In the Arms of the Galaxy

We have referred to our own Galaxy several times in this chapter as a spiral galaxy. We will discuss galaxy classification in the next chapter, but what are these spiral arms? When observed from afar, they appear to be arcs of bright emission, curving out from the center of the galaxy. There may be only two arms or more, and they may be loosely or tightly wound.

How do we know that the Milky Way also consists of spiral arms? Using the 21 cm hydrogen line discussed in the last chapter, the distribution of neutral hydrogen (by far the most abundant element in the Galactic disk or anywhere) has been plotted. Both position and velocity of the hydrogen clouds are required to make this plot, since it shows a dimension that is not on the sky, namely depth. These radio images confirm the spiral structure of the Milky Way.

These radio studies also confirm that we are far from the Galactic center, located inauspiciously on a cusp between two spiral arms. Spiral arms themselves are not that hard to account for, but their longevity is. In the following chapter, we will discuss how spiral arms might arise, and how they could possibly persist.

Star Words

A spiral galaxy is characterized by a distinctive structure consisting of a thin disk surrounding a Galactic bulge. The disk is dominated by bright curved arcs of emission known as spiral arms.

Star Words

A spiral galaxy is characterized by a distinctive structure consisting of a thin disk surrounding a Galactic bulge. The disk is dominated by bright curved arcs of emission known as spiral arms.

NGC 253 is a spiral galaxy experiencing a wave of star formation and is referred to as a starburst galaxy. (The NGC in the designation stands for New General Catalog.) This image shows both the whole visible disk of the galaxy and a detailed view of a region in its bulge.

(Image from J. Gallagher and NASA)

Electromagnetic Bulging Forming

In This Chapter

V Hubble's classification of galaxy types: spiral, elliptical, and irregular

V Spiral density waves

V Determining the distance of galaxies

V Galactic clusters and superclusters

V Calculating the mass of galaxies and galactic clusters

V Receding galaxies, expanding universe

V Hubble's Law

Even with the naked eye, it is clear that some "stars" are fuzzier than others. After the invention of the telescope, it became obvious that this was the case. Through a telescope eyepiece some are resolved into the disks of planets, some into regions where stars are forming, and others into collections of old stars. There was great disagreement about one class of objects long called spiral nebulae. These clearly were not stars, and yet there was no way to figure out how big they were without knowing how far away they were.

Before Edwin Hubble extended our conception of the size of the universe in the 1920s, these objects were classified as various nebulae and were thought to lie within the Milky Way, which was thought to be synonymous with the universe. Now, at last, we know that our Galaxy is but one among many, and that the spiral nebulae are other galaxies, some smaller, some bigger than our own, all containing hundreds of billions of stars. And we see galaxies no matter where we look in the sky. When we look deep enough, they are there. Some are so close that they are part of our "Local Group," while others are so distant that their light has barely had time to get to us since the universe began.

In this chapter and the next we talk about galaxies other than our own. Astronomers have two major goals in the exploration of other galaxies. The first is to understand the galaxies themselves and how they have evolved with time. But, it is equally important to use other galaxies to tell us more about the Milky Way, our home.

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