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MYSTERIES of the MILKY WAY Scientificamerican.Com Exclusive Online Issue No COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. MYSTERIES OF THE MILKY WAY ScientificAmerican.com exclusive online issue no. 15 Viewed from above, it appears as a giant pinwheel of diamond-studded smoke. From Earth, it cuts a pale swath of light across the summer sky. This is the Milky Way, our home galaxy, and it has captivated humankind for millennia. Galileo was the first to formally probe its secrets. Armed with a telescope of his own devising, he determined that the shimmering band was in fact composed of stars. Astronomers have been smitten ever since. This exclusive online issue brings together stunning Milky Way discoveries from the past decade. Leading scientists explain how our galaxy formed, how it continues to evolve and why only part of it is habitable. Other articles home in on particular Milky Way elements--our paradoxical sun, cosmic dust and the dynamic interstellar medium. Of course, as fascinating as our galactic neighborhood is, astronomers also yearn to peer beyond it. These efforts, too, are summarized here. We hope that after reading this issue, you’ll see our spectacular corner of the cosmos in a different light. —The Editors TABLE OF CONTENTS 2 How the Milky Way Formed BY SIDNEY VAN DEN BERGH AND JAMES E. HESSER; JANUARY 1993 Its halo and disk suggest that the collapse of a gas cloud, stellar explosions and the capture of galactic fragments may have all played a role 7 Our Growing, Breathing Galaxy BY BART P. WAKKER AND PHILIPP RICHTER; JANUARY 2004 Long assumed to be a relic of the distant past, the Milky Way turns out to be a dynamic, living object 17 Refuges for Life in a Hostile Universe BY GUILLERMO GONZALEZ, DONALD BROWNLEE AND PETER D. WARD; OCTOBER 2001 Only part of our galaxy is fit for advanced life 25 The Paradox of the Sun's Hot Corona BY BHOLA N. DWIVEDI AND KENNETH J. H. PHILLIPS; NEW LIGHT ON THE SOLAR SYSTEM, SPRING 2003 Like a boiling teakettle atop a cold stove, the sun's hot outer layers sit on the relatively cool surface. And now astronomers are figuring out why 33 The Gas between the Stars BY RONALD J. REYNOLDS; JANUARY 2002 Filled with colossal fountains of hot gas and vast bubbles blown by exploding stars, the interstellar medium is far more interest- ing than scientists once thought 43 The Secrets of Stardust BY J. MAYO GREENBERG; DECEMBER 2000 Tiny grains of dust floating in interstellar space have radically altered the history of our galaxy 48 Galaxies behind the Milky Way BY RENEÉ C. KRAAN-KORTEWEG AND OFER LAHAV; OCTOBER 1998 Over a fifth of the universe is hidden from view, blocked by dust and stars in the disk of our galaxy. But over the past few years, astronomers have found ways to peek through the murk 1 SCIENTIFIC AMERICAN EXCLUSIVE ONLINE ISSUE JULY 2004 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. How the Milky Way Formed Its halo and disk suggest that the collapse of a gas cloud, stellar explosions and the capture of galactic fragments may have all played a role by Sidney van den Bergh and James E. Hesser originally published in January 1993 ttempts to reconstruct how the mation has led other researchers to pos- In addition to a disk and a halo, the Milky Way formed and began tulate that several gas cloud fragments Milky Way contains two other subsys- Ato evolve resemble an archaeo- merged to create the protogalactic Mil- tems: a central bulge, which consists logical investigation of an ancient civi- ky Way, which then collapsed. Other var- primarily of old stars, and, within the lization buried below the bustling center iations on these themes are vigorously bulge, a nucleus. Little is known about of an ever changing modern city. From maintained. Investigators of virtually all the nucleus because the dense gas excavations of foundations, some pot- persuasions recognize that the births of clouds in the central bulge obscure it. tery shards and a few bones, we must stars and supernovae have helped shape The nuclei of some spiral galaxies, in- infer how our ancestors were born, how the Milky Way. Indeed, the formation cluding the Milky Way, may contain a they grew old and died and how they and explosion of stars are at this mo- large black hole. A black hole in the nu- may have helped create the living cul- ment further altering the galaxy’s struc- cleus of our galaxy, however, would not ture above. Like archaeologists, astron- ture and influencing its ultimate fate. be as massive as those that seem to act omers, too, look at small, disparate as the powerful cores of quasars. clues to determine how our galaxy and uch of the stellar archaeo- All four components of the Milky others like it were born about a billion logical information that Way appear to be embedded in a large, years after the big bang and took on Mastronomers rely on to deci- dark corona of invisible material. In their current shapes. The clues consist pher the evolution of our galaxy resides most spiral galaxies the mass of this in- of the ages of stars and stellar clusters, in two regions of the Milky Way: the visible corona exceeds by an order of their distribution and their chemistry— halo and the disk. The halo is a slowly magnitude that of all the galaxy’s visible all deduced by looking at such features rotating, spherical region that sur- gas and stars. Investigators are intensely as color and luminosity. The shapes and rounds all the other parts of the galaxy. debating what the constituents of this physical properties of other galaxies can The stars and star clusters in it are old. dark matter might be. also provide insight concerning the for- The rapidly rotating, equatorial region The clues to how the Milky Way de- mation of our own. constitutes the disk, which consists of veloped lie in its components. Perhaps The evidence suggests that our gal- young stars and stars of intermediate the only widely accepted idea is that the axy, the Milky Way, came into being as age, as well as interstellar gas and dust. central bulge formed first, through the a consequence of the collapse of a vast Embedded in the disk are the sweeping- collapse of a gas cloud. The central gas cloud. Yet that cannot be the whole ly curved arms that are characteristic of bulge, after all, contains mostly massive, story. Recent observations have forced spiral galaxies such as the Milky Way. old stars. But determining when and workers who support the hypothesis of Among the middle-aged stars is our sun, how the disk and halo formed is more a simple, rapid collapse to modify their which is located about 25,000 light- problematic. idea in important ways. This new infor- years from the galactic center. (When In 1958 Oort proposed a model ac- you view the night sky, the galactic cen- cording to which the population of stars ter lies in the direction of Sagittarius.) forming in the halo flattened into a SIDNEY VAN DEN BERGH and The sun completes an orbit around the thick disk, which then evolved into a JAMES E. HESSER both work at Domin- center in approximately 200 million thin one. Meanwhile further condensa- ion Astrophysical Observatory, National years. tion of stars from the hydrogen left over Research Council of Canada, in Victoria, British Columbia. Van den Bergh has a That the sun is part of the Milky Way in the halo replenished that structure. longtime interest in the classification and was discovered less than 70 years ago. Other astronomers prefer a picture in evolution of galaxies and in problems relat- At the time, Bertil Lindblad of Sweden which these populations are discrete ed to the age and size of the universe. He and the late Jan H. Oort of the Nether- and do not fade into one another. In received his undergraduate degree from lands hypothesized that the Milky Way particular, V. G. Berman and A. A. Princeton University and a doctorate in as- system is a flattened, differentially rotat- Suchkov of the Rostov State University tronomy from the University of Göttingen. ing galaxy. A few years later John S. in Russia have indicated how the disk Hesser’s current interests focus on the ages Plaskett and Joseph A. Pearce of Do- and halo could have developed as sepa- and compositions of globular star clusters, which are among the oldest constituents of minion Astrophysical Observatory ac- rate entities. the galaxy. He received his B.A. from the cumulated three decades’ worth of data These workers suggest a hiatus be- University of Kansas and his Ph.D. in atom- on stellar motions that confirmed the tween star formation in the halo and ic and molecular physics from Princeton. Lindblad-Oort picture. that in the disk. According to their mod- 2 SCIENTIFIC AMERICAN EXCLUSIVE ONLINE ISSUE JULY 2004 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. el, a strong wind propelled by superno- place, investigators found that many have its own chemical signature. Recent va explosions interrupted star forma- of the oldest stars and star clusters in work by one of us (van den Bergh) indi- tion in the disk for a few billion years. the galactic halo move in retrograde or- cates that such differences do indeed ap- In doing so, the wind would have eject- bits—that is, they revolve around the pear among the halo populations. ed a significant fraction of the mass of galactic center in a direction opposite to the protogalaxy into intergalactic space. that of most other stars.
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