This is an infrared image of what may be the most luminous star known, about 7000 pc distant in the direction of the Galactic center. The "Pistol Star," named for the shape of the nebula surrounding it, has up to 10 million times the luminosity of the Sun and is big enough to fill the diameter of Earth's orbit. The nebula itself (shown mostly in red) extends about 1 pc across, and is thought to be energized by this one star. The star is estimated to be very young, about a million years old; it will not last long given its prodigous rate of emission, probably exploding as a supernova in about another million years. The image was taken with the newly installed infrared camera on the Hubble Space Telescope; it looks fuzzy because the camera is somewhat out of focus. It is possible that, with better resolution, this object may well be shown to be several smaller stars in a cluster.
Studying this chapter will enable you to:
Summarize the composition and physical properties of the interstellar medium.
Describe the characteristics of emission nebulae and explain their significance in the life cycle of stars.
Discuss the nature of dark interstellar clouds.
Specify the radio techniques used to probe the nature of interstellar matter.
Discuss the nature and significance of interstellar molecules.
Stars and planets are not the only inhabitants of our Galaxy. The space around us harbors invisible matter throughout the dark voids between the stars. The density of this interstellar matter is extremely low—approximately a trillion trillion times less dense than matter in either stars or planets, far more tenuous than the best vacuum attainable on Earth. Only because the volume of interstellar space is so vast does its mass amount to anything at all. So why bother to study this near-perfect vacuum? We do so for three important reasons. First, there is as much mass in the "voids" among the stars as there is in the stars themselves. Second, interstellar space is the region out of which new stars are born. Third, it is the region into which some old stars explode at death. It is one of the most significant crossroads through which matter passes in our universe.
