpc across. The Virgo Cluster, the nearest major cluster of galaxies, is about 15,000 kpc away, while the edge of the observable universe lies some 300 times further.
To anchor this perspective, the small white circle around the Sun has a radius of 500 parsecs (1600 light years). This is our ``neighborhood'' in the Galactic ``city''. The majority of the stars you can see with your eye lie within this small area (Alpha Centauri, the nearest star system to our own, is 1.3 parsecs away). The circle also happens to mark about the maximum distance we can presently determine with trigonometric parallax measurements; outside the 500 parsec circle, distances can only be determined by indirect methods, part of the reason our galactography is still so primitive.
