Ngc 2903

HST images of six spiral galaxies, showing regions of star formation.These are false color images made through three separate filters.The red represents the Paschen line from H at 1.87 ^m. Blue shows near IR emission (1.4 to 1.8 ^m ). Green is a mixture of the two. [STScI/NASA]

HST images of six spiral galaxies, showing regions of star formation.These are false color images made through three separate filters.The red represents the Paschen line from H at 1.87 ^m. Blue shows near IR emission (1.4 to 1.8 ^m ). Green is a mixture of the two. [STScI/NASA]

Fig. 17.6. shows the distribution of tracers of young stars in a selection of spirals. In studying general trends, we can get a good idea of the distribution of young stars as we go farther out in the disks of spiral galaxies. The luminosity of the disk falls off sharply with r, the distance from the center. We can approximately fit the observed falloff with an exponential expression. That is, if L0 is the luminosity at the center, the L(r), the luminosity at radius r, is given by

Table 17.1. ellipticals.

Properties of spirals and

Property

Spirals

Ellipticals

Gas Dust

Young stars Shape

Stellar motions Color yes yes yes flat circular rotation blue some some none round random red

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