Published online by Cambridge University Press: 05 November 2013
1.1 Introduction
What does our Galaxy look like? We can compare the COBE image of our Galaxy, taken in the near-IR, with the visible image of the edge on spiral NGC 891. Our Galaxy would probably look much like NGC 891 if it were observed in visible light from far away (see Figure 1.1). The Milky Way is very clearly a disk galaxy: its disk is the primary component and is supported almost entirely by its rapid rotation. We also see a small central bulge which contributes about 20% of the total light. Some galaxies have much larger bulges. The small bulge of the Milky Way is a pointer to the events that occurred as it formed and evolved. We would like to understand how our Galaxy came to look like this.
Figure 1.2 shows schematically the five main components of the stellar galaxy. The thin disk and bulge are the main visible components. The thin disk is enveloped in a thicker thick disk which contributes only about 10% of the light of the disk. These thick disks are very common and their formation appears to be part of the formation process of disk galaxies. The stellar halo provides only about 1—2% of the total light but is very important for understanding how the Galaxy was assembled. The stars of the halo are metal-poor, mostly with abundances of [Fe/H] < —1.
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