The problem with living in the middle of the Galactic disk is that the interstellar medium blocks our view of most of the Galaxy. We are very fortunate that there is a way around this, provided by radio astronomy. This comes by way of the "21-cm" line, which arises from neutral hydrogen. It is a special sort of energy transition, involving a "spin flip" of the electron relative to the proton. The atom is in a lower energy state when the spins are not aligned - the energy difference is equal to a photon with wavelength of 21-cm. This transition is very weak ( a typical interstellar atom can stay in the higher energy state for millions of years), but since most of the Universe is hydrogen this radiation arises everywhere. Thus, the Galaxy is both transparent and visible in 21-cm radiation. The transparency makes it difficult to tell where along a line-of-sight the detected emission is coming from. Because it is a spectral line, one can make use of the Doppler shift to help with this problem. In general, the inner part of the Galaxy rotates at a different speed than the outer parts, so there will be some projected velocity difference between gas at different distances along the line-of-sight. Given a model for the Galactic rotation, one can use it to assign each velocity bin in an observed broadened spectral line to a different location. In this way, we can build up an overall map of the neutral hydrogen (Fig. 15.18). This map can be supplemented for nearby locations by observing star forming regions containing massive stars; we know how to assign distances to stars based on their main sequence location. Another useful molecule is carbon monoxide (CO), whose rotational changes can also give rise to radio photons. This is a good tracer of molecular clouds.
The picture which arises is that we live in a flattened disk of stars and gas, which is organized into spiral arms. We see many examples of similar structure for other galaxies. Indeed, the nearest major galaxy, M31 in Andromeda, we think looks very much like the Milky Way. The spiral structure may at first seem sensible, since the Galaxy rotates at different rates at different radii, it would take radial structures and wind them into spirals. The problem is that the spirals are never too tightly wound, even though the galaxies have had time for up to 100 rotations. The solution is that these arms are not composed of particular stars and clouds; they are "density waves". As on a freeway where there is an accident, the density of cars behind and up to the wreck is much higher than in front of it, but individual cars move through the density wave and then speed up away from it. So stars, including our Sun, find themselves moving a little slower while in the spiral arms and faster between them. The arms maintain themselves by their own extra gravity, and serve the function of moving angular momentum to the outer regions of the Galaxy, allowing it to slowly collapse. Due to the higher density in the arms, this is where molecular clouds and star formation tend to be concentrated. In other galaxies where we can look down from above, the spiral arms are clearly traced out by the young bright stars.