Interstellar Matter

Interstellar Matter

I

INTRODUCTION

Interstellar Matter, gas and dust between the stars in a galaxy. In our own galaxy, the Milky Way, we can see glowing gas and dark, obscuring dust between the galaxy’s many visible stars. This gas and dust makes up interstellar matter. Galaxies differ in the density of interstellar matter that they contain. Spiral galaxies, such as the Milky Way, have much more interstellar matter than elliptical galaxies, which have almost none. About 3 percent of the mass of the Milky Way Galaxy is interstellar gas, and 1 percent is interstellar dust. Stars make up the rest of the ordinary matter in the galaxy. Dark matter—a material that does not reflect or emit light or other forms of electromagnetic radiation—also makes up some of the mass of the galaxy. Astronomers consider interstellar matter separately from intergalactic matter, or matter between galaxies.

Hydrogen gas makes up most of the interstellar matter, but essentially all of the chemical elements occur in interstellar matter. About 90 percent of the atoms in space are hydrogen, about 9 percent helium, and less than 1 percent consists of all the other chemical elements. The interstellar matter is so spread out that the space it occupies would be considered a vacuum in laboratories on Earth.

II

DISTRIBUTION OF INTERSTELLAR MATTER

Most astronomers believe that the Milky Way Galaxy condensed out of a huge cloud of gas. Most of the interstellar gas that now exists is presumably left over from the formation of the galaxy. This gas consisted mainly of the lighter elements hydrogen and helium, but heavier elements joined the gas as the galaxy evolved. These heavier elements, which are the products of various stars, are released into interstellar space as a star evolves or when a star explodes at the end of its life. Nuclear fusion reactions inside massive stars form most of the moderately heavy chemical elements—that is, those elements with atomic weights between that of lithium and iron. Supernova explosions, which mark the end of the lives of massive stars (see Supernova), form the heaviest naturally occurring elements, such as silver and lead. Some of these heaviest elements are also produced inside binary star systems.

Red giant stars—large, bright, relatively cool stars that evolve from stars like the sun—produce interstellar dust particles as their atmospheres expand and cool. Small particles of silica and carbon form in the atmosphere and drift into interstellar space. Atoms collect on the surface of these particles, adding to the particle size and sometimes forming molecules.

A

Nebulas

Many of the most beautiful examples of interstellar matter are in the form of nebulas, regions of gas and dust scattered through the galaxy. Many nebulas emit or reflect light in the visible part of the electromagnetic spectrum, and so are visible when viewed through a telescope. French astronomer Charles Messier cataloged many nebulas in the mid-1700s. Amateur astronomers, as well as professionals, often study nebulas. The high resolution of the Wide Field and Planetary Camera 2 on the Hubble Space Telescope has allowed astronomers to image all types of nebulas much more clearly than before.

Nebulas glow for one of two reasons—reflection or emission. Reflection nebulas are composed mostly of dust. When a reflection nebula occurs near a star or a group of hot stars, light from the stars illuminates the gas and dust to produce wispy, bluish patches. Emission nebulas are composed mostly of ionized hydrogen—hydrogen atoms that have lost their electrons. Energy from nearby stars heats the gas, making it emit a reddish light. A special class of nebulas, known as planetary nebulas, are composed of gas given off by stars like the sun in a late stage of their lifetimes. They are called planetary nebulas because early astronomers noticed that they looked like the faint disks of distant planets.

B

Galactic Halo

Most known nebulas occur in the plane of the galaxy. The galactic halo is a huge sphere that surrounds the plane of the galaxy. Astronomers believe that the halo must contain about 90 percent of the total mass of the galaxy. A fraction of that mass occurs in visible matter—mostly globular star clusters. About half of the halo’s mass is probably made up of small stars that are dark. Such stars have used up their nuclear fuel or are not massive enough to begin nuclear reactions. The rest of the halo’s matter may be interstellar matter in the form of interstellar dust or weakly interacting particles. Weakly interacting particles are nuclear particles that participate only in the weak interaction, one of the four ways matter interacts (the others are the strong interaction, gravity, and electromagnetic interaction).

C

Other Galaxies

Astronomers and cosmologists are actively studying interstellar matter in galaxies other than the Milky Way. Irregular galaxies such as the Large and Small Magellanic Clouds—satellites of our own galaxy—often have much interstellar matter. Spiral galaxies in general also have large amounts of interstellar matter—spiral galaxies that appear edge-on from Earth show dark lanes, or long, narrow dark patches where interstellar dust appears dark in silhouette against radiation from farther away. The Hubble Space Telescope is powerful enough to make detailed images of emission nebulas in nearby spiral galaxies. Studying interstellar matter in other galaxies helps astronomers understand the structure of our own galaxy.

III

EFFECTS OF INTERSTELLAR MATTER

While astronomers can detect some interstellar matter directly, they can also detect interstellar matter by how it changes the radiation that travels through it. Astronomers can then study the interstellar matter by measuring how it changes this radiation. Interstellar matter blocks, reflects, and absorbs radiation. Astronomers detect interstellar matter in a wide variety of ways, using instruments that are sensitive in many parts of the electromagnetic spectrum, from radio waves to X rays. See also Electromagnetic radiation.

A

Interstellar Dust

Interstellar dust produces effects that are quite different from interstellar gas. Dust particles can block all of the light from a source, or they can just block certain wavelengths. Dust may also reflect light that hits it, making light from a single star appear diffuse and cloudy. Dust particles can also emit their own radiation if they absorb enough energy from other sources. Glowing dust particles can also be detected in the infrared, even if they are invisible in the visible light part of the spectrum.

A1

Extinction

Interstellar dust makes up only about 1 percent of interstellar matter. Sometimes, it has sufficient density to absorb enough light that astronomers can see the silhouette of a cloud of dust. At other times, it blocks only a percentage of the light from behind it, a process known by astronomers as extinction. The long, narrow dark lanes in the Milky Way as seen from Earth are examples of extinction. The amount of extinction is different for different wavelengths of light.

A2

Reddening

Starlight that does not get completely absorbed by interstellar dust can still be changed by the dust’s effects. As light passes through less dense patches of interstellar dust, the dust particles scatter some of the light. The dust particles are of a particular size that scatters light of short wavelengths more than light of long wavelengths. In the visible light area of the spectrum, this means that more of the original red light (with a long wavelength) than the original blue light (with a short wavelength) gets through the dust. This makes distant stars appear redder than they actually are. Astronomers call this process reddening. Reddening is not related to the red shift caused by the movement of distant galaxies.

A3

Infrared Radiation

Interstellar dust blocks visible light, but the light and other radiation from stars also warms the dust and makes it emit energy as infrared radiation. Most infrared radiation does not pass through Earth's atmosphere, so astronomers use observatories at high altitude such as the Mauna Kea Observatory in Hawaii or observatories in space to study infrared radiation. See also Infrared Astronomy.

A4

Reflection

Interstellar dust often surrounds newly formed stars. The dust reflects light from the stars to produce a reflection nebula, a fuzzy patch of bluish light. The Pleiades star cluster is an example of a reflection nebula. A cluster of stars surrounded by a cloud of dust makes up the Pleiades. The dust reflects and diffuses the light from the stars into several clouds of light.

B

Interstellar Gas

Gas does not block as much radiation as dust does, but astronomers can detect the presence of interstellar gas because of the radiation it emits and absorbs.

B1

Radio Emissions

Much of the interstellar gas is neutral hydrogen—that is, hydrogen in its lowest energy state (also known as its ground state). An atom of neutral hydrogen has two possible orientations, depending on a property—called spin—of the atom’s single electron. When a hydrogen atom switches between these two versions of the ground state, it gives off a photon, or a packet of electromagnetic radiation, with a wavelength of 21 cm (8.3 in). This wavelength is in the radio area of the electromagnetic spectrum and can be detected with a radio telescope.

Astronomers have used this 21-cm radiation to map the distribution of gas in space. If the gas is moving relative to Earth, the radiation it produces will have a slightly different wavelength. Gas moving away from Earth will seem to produce radiation with a slightly longer wavelength, while gas moving toward the planet will appear to produce slightly shorter wavelengths. This shift in wavelength arises from the relative movement between the source of the radiation and the observer on Earth, and it is called a Doppler shift (see Doppler Effect). Studying the movement of gas enables astronomers to study the galaxy’s structure and see how the galaxy rotates.

The ground-state hydrogen atom is not the only atom or molecule that emits radio waves. Since the 1960s, radio astronomers have discovered about 100 types of molecules in interstellar space that emit radio waves. The intensity of these emissions and their Doppler shifts have contributed to mapping the Milky Way Galaxy and to determining the composition of the Milky Way and other galaxies.

B2

Emission and Absorption Lines

Astronomers can also study interstellar gas by using the fact that atoms emit or absorb radiation (such as light) when they change from one energy level to another. Atoms emit radiation when they drop from one energy level to a lower energy level and absorb radiation when they jump to a higher level. In the case of interstellar gas, the radiation they absorb is provided by the light of nearby stars.

In a cloud of interstellar gas, many atoms will make the same energy level change at the same time, creating enough change in radiation to allow astronomers to study the gas. Astronomers study radiation from interstellar gas by separating the radiation into its different wavelengths, or its spectrum, much as a prism will separate white light into the colors of a rainbow (Spectroscopy). Atoms of a particular element at a particular energy level will only emit or absorb radiation at very specific wavelengths, or colors in the case of visible light. Many atoms making the same energy-level change will show up on the spectrum as bright or dark lines. The bright lines, caused by atoms emitting radiation, are called emission lines. The dark lines, caused by atoms absorbing radiation at a particular wavelength, are called absorption lines. If the cloud of gas is moving relative to Earth, the lines may be shifted by the Doppler effect. Astronomers use the wavelengths at which emission or absorption lines occur to determine the types of atoms present and the speed and direction of the movement of the cloud.

Emission and absorption lines are not limited to radiation in the visible light range. Neutral hydrogen produces emission and absorption lines at some ultraviolet and some radio wavelengths. Molecular hydrogen (H₂, two hydrogen nuclei sharing their electrons) emit and absorb in the ultraviolet part of the spectrum. Some of the gas in the interstellar medium is hot, about 100,000° C (about 200,000° F). Gas this hot emits radiation in the X-ray range. Astronomers can determine the gas’s temperature by analyzing its spectrum. See also X-Ray Astronomy.

Contributed By:
Jay M. Pasachoff

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