Interstellar Dust and Gas

We often think of the vast areas of space between the stars as being completely empty. However, this is not really true. Much of the space between the stars is filled with atomic and molecular gas (primarily hydrogen and helium) and tiny pieces of solid particles or dust (composed mainly of carbon, silicon and oxygen). In some places this interstellar material is very dense, forming nebulas. In other regions the gas and dust density is very low. The image to the right shows an infrared view of the gas and dust in our galaxy along the plane of our Milky Way galaxy.

W. Waller and F. Varosi (GSFC),

Most of this interstellar gas and dust originates from the death of stars which either exploded (supernova) or blew off their outer layers, returning their material to interstellar space. From this material, new stars are formed. Often, the gas and dust between the stars can be detected only in the infrared. Dust grains absorb visible and ultraviolet light which causes them to heat up and radiate in the infrared. Also, by using infrared detectors, astronomers can penetrate the often invisible interstellar gas and dust clouds and gain much information about their composition and structure. Below are two infrared images taken by NICMOS of material being ejected into space by dying stars.

Credits (Left Image): Rodger Thompson, Marcia Rieke, Glenn Schneider, Dean Hines (University of Arizona); Raghvendra Sahai (Jet Propulsion Laboratory); NICMOS Instrument Definition Team, and NASA/AURA/STScI Credits (Right Image): Credits: William B. Latter (Spitzer Science Center/Caltech) and NASA/AURA/STScI

A surprise discovery from the IRAS mission was that space is filled with faint wisps of dust which cannot be seen in visible light. This has been given the name "infrared cirrus" because it resembles the cirrus clouds in the Earth's atmosphere. Infrared cirrus is very cold (15-30 K or -433 to -406 F) and can only be detected in the infrared. Its temperature is due to dust grains being slightly heated by starlight.

IRAS image of cirrus at the south celestial pole and ISO-LWS spectra of infrared cirrus

Infrared astronomy has also resulted in the detection of several types of complex interstellar molecules. Astronomers using ISO discovered emission lines from interstellar water vapor in a variety of sources including star forming regions, planetary nebulae and near formed stars. ISO also discovered for the first time hydrogen cyanide ice molecules in a dusty cloud surrounding a newly forming star. Many of the most important spectral lines produced by interstellar gas falls within infrared wavelengths.

The Spitzer Space Telescope will conduct spectroscopic studies of interstellar dust and gas. Among these research investigations will be the important study of water, ices and organic molecules. Spitzer observations at near-infrared wavelengths will map the inner regions of our Milky Way, yielding important new information that is otherwise obscured by heavy concentrations of dust at visible wavelengths.