What Is the Interstellar Medium?

The Interstellar Medium

Author: 栗小白
Reviewer: 时光

Introduction

Our universe contains more than stars and planets. Matter of extremely low density—far thinner than the best vacuum we can create on Earth—pervades interstellar space. Known as the interstellar medium, this material occupies the vast expanses between planetary systems.

The Interstellar Medium

The interstellar medium consists mainly of gas and dust. In most cases, the elemental abundances in interstellar gas are much like those found in other celestial objects. Most of the gas—roughly 90% or more—is made up of hydrogen atoms or molecules, about 9% is helium, and the remainder consists of heavy elements. We know less about interstellar dust. Evidence from infrared observations suggests that it includes silicates, graphite, iron, and “dirty ice,” a mixture of water ice, ammonia, methane, and other compounds.

The interstellar medium has two important effects: extinction and reddening. The following sections explain how each one arises.

• Extinction
  Dust grains can absorb only light whose wavelength is less than or approximately equal to their radius. Shorter wavelengths are therefore blocked more strongly by dust. Interstellar dust grains are about 10^-7 m in diameter, comparable to the wavelengths of visible light, so interstellar dust dims starlight.

• Reddening
  Because the interstellar medium blocks short-wavelength radiation more effectively than long-wavelength radiation, higher-frequency photons from distant stars are more readily “seized” along the way. The star consequently appears redder than it really is, an effect called reddening. The principle is illustrated below:

Nebulae

Emission Nebulae

There are many kinds of nebulae, which appear as hazy patches in the night sky. An emission nebula is an extended cloud of hot, glowing interstellar gas. Emission nebulae are associated with star formation: hot O- and B-type stars heat and ionize the interstellar matter around them, creating an emission nebula. If a nebula blocks stars farther along our line of sight, we see a small black region against a bright background and call it a dark nebula. If objects inside the cloud, such as young stars, cause it to glow, however, we see a bright emission nebula.

Reflection Nebulae

In contrast to red emission nebulae, reflection nebulae are blue. Dust grains in a reflection nebula scatter the light emitted by its stars, giving the cloud a bluish appearance. An emission nebula forms because stars at or near the center of the gas produce large amounts of ultraviolet radiation. As ultraviolet photons escape outward from the stars, they ionize the surrounding gas. Electrons then recombine with atomic nuclei and emit visible light, making the gas cloud glow. Hydrogen predominates in these clouds, and photons emitted by hydrogen atoms fall in the red part of the electromagnetic spectrum, so the nebula appears reddish. Reflection nebulae, by contrast, are produced mainly when visible light is scattered by dust within the cloud.

Reflection nebulae shine by reflecting light from nearby stars. Because scattering is more efficient for blue light than for red light—the same process that makes the sky blue and sunsets red—reflection nebulae are usually blue.

Dark Nebulae

If there is no bright star near a cloud of gas and dust, the nebula will be dark, making it a dark nebula. A dark nebula may be dense enough to obscure the light of an emission or reflection nebula behind it, as in the case of the Horsehead Nebula, or to block background stars. Although dark nebulae emit no light of their own and have no nearby light to reflect, they absorb and scatter light originating behind them. Their silhouettes can therefore be found against the star-rich Milky Way and bright diffuse nebulae.

Interstellar Bubbles

The final concept is the interstellar bubble. Some regions of interstellar space are extraordinarily tenuous yet contain gas hotter than that in emission nebulae. Ultraviolet observations by space telescopes have revealed these superheated interstellar “bubbles.” They form the medium between clouds and occupy enormous volumes. The hot gas was probably created when remnants left by stars that exploded long ago expanded rapidly. These regions have high temperatures but appear faint because their matter is so sparse. Our Solar System lies within one such low-density region, which surrounds us and is known as the Local Bubble. The Local Bubble contains about 200,000 stars and spans roughly 100 pc. It was very likely formed by ancient supernova explosions whose energy heated the surrounding interstellar gas and drove that gas out of the Solar System’s immediate neighborhood.

Dark Dust Clouds

Emission nebulae and interstellar bubbles make up only a small part of the interstellar medium. In fact, most of the universe—more than about 80%—is empty space with no stars, only endless cold, at a temperature of roughly 100 K, and darkness. Yet another class of object lies hidden in this dark void: dark dust clouds. These clouds are colder than their surroundings, reaching temperatures of only a few tens of kelvins, but are thousands to millions of times denser. They are therefore also known as “dense interstellar gas clouds.”

The 21 cm Line

The 21-centimeter line of neutral hydrogen is an important tool for studying the distribution of neutral hydrogen in interstellar space and the structures of the Milky Way and other galaxies. It allows us to detect the radiation emitted by cold, neutral interstellar gas anywhere in space. The method relies on low-energy radiation produced by the interstellar gas itself.

How is the 21 cm line produced? All matter in the universe tends toward the lowest possible energy state, and the interstellar medium is no exception. In a slightly excited hydrogen atom, the electron and proton spins point in the same direction. The atom can return to its ground state, in which the spins point in opposite directions, by releasing a photon whose energy equals the difference between the two states.

The energy difference between these levels is extremely small, so the emitted photon has low energy and a long wavelength: 21.1 cm. Astronomers also call this 21 cm radiation. Its wavelength is far greater than the size of interstellar dust grains, so it is not scattered by interstellar dust before reaching Earth. Twenty-one-centimeter radiation can probe interstellar space thousands of parsecs away and is profoundly important for studying the structure of the Milky Way and other galaxies.

References: Astronomy Today and Baidu Baike