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Giant molecular clouds have densities of hundreds to thousands of atoms per cm 3 , much denser than interstellar space is on average. As a result, though they account for a very small fraction of the volume of interstellar space, they contain a significant fraction—20–30%—of the total mass of the Milky Way’s gas. Because of their high density, molecular clouds block ultraviolet starlight, the main agent for heating most interstellar gas. As a result, they tend to be extremely cold, with typical temperatures near 10 K (−263 °C). Giant molecular clouds are also the sites where new stars form, as we will discuss below.
It is in these dark regions of space, protected from starlight, that molecules can form. Chemical reactions occurring both in the gas and on the surface of dust grains lead to much more complex compounds, hundreds of which have been identified in interstellar space. Among the simplest of these are water (H 2 O), carbon monoxide (CO), which is produced by fires on Earth, and ammonia (NH 3 ), whose smell you recognize in strong home cleaning products. Carbon monoxide is particularly abundant in interstellar space and is the primary tool that astronomers use to study giant molecular clouds. Unfortunately, the most abundant molecule, H 2 , is particularly difficult to observe directly because in most giant molecular clouds, it is too cold to emit even at radio wavelengths. CO, which tends to be present wherever H 2 is found, is a much better emitter and is often used by astronomers to trace molecular hydrogen.
The more complex molecules astronomers have found are mostly combinations of hydrogen, oxygen, carbon, nitrogen, and sulfur atoms. Many of these molecules are organic (those that contain carbon and are associated with the carbon chemistry of life on Earth.) They include formaldehyde (used to preserve living tissues), alcohol (see the feature box on Cocktails in Space ), and antifreeze.
In 1996, astronomers discovered acetic acid (the prime ingredient of vinegar) in a cloud lying in the direction of the constellation of Sagittarius. To balance the sour with the sweet, a simple sugar (glycolaldehyde) has also been found. The largest compounds yet discovered in interstellar space are fullerenes , molecules in which 60 or 70 carbon atoms are arranged in a cage-like configuration (see [link] ). See [link] for a list of a few of the more interesting interstellar molecules that have been found so far.
| Some Interesting Interstellar Molecules | ||
|---|---|---|
| Name | Chemical Formula | Use on Earth |
| Ammonia | NH 3 | Household cleansers |
| Formaldehyde | H 2 CO | Embalming fluid |
| Acetylene | HC 2 H | Fuel for a welding torch |
| Acetic acid | C 2 H 2 O 4 | The essence of vinegar |
| Ethyl alcohol | CH 3 CH 2 OH | End-of-semester parties |
| Ethylene glycol | HOCH 2 CH 2 OH | Antifreeze ingredient |
| Benzene | C 6 H 6 | Carbon ring, ingredient in varnishes and dyes |
The cold interstellar clouds also contain cyanoacetylene (HC 3 N) and acetaldehyde (CH 3 CHO), generally regarded as starting points for amino acid formation. These are building blocks of proteins, which are among the fundamental chemicals from which living organisms on Earth are constructed. The presence of these organic molecules does not imply that life exists in space, but it does show that the chemical building blocks of life can form under a wide range of conditions in the universe. As we learn more about how complex molecules are produced in interstellar clouds, we gain an increased understanding of the kinds of processes that preceded the beginnings of life on Earth billions of years ago.
Interested in learning more about fullerenes, buckyballs, or buckminsterfullerenes (as they’re called)? Watch a brief video from NASA’s Jet Propulsion Laboratory that explains what they are and illustrates how they were discovered in space.
Among the molecules astronomers have identified in interstellar clouds is alcohol, which comes in two varieties: methyl (or wood) alcohol and ethyl alcohol (the kind you find in cocktails). Ethyl alcohol is a pretty complex molecule, written by chemists as C 2 H 5 OH. It is quite plentiful in space (relatively speaking). In clouds where it has been identified, we detect up to one molecule for every m 3 . The largest of the clouds (which can be several hundred light-years across) have enough ethyl alcohol to make 10 28 fifths of liquor.
We need not fear, however, that future interstellar astronauts will become interstellar alcoholics. Even if a spaceship were equipped with a giant funnel 1 kilometer across and could scoop it through such a cloud at the speed of light, it would take about a thousand years to gather up enough alcohol for one standard martini.
Furthermore, the very same clouds also contain water (H 2 O) molecules. Your scoop would gather them up as well, and there are a lot more of them because they are simpler and thus easier to form. For the fun of it, one astronomical paper actually calculated the proof of a typical cloud. Proof is the ratio of alcohol to water in a drink, where 0 proof means all water, 100 proof means half alcohol and half water, and 200 proof means all alcohol. The proof of the interstellar cloud was only 0.2, not enough to qualify as a stiff drink
Interstellar gas may be hot or cold. Gas found near hot stars emits light by fluorescence, that is, light is emitted when an electron is captured by an ion and cascades down to lower-energy levels. Glowing clouds (nebulae) of ionized hydrogen are called H II regions and have temperatures of about 10,000 K. Most hydrogen in interstellar space is not ionized and can best be studied by radio measurements of the 21-centimeter line. Some of the gas in interstellar space is at a temperature of a million degrees, even though it is far away in hot stars; this ultra-hot gas is probably heated when rapidly moving gas ejected in supernova explosions sweeps through space. In some places, gravity gathers interstellar gas into giant clouds, within which the gas is protected from starlight and can form molecules; more than 200 different molecules have been found in space, including the basic building blocks of proteins, which are fundamental to life as we know it here on Earth.
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