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What is Astrochemistry?

MM SgrB2 ImageCredit and Copyright

Astrochemistry is the study of chemistry in space. More specifically, it is the study of the chemical interactions between the gases and dust interspersed between the stars.

For many years astronomers possessed little knowledge of the composition of interstellar space. Optical astronomy revealed only stars, galaxies, and nebulae. Darkness appeared to reign between the stars, as if nothing was there. With the arrival of radioastronomy in the 50s and 60s, astounding discoveries began to emerge.

Observation of molecular hydrogen's 21 centimeter spectral line, revealed an abundance of hydrogen between the stars. Up until that time, optical astronomy pointed to an absence of material; the presence of so much gas in interstellar space was inconceivable.

Since the discovery of molecular hydrogen, many more types of molecules have been detected. Some do not exist on Earth, while others abound, particularly hydrogen, carbon monoxide, ammonia, and water. Scientists are now expanding their search to more complex molecules, carbon-rich compounds that may hold the key to how life began on our planet.

Molecular Chemistry in Space

HNO molecules in SgrB2

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Giant clouds of gas and dust called Giant Molecular Clouds or GMC's contain enormous numbers of molecules. These clouds stretch across vast distances, up to a light year or more across. Throughout most of their volume, pressures, densities and temperatures are exceedingly low, a tiny fraction of those found here on Earth. Comparing molecular processes in GMC's with those on Earth can provide insights into how our planet's chemistry evolved given its unique environment.

Lew Snyder, University of Illinois, on-camera
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QuickTime Movie (1.2 MB); Sound File (783K); Text

Molecular evolution in space involves several chemical reactions, each tending to yield more complex molecules than the previous. Forged in the cores of stars, then returned to the interstellar medium during stardeath, elements such carbon, oxygen, hydrogen, and nitrogen combine to form hydrogen cyanide, water, and ammonia. Evidence is mounting that these molecules could, in turn, combine to produce simple amino acids, one of the main chemical building blocks of life.

From Amino Acids to Life

For decades, astronomers have debated whether the molecules of life were formed in the depths of space, or evolved amidst the violent volcanic eruptions and severe lighting storms that raged on the young Earth. If amino acids evolve in interstellar space, then how much more likely are they, and perhaps even carbon-based life itself, to be found elsewhere in the universe. Conversely, if the molecules required for biochemistry evolved exclusively on Earth, then life is surely a one-time deal, maybe never to be repeated in the cosmos.

Lew Snyder, on-camera
Movie/Sound Byte
QuickTime Movie (962K); Sound File (639K); Text

While these questions remain hotly debated, evidence is mounting that at least some of life's precursor molecules were formed between the stars. Embedded in meteorites and moon rocks, some amino acids may have been first created in interstellar space, then frozen in meteors which bombarded the Earth during its early history. In hopes of unmasking more evidence, astronomers are searching for amino acids in the cold, molecular gas found in some regions of our own Milky Way galaxy.

One such region is Sagittarius B2, commonly referred to as Sgr B2. A giant, star-forming molecular cloud some 30 thousand light years away, SGr B2 lies near the center of our galaxy. Containing an abundance of carbon- and nitrogen-rich molecules, Sgr B2 presents a perfect target for the search.

Employing the BIMA array, astronomers are probing this region for glycine, one of the smallest, simplest amino acids. The BIMA array's flexible spectrometer and high spectral resolution allow astronomers to map the precise abundances and locations of the molecules present in Sgr B2, as well as their velocities. Taken together, this information will help researchers ascertain how simple molecules such as hydrogen cyanide (HCN), water (H2O), ammonia (NH3), and formaldehyde (H2CO) might combine in space to yield glycine and other amino acids.

More Windows on Astrochemistry

To obtain a full understanding of molecular evolution in space, one that may settle the question of whether the precursor molecules of life, or even life itself, could have evolved in the hostile environment of the young Earth, or if these same molecules could have been transported here by comets and meteorites, radioastronomers cannot rely upon results obtained solely from millimeter observations. Comparison with observations in other wavebands, especially the centimeter and infrared regions of the electromagnetic spectrum, provide important details about the physical conditions favoring chemical evolution in space. All of these studies involve detecting a variety of tracer molecules.

Cm Star Forming Region: SgrB2

Cloaked deep within GMC's, newborn stars emit radiowaves in the centimeter waveband. This radiation reveals the presence of hot, ionized gases close to the surfaces of stars and also surrounding them. From centimeter emissions, astrochemists know where the star is located and how much energy it gives off to the surrounding molecular gases within the GMC.

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Infrared observations of tracer gases provide a measure of how hot the enveloping gas has become. Its temperature indicates how much energy is available to drive chemical reactions thought to yield complex molecules, including perhaps amino acids.

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NCSA. Last modified, 11/11/95