Recent discoveries in vast interstellar dust clouds permeating the universe and in nebula have revealed hints of organic matter that could be created naturally by stars, according to researchers in a 2011 study at the University of Hong Kong. The discovery team observed stars at different evolutionary phases and found that they are able to produce complex organic compounds and eject them into space, filling the voids between stars.
The compounds are so complex that their chemical structures resemble the makeup of coal and petroleum, the study's lead author, Sun Kwok of the University of Hong Kong, said. Kwok and his colleague Yong Zhang, also of the University of Hong Kong, studied a set of well-known but mysterious infrared emissions found in stars, interstellar space and galaxies. These phenomena, which are collectively called Unidentified Infrared Emission (UIE) features, have been known for 30 years, but the exact source of the emissions has not been identified, and remains a broad assumption.
Such chemical complexity was thought to arise only from living organisms, but the results of the new study show that these organic compounds can be created in space even when no life forms are present. In fact, such complex organics could be produced naturally by stars, and at an extremely rapid pace.
"What impressed me most is that complex organics are easily formed by stars, they are everywhere in our own galaxy and in other galaxies," Kwok told Space.com. "Nature is much more clever than we had imagined."
"In the astronomy community, it has been commonly assumed that the UIE features are emitted by (polycyclic aromatic hydrocarbon, or PAH) molecules, which are simple, purely aromatic, molecules made of carbon and hydrogen," Kwok said. Their findings have overturned this assumption.
"I have been suspecting this for many years," Kwok said. "Now we think we have the evidence."
The researchers observed stars at different phases of stellar evolution — first low- to medium-mass stars, then stars in the protoplanetary nebula phase, which is a short-lived episode during a star's rapid evolution, and finally stars in the planetary nebula phase, which is characterized by an expanding shell of ionized gas that is ejected by certain types of stars late in their life.
Kwok and his colleague found that characteristics of the Unidentified Infrared Emission features could not be detected in low- to medium-mass stars. But, the astronomers found that the emissions began to appear in stars in the protoplanetary nebula stage and grew stronger as the stars matured into the planetary nebula phase.
"Since we know their dynamical and evolutionary ages of these objects (dynamical age is how fast the nebula will disperse, and evolutionary age is how fast the star is evolving), we can put constraints on the chemical time scales," Kwok said. "Since the dynamical/evolution ages are of the order of thousands of years, the appearance of the spectral features suggests that the organic compounds are made on time scales shorter than thousands of years."
"Their spectra changed from a pure gas spectrum to a dust spectrum on a matter of days or weeks," Kwok added. "The sudden appearance of the features suggests that organic dust can be made extremely quickly."
The image at the top of the page shows the long tail of interstellar dust shines in the reflected light of stars in this view of a nebula in the constellation Corona Australis (the southern crown). In some parts the dust accumulates to form dense molecular clouds from which it is thought young stars are born.
Image credit: NASA / JPL-Caltech / SSI and David Malin