The interstellar medium—the space between the stars—consists of nearly empty space. It is the vacuum of the universe. It would be totally empty if not for a smattering of gas atoms and tiny solid particles—interstellar matter.
On average, the interstellar matter in our region of the galaxy consists of about one atom of gas per cubic centimeter and 25 to 50 microscopic solid particles per cubic kilometer. In contrast, the air at sea level on Earth contains about 1,019 molecules of gas per cubic centimeter.
In some regions of space, however, the concentration of interstellar matter is thousands of times greater than average. Where there is a large enough concentration of gas and particles (also called cosmic dust), clouds form. Most of the time these clouds are so thin they are invisible. At other times they are dense enough to be seen and are called nebulae (plural for nebula).
Cosmic dust accounts for only 1 percent of the total mass in the interstellar medium; the other 99 percent is gas. Scientists believe the dust is primarily composed of carbon and silicate material (silicon, oxygen, and metallic ions), possibly with solid carbon dioxide and frozen water and ammonia. A dark nebula is a relatively dense cloud of cosmic dust. The nebula is dark because much of the starlight in its path is either absorbed or reflected by dust particles. When starlight is reflected, it shines off in every direction, meaning only a small percentage is sent in the direction of Earth. This process effectively blocks most of the starlight from Earth's view.
Even individual particles of cosmic dust affect the quality of starlight. Random dust particles absorb or reflect some light from various stars, causing them to appear far dimmer than they actually are. Scientists have theorized that without the presence of cosmic dust, the Milky Way would shine so brightly that it would be light enough on Earth to read at night.
Most dark nebulae resemble slightly shimmering, dark curtains. However, in cases where a dense cloud of dust is situated near a particularly bright star, the scattering of light may be more pronounced, forming a reflection nebula. This is a region where the dust itself is illuminated by the reflected light.
Cosmic dust: Solid, microscopic particles found in the interstellar medium.
Interstellar medium: The space between the stars, consisting mainly of empty space with a very small concentration of gas atoms and tiny solid particles.
Light-year: Distance light travels in one year, about 5.9 trillion miles (9.5 trillion kilometers).
Nebula: An interstellar cloud of gas and dust.
Red giant: Stage in which an average-sized star (like our sun) spends the final 10 percent of its lifetime. Its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.
In contrast to solid particles, interstellar gas is transparent. Hydrogen accounts for about three-quarters of the gas. The remainder is helium plus trace amounts of nitrogen, oxygen, carbon, sulfur, and possibly other elements.
While interstellar gas is generally cold, the gas near very hot stars is heated and ionized (electrically charged) by ultraviolet radiation given off by those stars. The glowing areas of ionized gas are called emission nebulae. Two well-known examples of emission nebulae are the Orion nebula, visible through binoculars just south of the hunter's belt in the constellation of the same name, and the Lagoon nebula in the constellation Sagittarius. The Orion nebula is punctuated by dark patches of cosmic dust.
Interstellar space also contains over 60 types of polyatomic (containing more than one atom) molecules. The most common substance is molecular hydrogen (H 2 ); others include water, carbon monoxide, and ammonia. Since these molecules are broken down by starlight, they are found primarily in dense, dark nebulae where they are protected from the light by cosmic dust. These nebulae—known as molecular clouds—are enormous. They stretch across several light-years and are 1,000 to 1,000,000 times as massive as the Sun.
Scientists have proposed various theories as to the origins of interstellar matter. Some matter has been ejected into space by stars, particularly from stars in the final stages of their lives. As a star depletes the supply of fuel on its surface, the chemical composition of the surrounding interstellar medium is altered. Massive red giant stars have been observed ejecting matter, probably composed of heavy elements such as aluminum, calcium, and titanium. This material may then condense into solid particles, which combine with hydrogen, oxygen, carbon, and nitrogen when they enter interstellar clouds.
It is also possible that interstellar matter represents material that did not condense into stars when the galaxy formed billions of years ago. Evidence supporting this theory can be found in the fact that new stars are born within clouds of interstellar gas and dust.