Spiral structure in the Milky

Early photographic surveys revealed a number of nebulae with a spiral appearance. In Chapter 17 we will discuss the reasons for believing that these spiral nebulae are distant galaxies, distinct from our own, rather than part of the interstellar medium in our galaxy. Some examples of spiral galaxies are shown in Fig. 17.3. Given that many other galaxies seem to have a spiral appearance, it is reasonable to wonder whether our galaxy is also a spiral. Unfortunately, from our vantage point within the galaxy, it is very difficult to see the overall pattern. In other galaxies, the spiral pattern appears in the disk.

However, we can detect certain similarities between our galaxy and spiral galaxies. In particular, spiral galaxies have a significant amount of gas and dust. The amount of gas and dust in our galaxy is comparable to that in other spirals. For this reason, astronomers have been encouraged in trying to unravel the spiral structure of our galaxy.

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