Wrapped in a thick cloud of gas and dust, the swaddling clothes of celestial nativity, an infant star in the Milky Way is growing and attracting the attention of astronomers, for the most part confirming their theories about a critical stage in the act of stellar creation.
This nascent star is not far away by cosmic standards, only 800 light-years
from Earth, in the direction of the constellation Aquila.
Diagram by Nigel Holmes, published in the New York Times
It is not that big, only half as massive as the Sun, though greater size may come with maturity. And it is not that unusual. While other stars are aging and collapsing in death, some with a bang, some a whimper, the universe is always replenishing itself with new stars.
First stage of star formation. Diagram by Nigel Holmes, published in the New York Times
Second stage of star formation. Diagram by Nigel Holmes, published in the New York Times
But for years astronomers have been observing the birthplace of this particular star, an interstellar gas cloud of a type, called a Bok globule, that is often the site of star formation. They cannot see the new star embedded in the opaque cloud, but know from infrared observations that it is there and must be about 150,000 years old, not yet a true star. They call it a protostar, because it has not reached the core densities and temperatures required to ignite nuclear fusion. The heat it radiates is generated from the gravitational energy released as the cloud gases contract.
Now, with the increasing power of radio telescopes, scientists with the National Aeronautics and Space Administration have obtained the first images of the star-forming process at this early stage. The radio images show material still collapsing onto this protostar, something never before illustrated.
Third stage in the birth of a star and solar system. Diagram by Nigel Holmes, published in the New York Times
By detecting the radio emissions of a certain molecule, the scientists were able to trace material from the outer gas cloud falling toward the protostar and its rotating disk. It is a phenomenon scientists had predicted, but had never been able to observe so clearly.
Their achievement has afforded astronomers a revealing glimpse of the dynamic forces contributing to the growth of a new star and its surrounding disk of gas and dust out of which planets could emerge. The findings, scientists said, should also provide insights into conditions in the early solar system before planets began forming. Their new observational prowess could lead to identifying the likely chemical composition of the Sun's preplanetary environs.
Final stage in the birth of a solar system. Diagram by Nigel Holmes, published in the New York Times
The first direct images of this so-called in-falling material were produced by combining radio observations made over the last three years by the NASA Deep Space Network antenna at Goldstone, Calif., and the Very Large Array, a collection of radio telescopes outside Socorro, N.M., operated by the National Radio Astronomy Observatory. The results were reported in the current issue of Astrophysical Journal Letters by Dr. Thangasamy Velusamy, Dr. Thomas B.H. Kuiper and Dr. William D. Langer, astrophysicists at the Jet Propulsion Laboratory in Pasadena, Calif.
``This is confirmation of the general theory of protostar formation,'' Langer said in an interview.
Astrophysicists had been actively searching for this confirmation for at least a decade. They already understood the initial stage of star formation. A cloud of gas becomes so dense that gravity causes it to collapse, producing a protostar at the center.
They also knew that in the final stages, after an evolutionary period of perhaps a million years, the cloud collapse ceases, the core object further contracts until its density can support nuclear fusion and thus a shining star is born. But they could only make educated guesses about the details of what was happening in between.
``It's extremely hard to find unambiguous evidence of the continuing infall of material on a protostar,'' said Dr. Anneila I. Sargent, an astronomer at the California Institute of Technology in Pasadena. ``If they are really seeing the collapse in a star-forming cloud, that's tremendously important. It's very exciting.''
Like Sargent and others who have been conducting similar studies, Neal J. Evans, an astronomer at the University of Texas at Austin, said that previous research had been producing data that ``basically support the interpretation of in-falling material.'' It would be important, he said, to examine many more regions of protostar development to be certain that the theory has been confirmed and to understand more details of this stage in star formation.
The scientists at the Jet Propulsion Laboratory focused on the protostar in the gas cloud designated B335. It is one of the Bok globules, named after Dr. Bart J. Bok, the late Dutch-American astronomer who in 1947 was one of the first to point out their potential role as star-formation sites. Astronomers now know that the vast majority of stars in the Milky Way are born in objects known as giant molecular clouds, though Dr. Robert L. Dickman, an astrophysicist at the University of Massachusetts in Amherst, notes that Bok globules, being simpler structures, ``offer the best prospects for understanding the still obscure process of star birth.''
Hydrogen constitutes the preponderance of the gas in these globules, but there are also sufficient traces of oxygen, carbon, nitrogen and sulfur to form dozens of complex molecules. One of these, the chemical dicarbon monosulfide, served as the tracer in determining the motion of gases falling toward and feeding the protostar and its surrounding disk. Emissions from the molecules in radio wavelengths, though extremely weak, were detected and plotted by radio telescopes.
The roughly spherical region of the gas cloud whose image was obtained by the radio astronomers extended over a distance about 7,000 times that of Earth's orbit around the Sun. That distance is comparable to the outermost regions of the solar system, far beyond the planets and out where comets are formed.
Examining the images, the astronomy team led by Velusamy searched for evidence that material was still falling straight in toward the protostar, as most theorists assumed. Or was it spiraling in? Or had it already stopped?
In their report, the astronomers said the telescopes detected a radial flow of the tracer molecules, arising primarily from the outer parts of the collapsing cloud and extending in about half of the cloud's radius. The scientists were not sure why there was an absence of the molecules in the deeper interior of the cloud. But they concluded that the velocity and pattern of the in-falling material supported ``the evidence for inside-out collapse'' and was consistent ``with accretion onto a rotating central disk.''
Figure 2 from the paper in the Astrophysical Journal Letters. The image on the left was made with the NASA Goldstone 70-m antenna. The antenna beam is too large to see the central hole in the emission. The center image was made with the NRAO VLA, which is sensitive only to the small scale structure, and shows only a hint of the real distribution. The image on the right was made by combining the two data sets, and shows the circular distribution arsing from CCS in the outer parts of the infalling envelope. The circle shows the outer boundary of the infalling envelope.
The apparent inside-out collapse has been a source of confusion for theorists and observers alike. More than a decade ago, astronomers began finding that nearly all newborn stars go through a phase in which they seem to be rejecting mass at the same time they were also presumably drawing in mass from the cloud collapse. This was a common occurrence. Astronomers repeatedly observed two jets of gas shooting out at opposite sides and perpendicular to the disk of rotating matter around the protostar. The rotational forces were apparently twisting the magnetic field lines, producing winds carrying gas out in powerful jets.
The outflow phenomenon was as frustrating as it was fascinating. The jets were easily detected by radio telescopes and tended to obscure the view of other star-formation processes, like the inflow of material. So astronomers naturally devoted most of their time and thought to the jets they could see but could not explain, rather than on the inflowing material they hoped to see but could not.
The new research promises to draw new attention to questions related to the intermediate stage of star formation, the time after the initial collapse of the gas sphere into a protostar and before the fall of material ceases and planetary formation becomes possible.
One particular observation by the radio astronomers is already pointing to new avenues of study. They reported an asymmetric clumpy distribution of the trace emissions, which implies that the physical conditions of the collapsing cloud were not spherical and that the gas falling onto the circumstellar disk may be episodic. It was a result that especially intrigued Evans of the University of Texas.
``This is showing that material around the young star is not completely smooth,'' he said. ``If the region is really lumpy and there's still infall of material, it may change the way in which the star grows. The results show our theory seems to work pretty well. But how lumpy and how episodic the inflow feeding a new star, are questions that will occupy us for a long time.''
As a result of the new findings about B335 and many other observational efforts, Sargent said, the whole field of protostar research ``is probably going to take off in the next 10 years.''
The result could be a more profound understanding not only of how stars are formed, but planets as well. The new findings, Langer said, suggest phenomena that match the theory of the formation of the solar system 4.6 billion years ago. A large gas cloud collapsed to form a star with an attendant circumstellar disk. Over time, planets accreted from the matter in the disk and orbited the Sun.
Astrophysicists have reasons to think this was not a singular phenomenon, notably the photographs and other data from stars like Beta Pictoris showing possibly preplanetary disks of gas and dust rotating around them. This may not happen to the smaller protostar in B335, but its career has hardly begun, and astronomers acknowledge that they will need more than one set of radio images to understand what makes a young star.
This text was obtained via America OnLine.
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