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JON A. MORSE Space Telescope Science Institute AND NASA

Complex jet patterns,as evident in Herbig-Haro 47,can arise because of variations in the outflow rate and the gravitational effect of companion stars.

But similar disks in milder environments of the Space Telescope Science Institute, should indeed survive long enough to turned Hubble on Herbig-Haro 30, give birth to planets. which consists of a pair of oppositely

With the discovery of all the basic directed jets. To our surprise, the images components of the modern version of revealed two small cusp-shaped nebulae

Laplace's theory—spinning clouds, out- where the source of the jets should be.

flows, disks—astronomers could begin Cutting across the nebulae is a dark to study the relationships among them. band. It soon became clear that we

My colleagues and I, along with anoth- were looking at a disk perpendicular to er group led by Christopher J. Burrows the jets. As seen from our edge-on view, the disk obscures the central star. The nebulae are dust clouds illuminated by starlight. Jets stream outward, culminating in the Herbig-Haro objects. The jigsaw puzzle of star formation was coming together.

In active galaxies, disks are crucial to the formation of jets. But how does this process work for an embryonic star? An intriguing coincidence has provided a crucial clue. All the jets and flows located near Herbig-Haro 30, with one odd exception, have roughly the same orientation. In fact, they are aligned with the magnetic field of the parent cloud. This seems to support ingenious suggestions—made by Ralph E. Pudritz and Colin A. Norman, both then at the University of Cambridge, and by Frank H. Shu of the University of California at Berkeley—for how magnetic fields could drive an outflow from a young star.

Astronomy abounds with examples of magnetic fields guiding ionized gas. For example, auroras are caused by charged particles that stream down the earth's magnetic field lines and hit the upper atmosphere. In the same way, ionized particles from a circumstellar disk could attach themselves to the field lines of either the disk or the star. Because the disk is spinning, the particles would experience a centrifugal force and would thus be flung out along the field lines. More matter would flow in to replace what was lost, and so the process would continue. Although most of the matter would end up being accreted by the star, some 10 percent might be ejected. In computer simulations the process proceeds in fits and starts, which would account for the knotty structures seen in many jets.

Nebulous No More

The realization that jets are integral to star formation may solve several of the theoretical puzzles. As particles travel outward, they carry angular momentum away from their source—which would partially explain why mature stars such as the sun rotate so slowly. Jets may also churn up the surrounding cloud, supplying the necessary turbulent support to slow down its collapse.

At the same time, many questions remain. For example, only about 50 percent of optically visible young stars are found to have disks. The other stars presumably had disks as well, but these disks may have already coalesced into planets. Observers, however, have been unable to confirm this. Another problem in star formation is the distribution of stellar masses. Why is the ratio of high-to low-mass stars pretty much the same irrespective of location in the galaxy? This ratio seems to be a fundamental property of the way molecular clouds fragment, but for unknown reasons. On a related note, researchers know little about the early life of high-mass stars— partly because they are rarer, partly because they evolve faster and are difficult to catch in the act of forming.

With these caveats, astronomers can now sketch out nature's recipe for stars. They form in interstellar clouds that consist largely of the ashes of earlier generations of stars. The dust was manufactured in the cool winds and outer atmospheres of stars as they approached the ends of their lives. The clouds are also laced with heavy elements such as iron and oxygen that were forged deep in the nuclear furnaces of bygone stars. Magnetic fields or turbulent motions hold up the clouds, but eventually they collapse under their own weight, perhaps because the magnetic fields leak away, the turbulence dissipates or a supernova goes off nearby. As the material falls in, the clouds fragment into cloudlets, each of which settles into a primitive star system. In massive molecular cores, such as those that gave rise to the cluster in the Orion Nebula, these systems are spaced every few light-weeks (as opposed to light-years) apart. Most stars in the galaxy, including the sun, probably formed in such clusters.

Jets carry away angular momentum and allow the accretion to continue. Our sun must once have had narrow jets that stretched for several light-years. What turned them off is not certain. The store of infalling material may simply have run out. Some of it may have been driven away by the outflows; if so, the jets may have served to limit the sun's final mass. Around the same time, large dust grains were beginning to stick together to form planetesimals, the building blocks of the planets. The planetesi-mals swept up any remaining gas, further choking off the jets. The outflows from the sun and its stellar contemporaries blew away the leftover gas and dust that threaded the space between them. This weakened the gravitational glue that bound them together, and over a few million years the stars dispersed. Today the nearest star to the sun is about four light-years away.

Two centuries after Laplace put forward his nebular hypothesis, the pieces are beginning to fall into place. Studies of young stars suggest not only that planet formation is going on today but that planets are very common throughout our own and other galaxies. E9

Magnetic, Peripatetic mn

The Author

THOMAS P. RAY told his high school career adviser that he wanted to become an astronomer. Her reply: "That's a great idea, but what real job would you like?" Today he is gainfully employed as a professor at the Dublin Institute for Advanced Studies, having also worked at the University of Sussex and the Max Planck Institute for Astronomy in Heidelberg, Germany. Ray has been the principal or co-investigator on numerous Hubble observations of jets from young stars. His other interests include quasars, comets, archaeoas-tronomy (the study of sites such as Stonehenge), sailing and Guinness.

Further Information

In Darkness Born: The Story of Star Formation. Martin Cohen. Cambridge University Press, 1987.

Young Stars and Their Surroundings. C. Robert O'Dell and Steven V. W. Beckwith in Science, Vol. 276, No. 5317, pages 1355-1359; May 30, 1997.

Jets: A Star Formation Perspective. Thomas P. Ray in Astrophysical Jets, Open Problems. Edited by Silvano Massaglia and Gianluigi Bodo. Gordon and Breach Scientific Publishers, 1998.

The Origin of Stars and Planetary Systems. Edited by Charles J. Lada and Nikolaos D. Kylafis. Kluwer Academic Publishers, 1999.

Protostars and Planets IV. Edited by Vince Mannings, Alan P. Boss and Sara S. Russell. University of Arizona Press, 2000.

Star Factories: The Birth of Stars and Planets. Ray Jayawardhana. Raintree/Steck Vaughn, 2000. (Recommended for ages 11-14.)

For links to World Wide Web sites, visit astro.caltech.edu/~lah/starformation.html

The generation of jets may begin when material—a mixture of ions, atoms, molecules and dust—rains onto the circumstel-lar disk along magnetic field lines.
As the disk contracts under gravity, the lines (which are frozen into the material) are pulled in,taking on an hourglass shape.

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When the field lines are bent to an angle of 30 degrees from the perpendicular, centrifugal force overcomes gravity and flings material outward along the lines.

When the field lines are bent to an angle of 30 degrees from the perpendicular, centrifugal force overcomes gravity and flings material outward along the lines.

The inertia of the swirling material twists the field lines into a helix, which helps to channel the outward-flowing material in a vertical direction.

Is Global Warming Harmful to Health?

Computer models indicate that many diseases will surge as the earth's atmosphere heats up. Signs of the predicted troubles have begun to appear by Paul R. Epstein

Today few scientists doubt the atmosphere is warming. Most also agree that the rate of heating is accelerating and that the consequences of this temperature change could become increasingly disruptive. Even high school students can reel off some projected outcomes: the oceans will warm, and glaciers will melt, causing sea levels to rise and salt water to inundate settlements along many low-lying coasts. Meanwhile the regions suitable for farming will shift. Weather patterns should also become more erratic and storms more severe.

Yet less familiar effects could be equally detrimental. Notably, computer models predict that global warming, and other climate alterations it induces, will expand the incidence and distribution of many serious medical disorders. Disturbingly, these forecasts seem to be coming true.

Heating of the atmosphere can influence health through several routes. Most directly, it can generate more, stronger and hotter heat waves, which will become especially treacherous if the evenings fail to bring cooling relief. Unfortunately, a lack of nighttime cooling seems to be in the cards; the atmosphere is heating unevenly and is showing the biggest rises at night, in winter and at latitudes higher than about 50 degrees. In some places, the number of deaths related to heat waves is projected to double by 2020. Prolonged heat can, moreover, enhance production of smog and the dispersal of allergens. Both effects have been linked to respiratory symptoms.

Global warming can also threaten human well-being profoundly, if somewhat less directly, by revising weather patterns—particu-larly by pumping up the frequency and intensity of floods and droughts and by causing rapid swings in the weather. As the atmosphere has warmed over the past century, droughts in arid areas have persisted longer, and massive bursts of precipitation have become more common. Aside from causing death by drowning or starvation, these disasters promote by various means the emergence, resurgence and spread of infectious disease.

That prospect is deeply troubling, because infectious illness is a genie that can be very hard to put back into its bottle. It may kill fewer people in one fell swoop than a raging flood or an extended drought, but once it takes root in a community, it often defies eradication and can invade other areas.

The control issue looms largest in the developing world, where resources for prevention and treatment can be scarce. But the technologically advanced nations, too, can fall victim to surprise attacks—as happened last year when the West Nile virus broke out for the first time in North America, killing seven New Yorkers. In these days of international commerce and travel, an infectious disorder that appears in one part of the world can quickly become a problem continents away if the disease-causing agent, or pathogen, finds itself in a hospitable environment.

Floods and droughts associated with global climate change could undermine health in other ways as well. They could damage crops and make them vulnerable to infection and infestations by pests and choking weeds, thereby reducing food supplies and potentially contributing to malnutrition. And they could permanently or semipermanently displace entire populations in developing countries, leading to overcrowding and the diseases connected with it, such as tuberculosis.

Weather becomes more extreme and variable with atmospheric heating in part because the warming accelerates the water cycle: the process in which water vapor, mainly from the oceans, rises into the atmosphere before condensing out as precipitation. A warmed atmosphere heats the oceans (leading to faster

WOMAN RINSES RICE in floodwaters outside her hut in Madagascar. Heavy floods earlier this year there and to the west in Mozambique led to outbreaks of cholera (a waterborne disease) and malaria (transmitted by mosquitoes). At the right, a mother in Mozambique holds her child, who is feared to have malaria; at the far right, the body of a cholera victim in Madagascar is placed in a coffin. As global warming increases, it is expected to generate more frequent and devastating floods and droughts around the world—and more of the infectious diseases those conditions promote.

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