Through the course of the 20th Century, the wide array of complex structures in other galaxies, with their beautiful spiral arms, etc, were systematically classified. However, studies of our own Milky Way were hampered by several complicating factors:
- Our location within the system means that we can see parts of it in almost every direction, so any study of its overall structure requires a huge survey of the whole sky.
- The Milky Way is full of “dust.” This soot-like material is flung out from stars as they age. It distorts and obscures our view of much of the galaxy, creating the dark lanes that we see cutting across the Milky Way.
- Since different parts of the Milky Way are at different distances, we have a problem of perspective: is a structure that we observe to be small intrinsically little and quite nearby, or is it on the far side of the Galaxy and enormous?
The first two of these problems have largely been overcome by all-sky surveys using near-infrared light. By covering the entire sky, the structure of the whole Galaxy can be mapped out. Further, dust absorbs infrared light far less than optical light, so these studies are less affected by the obscuration that dominates our view of the Milky Way at optical wavelengths. As you can see from the picture below, these new data totally revolutionise our view of the Galaxy.
Infrared survey of the sky showing the Milky Way (and the neighbouring Magellanic Clouds). This map was produced by measuring the positions of some 100,000,000 stars in the 2MASS Survey (a joint project of the University of Massachusetts and IPAC/Caltech, funded by the NSF and NASA).
Even with all-sky infrared data, there is still not enough information to construct a complete model of the Milky Way, as we must somehow add the third dimension to this two-dimensional picture. Fortunately, there are several techniques that astronomers can draw on to infer the three-dimensional shape of the various elements that make up the Galaxy:
- Perspective. It is possible to use the appearance of different parts of the Milky Way to figure out their intrinsic shapes. For example, the central bright “bulge” component in the infrared image appears slightly distorted with the left side larger than the right side. This implies that this component is bar-shaped, and oriented such that its left-hand end is closer to us (and so appears larger) than its right-hand end.
- Standard Candles. If we know the intrinsic brightness of an object, then we can use how bright it appears to measure its distance from us, and hence map out the third dimension. This technique is particularly useful for studying the distribution of star clusters, where the properties of the constituent stars can be used as such standard candles.
- Kinematics. In addition to the position of an object on the sky, astronomers can measure its line-of-sight velocity by the Doppler shift in the light that it emits. These extra data tell us something about the distance to the object, and how it is orbiting around the centre of the Galaxy. As you can see from the figure below, this information is quite complex to interpret, but careful modelling of the features in this type of diagram allows astronomers to measure the properties of the Galaxy’s spiral arms as well as its central bar.
Diagram showing the distribution of hydrogen gas in the plane of the Milky Way. The horizontal axis shows the angle between the gas’ location and the centre of the Galaxy. The vertical axis shows how fast the gas is moving along the line of sight. (Data courtesy of Dr D. Hartmann).
It is by using these techniques that it is now possible to produce a realistic three-dimensional map of our home galaxy, the Milky Way.