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14.11 The Interstellar Medium

Interstellar material includes gas (that is, atoms and molecules), microscopic dust grains, and possibly larger objects. Atoms of interstellar gas were discovered in 1904, when German astronomer Johannes Hartmann detected their absorption lines. While studying the spectrum of a binary star system, he accidentally discovered absorption lines caused by ionized interstellar calcium atoms that are between the Earth and this binary system. Absorption lines due to other elements, like sodium, were soon discovered. Since these lines had different Doppler shifts than the stars in whose spectra they appear, the atoms that cause the absorption must lie in the space between stars. Further studies have convinced astronomers that, although the most prominent absorption comes from atoms such as calcium and sodium, the most common interstellar gas is the ubiquitous hydrogen. Like the Sun and stars, interstellar gas is about three-fourths hydrogen and nearly one-fourth helium by mass. 

The most important tracer of interstellar gas is a spectral line that is emitted at radio wavelengths. In 1944, the Dutch astronomer H. C. van der Hulst predicted that cold hydrogen gas would have a spectral feature at a wavelength of 21-cm, caused by a change in the spin of hydrogen atoms’ electrons. A low-energy radio photon is emitted when the atom flips"" between two nearly equal quantum states. Harvard astronomers detected the predicted emission in 1951 using radio equipment. The 21-cm emission line of atomic hydrogen is extremely important in astronomy for several reasons. First, the widespread detection of this line confirms that hydrogen is the main constituent of interstellar gas. Second, the long wavelength and low energy of the spectral line is a sign that the hydrogen is at a very low temperature (cold material will emit long wavelength spectral features). The 21-cm emission comes from gas as cold as 10 or 20 K. Last, the long wavelength of this radiation means that it can penetrate much greater distances through the interstellar gas and dust than ordinary, visible wavelengths of light.

By 1940, astronomers at Mount Wilson Observatory in California had detected another component of interstellar material: interstellar molecules. In the cold and low-density environment of an interstellar cloud, molecules have many low-energy transitions that create spectral features in the infrared or radio parts of the spectrum. Astronomers adopted the infrared and radio technology that had been developed during World War II, and after the war they turned it to the search for molecules in space. They quickly discovered molecules using the new technology, and the use of these new detectors on large telescopes sparked an explosion of interstellar discovery in the late 1960s and 1970s.

More than 130 different interstellar molecules have been cataloged. The most important in astrophysical processes are molecular hydrogen (H2) and carbon monoxide (CO). Four atoms recur in these large molecules — carbon, hydrogen, oxygen, and nitrogen — the "building blocks of life!" It is intriguing that two of the detected molecules, methylamine (CH3NH2) and formic acid (HCOOH), can react to form the amino acid glycine (NH2CH2COOH). These large molecules can join to form the huge protein molecules that occur in living cells. Two exciting questions have come from research on interstellar molecules. First, does the existence of complex, carbon-rich molecules in space suggest that life could have originated elsewhere in space? Second, how do these molecules form? They are found both in ordinary interstellar gas, where collisions between atoms are extremely rare, and in denser regions, such as the clouds in which stars form.

Interstellar grains, or dust grains, are even bigger than interstellar molecules. Typical grains are about the size of smoke particles in our air. Grains account for about 1% of the total interstellar mass, although there is only 1 for every 1012 hydrogen atoms or molecules. These tiny particles range in length from 1/100 to twice the wavelength of visible light and they are often elongated. Grain composition has long been debated. In 1974, Sri Lankan astrophysicist Chandra Wickramasinghe proposed that grains are made of a form of carbon, like soot, condensed in cooling gas blown off by carbon-rich giant stars. Many different carbon compounds, silicates, and ices have in fact been identified by spectroscopy, especially for grains in star-forming regions. The grains may also contain iron, since elongated grains align parallel to each other, even though they are widely separated in space. Astronomers believe they are aligned by the weak magnetic field that permeates interstellar space, just as iron filings are aligned by a bar magnet.

With a composition of silicates, metals, and ices, interstellar grains resemble the dust in our primordial Solar System. Intriguingly, reactions initiated by ultraviolet light in these materials can create complex organic molecules on grain surfaces. These microscopic particles are tiny laboratories that play an important but still poorly understood role in interstellar chemistry. Even though the tiny dust grains are rare compared to hydrogen atom, they have a big effect on starlight.

The distribution of gas and dust in our galaxy is responsible for blocking our visible light view through the galactic disk. In other (typically spiral) galaxies this material can form opaque "dust lanes," that while beautiful are scientifically annoying because of what they prevent us from seeing. The study of interstellar materials is a still young area of research and new molecules are still being discovered, and new ways of compensating for the light the gas and dust blocks are still be mapped.

Astropedia Image
An electron orbiting a proton with parallel spins (pictured) has higher energy than if the spins were anti-parallel. Click here for original source URL.

", the widespread detection of this line confirms that hydrogen is the main constituent of interstellar gas. Second, the long wavelength and low energy of the spectral line is a sign that the hydrogen is at a very low temperature (cold material will emit long wavelength spectral features). The 21-cm emission comes from gas as cold as 10 or 20 K. Last, the long wavelength of this radiation means that it can penetrate much greater distances through the interstellar gas and dust than ordinary, visible wavelengths of light.

 

By 1940, astronomers at Mt. Wilson Observatory in California had detected another component of interstellar material: interstellar molecules. In the cold and low-density environment of an interstellar cloud, molecules have many low-energy transitions that create spectral features in the infrared or radio parts of the spectrum. Astronomers adopted the infrared and radio technology that had been developed during World War II, and after the war they turned it to the search for molecules in space. They quickly discovered molecules using the new technology, and the use of these new detectors on large telescopes sparked an explosion of interstellar discovery in the late 1960s and 1970s.

More than 100 different interstellar molecules have been cataloged. The most important in astrophysical processes are molecular hydrogen (H2) and carbon monoxide (CO). Four atoms recur in these large molecules — carbon, hydrogen, oxygen," and nitrogen — the ""building blocks of life""! It is intriguing that two of the detected molecules"," methylamine (CH3NH2) and formic acid (HCOOH)"," can react to form the amino acid glycine (NH2CH2COOH). These large molecules can join to form the huge protein molecules that occur in living cells. Two exciting questions have come from research on interstellar molecules. First", does the existence of complex, carbon-rich molecules in space suggest that life could have originated elsewhere in space? Second, how do these molecules form? They are found both in ordinary interstellar gas, where collisions between atoms are extremely rare, and in denser regions, such as the clouds in which stars form.

Astropedia Image
Trapezium in Orion, as seen by the Hubble Space Telescope. Click here for original source URL.

Astropedia Image
Chandra Wickramasinghe. Click here for original source URL.

Interstellar grains, or dust grains, are even bigger than interstellar molecules. Typical grains are about the size of smoke particles in our air. Grains account for about 1% of the total interstellar mass, although there is only 1 for every 1012 hydrogen atoms or molecules. These tiny particles range in length from 1/100 to twice the wavelength of visible light and they are often elongated. Grain composition has long been debated. In 1974, Sri Lankan astrophysicist Chandra Wickramasinghe proposed that grains are made of a form of carbon, like soot, condensed in cooling gas blown off by carbon-rich giant stars. Many different carbon compounds, silicates, and ices have in fact been identified by spectroscopy, especially for grains in star-forming regions. The grains may also contain iron, since elongated grains align parallel to each other, even though they are widely separated in space. Astronomers believe they are aligned by the weak magnetic field that permeates interstellar space, just as iron filings are aligned by a bar magnet.

Astropedia Image
Star forming region NGC 3603 showing young stars surrounded by gas and dust. Click here for original source URL.

With a composition of silicates, metals, and ices, interstellar grains resemble the dust in our primordial solar system. Intriguingly, reactions initiated by ultraviolet light in these materials can create complex organic molecules on grain surfaces. These microscopic particles are tiny laboratories that play an important but still poorly understood role in interstellar chemistry. Even though the tiny dust grains are rare compared to hydrogen atom," they have a big effect on starlight.

Astropedia Image
Star forming region NGC 2264, also known as the Cone Nebula. Click here for original source URL.

The distribution of gas and dust in our galaxy is responsible for blocking our visible light view through the galactic disk. In other (typically spiral) galaxies this material can form opaque ""dust lanes"," that while beautiful are scientifically annoying because of what they prevent us from seeing. The study of interstellar materials is a still young area of research and new molecules are still being discovered, and new ways of compensating for the light the gas and dust blocks are still be mapped.