Bonus material for Where Do Chemical Elements Come From? by Carolyn Ruth
After chemical elements have formed in outer space, they combine with one another to
form molecules. These molecules form where it is cold and dense, not in the intense heat
of supernovae or their remnants. Also, when a supernova spews its newly made elements
into space, the elements become part of an enormous cloud of gas and dust. In this cloud,
the chemical elements gather to form masses that presumably become new stars and
planets. Our solar system was likely formed that way.
Molecules—and maybe life?—from space
Lucy Ziurys, a professor of astronomy and chemistry at the University of Arizona,
Tucson, and colleagues have used two telescopes operated by the Arizona Radio
Observatory—one located on Kitt Peak, 50 miles southwest of Tucson, and the other
located on Mt. Graham, near Safford—to discover 17 interstellar molecules and radicals
(atoms, molecules, or ions with unpaired electrons), including glycolaldehyde (HOCH2CH=O), the first sugar found in space and phosphorus-bearing radicals—such as PO,
CCP, and PN. “Some of these molecules and radicals had never been before detected and
may help us understand how chemicals evolve in space and may contribute to the origin
of life,” Ziurys says.
To identify these chemical compounds, Ziurys and her team study radiation emitted by
them. When molecules bump into each other, they absorb energy. This energy is then
released in the form of microwave radiation. The scientists use a device called a
spectrometer that analyzes microwave radiation and produces a series of curves with
peaks called a molecular spectrum that is specific to each type of molecule. Examples of
such spectra and their corresponding molecules are shown in figure.
Molecular spectra obtained from molecules and radicals—parts of molecules—
detected in a cloud of gas resulting from the explosion of the star IRC+10216.
Astronomers have a directory of spectra for all known atoms and molecules. When they
receive radiation from outer space, they analyze the spectrum of that radiation and then
compare it to what they have in their directory. When they find a match, they know what
atom or molecule in outer space emitted the radiation.
But many times, the emission spectra analyzed by Ziurys and her team did not
completely match those available in the directories. To understand what chemicals
produced those spectra, the scientists are now trying to create conditions in the laboratory
that are similar to those in outer space. Doing this will allow them to produce exotic,
short-lived outer-space chemical compounds in detectable concentrations. Then, they will
study the radiation they emit, thus producing these compounds’ fingerprints. Their hope
is that these fingerprints will match some of the emission spectra produced by yetunidentified chemicals.
Our stellar origins
The cloud of gas and dust created by a supernova is called an interstellar cloud. The gas
is made of 90% hydrogen, 9% helium, and 1% heavier atoms. The dust contains silicates
(compounds made of silicon), carbon, iron, water ice, methane (CH4), ammonia (NH3),
and some organic molecules such as formaldehyde (H2CO).
Interstellar clouds are found so often between stars in our galaxy that astronomers think
that all stars and planets have formed from them. Although astronomers are still working
out the details of how stars and planets come to be, most of them agree that it happens
according to a process called the solar nebula hypothesis.
According to this hypothesis, inside the interstellar cloud, atoms from the explosion first
accumulate and interact with the dust particles, forming larger particles. As these
particles grow larger, they start producing a gravitational pull that attracts even more
particles, creating big clumps of matter.
As the cloud is rotating while all this is happening, it collapses inward, going from a
more or less spherical shape to a disk-like shape. “The cloud flattens like a chef flattens
pizza dough by tossing it into the air with a spin,” says Clinton Jones, professor of
chemistry and astronomy at Mercyhurst College, Erie, Pa.
A big clump at the center of the disk becomes a new star, while smaller clumps inside the
disk become planets. The reason why only the central clump becomes a star is that, as the
disk rotates, the speed of the particles is the highest in the middle. As more and more
particles accumulate on the central clump, heat builds up and gravitational energy is
transformed into kinetic energy. This raises the temperature even more, and ultimately
hydrogen nuclei fuse with each other to create helium and more energy. This additional
energy becomes the star’s “fuel,” which will sustain the star for as long as its hydrogen
reserves are available.
According to the solar nebula hypothesis, planet formation is a normal part of star
formation. If our sun and planets derived from interstellar clouds, it is likely that other
planetary systems formed in the same way. There are other explanations for solar system
formation, though. They differ mainly in the way in which the contracting process occurs,
but they are all based on the same overall mechanism: The sun and planets form from the
contraction of an interstellar cloud under its own gravitational pull, and the rotation of the
cloud is responsible for forming the disk.
—Carolyn Ruth