Yet we are also familiar with the concept of our home galaxy being a flattish disk with multiple spiral arms, even though no such photographic images of it from this angle actually exist.
So... is it a ribbon or spiral disc? This apparent contradiction can be explained if you consider the position of the Sun (and hence the Earth) in the galaxy. Evidence suggests that we are located in one of the spiral arms, about three fifths of the distance between the centre and the edge of the galaxy. Clearly if we are actually inside the disk of the galaxy, it will be impossible from our viewpoint to see it in all its spiral glory. Instead, if we look out parallel to the disc in any direction, we are looking at the disc edge on, so we see it as a band of stars. They appear much more densely packed in the direction of the galactic centre, compared to the opposite direction, because we are looking through more than half of this side of the disc, plus the central core and all of the opposite side of the disc. Conversely, if we look 'up' or 'down', away from the plane of the disc, we see far fewer stars, as the disc is thin and we are looking out beyond its top and bottom surface to the much emptier space beyond.
Movements of stars
Seeing our galaxy as a band makes sense, but how do we know about the spiral arms? This basically involves measuring the movement, in terms of both direction and velocity, of huge numbers of stars in different parts of sky, and also their distance from us. Stars may appear to be in fixed positions on the sky, but in fact with very precise measurements it is possible to see how they are actually moving in relation to us and to each other.
By collating all this information for many, many stars it is possible to build up a picture of how stars in various regions are moving in relation to each other and us, and of the overall movement of the galaxy. The results show that there is clear rotational movement around the centre of the galaxy, just as has also been observed in other galaxies known to be spirals. In addition, astronomers have found that most of the stars in the galaxy disc, and especially clusters of young stars, appear to be arranged in spiral paths.
There is still a lot of debate about the exact structure of the Milky Way, including the number of spiral arms. The task of mapping the layout and movement of our galaxy is further complicated by the thick regions of dust that lie within the disk, which blocks optical wavelengths of light from objects behind them. However, telescopes sensitive to different wavelengths, especially infra-red, are able to see through this dust, providing fascinating information about many features out there, so they have a major part to play in mapping the galaxy.
The most recent procect focussing on the Milky Way is the European Space Agency's Gaia mission, which is currently engaged in the mammoth task of mapping more than a thousand million stars to exquisite accuracy – when all this data is collected over the next five years, and then analysed, a even more detailed picture of the Milky Way's structure should emerge.
http://www.nasa.gov/jpl/charting-the-milky-way-from-the-inside-out
http://www.eso.org/public/news/eso1606/
http://www.universetoday.com/120183/astronomers-bypass-visible-light-to-map-the-galaxys-structure/
http://www.esa.int/Our_Activities/Space_Science/Gaia/Gaia_overview
1. ESO / APEX / ATLASGAL Consortium / NASA / GLIMPSE Consortium / ESA / Planck
2. NASA/JPL-Caltech/R. Hurt
3. ESO/ATLASGAL consortium/NASA/GLIMPSE consortium/VVV Survey/ESA/Planck/D. Minniti/S. Guisard. Acknowledgement: Ignacio Toledo, Martin Kornmesser