A map of the Milky Way deduced from spiral arm tracers is to the left. Spiral arms can be located with 21-cm line radiation, HII regions, O and B stars. In the Milky Way, visible light tracers like HII regions or O and B stars can only locate nearby spiral arms but dust prevents using these tracers on a wide scale in the Galaxy. 21-cm line radiation from atomic hydrogen is at such a long wavelength that it can penetrate dust and delineate arms on the other side of the galaxy. Because it is an emission line, it can be used to measure radial velocities. The velocities can be used to determine where along the line of sight the gas lies that is producing the line -- for example,  the parts of the galaxy closest to the center are rotating around the center at specific velocities set by the gravitational field -- analogous to how the inner planets in the Solar System orbit the Sun at specific speeds and with specific periods given by Kepler's Laws. From http://antwrp.gsfc.nasa.gov/apod/ap050825.html, Astronomy Picture of the Day, R. Hurt, Glimpse Team

We get enough of a hint of spiral structure to prove that we live in a spiral galaxy, but detailed studies of spiral arms are best made in galaxies we can "see" better.

Here is what we see when we look at another barred Sc galaxy "face-on" (which of course we cannot do with the Milky Way!).   We conclude that the Milky is a barred Sc galaxy. Although it is difficult for us to see its grand design, because we live inside it we can study many aspects of it in far more detail than we can for other galaxies, deducing properties of spiral galaxies in general.

1.) bulge (roughly spherical system of old stars surrounding the nucleus)

2.) disk (flat system centered on bulge that contains the spiral arms, with their molecular clouds and young stars)

3.) halo (large spherical system of old stars containing globular clusters, some isolated stars, and surrounding the whole visible galaxy)

4.) nucleus (small region in the very center that may contain black hole) - called the "Galactic Center" for the Milky Way

5.) dark matter

Stellar Populations and Correlation of Stellar Properties with Location in the Galaxy

Population I, Population II are names given to broad categories of stars in the galaxy, according to where they are found, what kinds of orbits they circulate on, how rich they are in heavy elements, and their typical ages:

	Population I	Intermediate	Population II
Orbits	Circular	Elongated	Very elliptical
Distribution	patchy	slightly patchy	smooth
Shape	spiral arms	disk	spherical
Central concentration	none	slight	strong
Thickness (pc)	120	400	2000
Elements heavier than hydrogen and helium (%)	3-4	0.4-2	0.4 or less
Total Mass(M)	2x109	5x1010	2x1010
Age (yr)	108	109	1010
Typical Object	Molecular Clouds, Open clusters, HII regions, OB stars	Sun	Globular clusters, RR Lyrae star

Population I includes the younger stars in the disk/plane of the galaxy. Because these stars formed recently, they have all be enriched in heavy elements produced in previous generations of stars.

Population II is the older stars that tend to lie around the center and in globular clusters, and hence have orbits that take them well out of the disk/plane. Many of these stars were among the first to form, and hence they tend to be almost pure hydrogen and helium, not enriched by previous generations of stars because there were no previous generations. From Gene Smith, http://casswww.ucsd.edu/public/tutorial/Galaxies.html

Summary

If we put all these lines of evidence together, we get a consistent picture of the Milky Way as a typical spiral galaxy.

A view of a very similar galaxy (M109) as our Galaxy would look from outside. It has a central  bulge and around it a distribution of globular clusters and a diffuse halo of stars (all population II). Around the bulge is the disk, with young O and B stars, emission nebulae, open clusters, and other markers for population I. Gas lies generally in the plane and extends well beyond where there is much starlight. The dark matter extends well outside any of the visible galaxy into and beyond the halo. (by G. Rieke, based on an image from Bill Keel, http://www.astr.ua.edu/keel/)

The Milky Way formed from a large, rotating cloud which collapsed; the old halo stars outline the original shape of the cloud since they formed first.