Figure 11-1 Stars and the Interstellar Medium (a) The stars in the constellation Orion can easily be seen with the naked eye (see also Figure 1-4). A careful examination of the middle “star” in the “sword” of Orion (which hangs down below his belt) shows that it is not circular. This is actually the center of the Orion Nebula (also called the Great Nebula of Orion). This central region of the Orion Nebula is composed of several stars (the Trapezium) and surrounding gas and dust. This middle “star” in the sword appears fuzzy due to visible light emitted by a cloud of gas and dust surrounding the Trapezium stars there. Little other interstellar gas and dust can be seen in this visible-light image, although the leftmost star in Orion’s belt, Alnitak, is surrounded by gas and dust (see Figure 11-3). (b) The same region of the sky in a false-color infrared image taken by the Infrared Astronomical Satellite. Gases are seen here to exist in more areas than can be detected in visible light.