The Nature
of the Galaxy

The band of light that we call the Milky Way is actually the plane of the disk of our spiral galaxy (see this representation). It contains approximately 200,000,000,000 stars, mostly grouped into a flattened disk with a bulge at its centre. The visible matter of the galaxy is approximately 100,000 light years across, with the Sun in the disk about 30,000 light years from the center.

The Milky Way in Visible Light

The Milky Way that we see in the night sky corresponds to looking out along the plane of the disk; in other directions we look out of the plane and therefore see far fewer stars.

The adjacent figure shows a telescopic view of the Milky Way in the constellation Scorpius. The center of the galaxy is in the left middle of this figure, but it is obscured by the gas and dust lying between us and the center (the dark bands in the figure). We shall find that gas and dust are common in spiral galaxies like the Milky Way.

The Milky Way at IR Wavelengths

Thus, in visible light we cannot see the center of the galaxy. However, visible light is not the only wavelength at which modern astronomers can make observations. The IR image below was obtained from the Diffuse InfraRed Background Experiment (DIRBE) of the NASA Cosmic Background Explorer (COBE). Here is further explanation of the image.

Visible light is strongly absorbed by the intervening gas and dust, but infrared light is transmitted well enough to give us a clear view of the central region. Thus, we see the side of the bulge surrounding the galactic core from our vantage point in the disk about 30,000 light years from the center.

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