Light passing through a dust cloud may not be completely blocked, although all wavelengths of light passing through will be dimmed somewhat. The Horsehead Nebula, seen to the left, is an example of this. These dark clouds are known as dark nebulae. If the dust is thick enough, the light will be completely blocked, leading to dark areas. When light from other stars passes through the dust, a few things can happen. The particles are irregularly shaped, and are composed of silicates, carbon, ice, and/or iron compounds. These dust particles are extremely small, just a fraction of a micron across, which happens to be approximately the wavelength of blue light waves. Interstellar dust is not like the dust that you might find under your bed it is made of very different substances. Examples of emission nebulae are the Trifid Nebula or the Orion Nebula (seen in this photograph). Visible light is emitted when electrons recombine with the ionized hydrogen, which is seen as beautiful red colors of emission nebulae. This radiation ionizes the surrounding clouds of gas. The ionized hydrogen is produced when large amounts of ultraviolet radiation are released by hot newly-formed stars. The neutral and molecular forms emit radiation in the radio band of the electromagnetic spectrum. The cold clouds of neutral or molecular hydrogen are the birthplace of new stars if they become gravitationally unstable and collapse.
(For comparison, the air we breathe has a density of approximately 30,000,000,000,000,000,000 molecules per cubic centimeter.) Even though the interstellar gas is very dilute, the amount of matter adds up over the vast distances between the stars. This gas is extremely dilute, with an average density of about 1 atom per cubic centimeter. The interstellar gas consists partly of neutral atoms and molecules, as well as charged particles, such as ions and electrons. In total, approximately 15% of the visible matter in the Milky Way is composed of interstellar gas and dust.Īpproximately 99% of the interstellar medium is composed of interstellar gas, and of its mass, about 75% is in the form of hydrogen (either molecular or atomic), with the remaining 25% as helium. These regions have very low densities and consist mainly of gas (99%) and dust. Although the interstellar regions are more devoid of matter than any vacuum artificially created on earth, there is matter in space. Many people imagine outer space to be a complete vacuum, devoid of any material.
Simply put, the interstellar medium is the material which fills the space between the stars.
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