Recent Astronomical Discoveries

From the Atom to the Universe: Recent Astronomical Discoveries

Jeremiah P. Ostriker

Astronomy starts at the point to which chemistry has brought us: atoms. The basic stuff of which the planets and stars are made is the same as the terres- trial material discussed and analyzed in the ½rst set of essays in this volume. These are the chemical ele - ments, from hydrogen to uranium. Hydrogen, found with oxygen in our plentiful oceanic water, is by far the most abundant element in the universe; iron is the most common of the heavier elements. All the combinations of atoms in the complex chemical compounds studied by chemists on Earth are also pos- sible components of the objects that we see in the cosmos. Although almost all of the regions that we astronomers study are so hot that the more compli - JEREMIAH P. OSTRIKER, a Fellow cated compounds would be torn apart by the heat, of the American Academy since some surprisingly unstable organic molecules, such as 1975, is the Charles A. Young Pro - cyanopolyynes, have been detected in cold re gions of fes sor Emeritus of Astrophysics at space with very low density of matter. Nev ertheless, Princeton University and Professor the astronomical world is simpler than the chemical of Astronomy at Columbia Univer- world of the laboratory or the real biolog ical world. sity. His research interests concern dark matter and dark energy, gal - But the enormous spatial and temporal extent of axy formation, and quasars. His the cosmos allows us–and in fact forces us–to ask publications include Heart of Dark- questions that would seem offbeat to a chemist. ness: Unraveling the Mysteries of the Where do the chemical elements come from? Pre- Invisible Universe (with Simon Mit- cisely how, where, and when were they made? Do ton, 2013) and the volumes Forma- the abundances of the elements change with time? tion of Structure in the Universe (edited Does alternative “matter” that is not made of the or- with Avishai Dekel, 1990) and Un - solved Problems in Astrophysics (edited dinary chemical elements exist and exert gravity in with John Bahcall, 1997). He was the the universe? We in the trades of astronomy and as - recipient of the U.S. National Med- trophysics must ask ourselves these questions–and al of Science in 2000. they are only the beginning.

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00306 by guest on 28 September 2021 From the In our ½rst essay, “Reconstructing the Amazingly, much of the discovery work Atom to the Cosmic Evolution of the Chemical Ele- has been done with relatively small tele- Universe ments,” Anna Frebel asks precisely the ½rst scopes, using the astronomical equivalent of these questions. A discoverer of some of crowdsourcing (a movement that Bakos of the oldest and most metal-poor stars, has helped lead). The search may soon she tells us how we have found out where reach the point where we will be able to and when the nuclear cooking of the ele- use large telescopes to analyze the spectra ments occurred and precisely which cos- from the most interesting newly discov- mic explosions spewed out which of our ered planets to tell if any of them have familiar elements, from the sodium in salt atmospheres like ours on Earth; and then to the gold in our jewelry. She also intro- the question of “life on other worlds” will duces some of the remaining mysteries of become the province of science rather than element creation in the early universe: science ½ction. what do we not know? Next up the scale from planets is, of Let us move beyond standard units of course, the domain of the stars–those ½xed ordinary matter to some larger objects in points of light in the sky that the ancients the universe. The Earth, our beloved plan- cataloged and arranged into houses or et, is but a grain of sand on the scale of the constellations. By the eighteenth century cosmos; however, it is a respectable body it was known that they were not, in fact, in our solar system of eight normal planets. ½xed, but varied in brightness and moved For literally thousands of years the (albeit slowly) on their own paths across bright er of these “wanderers” (the mean - the sky, their current positions by then ing of the Greek word “πλανεται,” or being signi½cantly different from those “plan etai”) puzzled our ancestors, who be - recorded by the classical astronomers. By lieved that their motions among the ½xed the twentieth century, the enigmatic “neb- stars foretold events on earth. The revolu- ulae,” including the common spiral nebu- tionary discoveries of Tycho, Kepler, Ga - la, were found to be simply giant assem- lileo, and Newton identi½ed our position blages of stars and galaxies–“billions and as the third orbiter of the sun, follow ing, billions of stars,” as incanted by the won- along with our companion planets, pre- derful popularizer of science of the last cisely the paths predicted by Newton’s century, Carl Sagan. We live in such a gal- laws of gravity and motion. But are there axy–the Milky Way–and our neighbor other planets outside of our solar system? Andromeda, seen in the northern hemi- Are the stars that we see in the night sky sphere in the winter sky, is another ½ne orbited by their own planets? When I was example of a typical spiral galaxy. The a graduate student, this was a subject of greatest classical astronomer, Hipparchus, speculation; there was no factual knowl- constructed a catalog of ½xed stars that edge. But in the last decades several inde- had fewer than one thousand entries. By pendent techniques have been developed the early twentieth century, the standard that tell us without equivocation that ex - HD catalog of bright stars (named for the trasolar planets are common! In fact, most American astrono mer Henry Draper) con- stars probably have planets orbiting them. tained over two hundred thousand entries. In “Exoplanets, 2003–2013,” Gáspár Bakos The Messier catalog of nebulae published lays out the dramatic tale of how we have in 1775 contained slightly more than one recently found a startling variety of new hundred objects; by the end of the nine- planets: fat and thin, in round and in el- teenth century, the similar ngc catalog liptical orbits, massive and lightweight. contained nearly ten thousand galaxies

68 Dædalus, the Journal ofthe American Academy of Arts & Sciences

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00306 by guest on 28 September 2021 and clusters. But now we live in the age of whose individual masses typically range Jeremiah P. electronic detectors, huge telescopes, giant from four million solar masses at the center Ostriker computers, and enormous databases. A of our own galaxy to six billion solar masses gigantic explosion of information has oc - at the center of the giant elliptical galaxy curred in our age of “big data,” as outlined M87. The processes by which these mega- in the essay by Michael Strauss, “Map - monsters were formed are under intense ping the Universe: Surveys of the Sky as investigation and are still quite uncertain. Discovery Engines in Astronomy.” Strauss, But the saga of how we discovered these a leader of the Sloan Digital Sky Survey extraordinary objects and what we now (the largest such survey completed to date), know about them can be told; and we are notes that catalogs now contain over a bil- fortunate to have had one of the discover- lion stars and gal axies; and they are grow- ers of quasars, Scott Tremaine, author the ing–if the reader will forgive the pun–at essay “The Odd Couple: Quasars and Black an astronomical rate! Couple this with the Holes.” We are just learning that there ap- new tools available for querying databases pear to be close relations between galaxies (such as Goo gle) and one can imagine the and their resident central massive black rate of discovery. holes, but how and why these relations Massive black holes are much heavier were formed remains a total mystery. (and stranger) than any star. The ½rst so - Moving farther up the cosmic scale, we lution indicating the possibility of the ex - ½nd galaxies, which are typically a thou- istence of black holes was obtained shortly sand times the mass of the black holes after Einstein invented general relativity that they harbor at their centers. Galaxies in 1915, but it was decades before their are the basic building blocks of the uni- character was understood and still longer verse. While it is true that they are collec- before black holes were found in nature. tions of stars, they also seem to be em bed - Given their common name by the visionary ded in massive halos of mysterious “dark physicist John Wheeler in 1967, they can matter,” the total weighing in typ ically at form when massive stars collapse to such a roughly a trillion solar masses with most of small size that gravity overwhelms any it in the mysterious dark component. The pressure or nuclear forces, crushing the visible galaxies were taken for granted by star into a singularity from which nothing, Hubble and the early twentieth-century as- not even light, can escape. But gaseous tronomers as being simply “there,” but by matter falling into black holes is heated as the 1960s, the realization had spread that it is compressed and will copiously emit they must be evolving with time, and in light before it disappears into the abyss. fact their formation itself was a subject that This makes black holes visible to astron- must be addressed. Luckily, our increas - omers, and many have been found in our ingly powerful telescopes can see farther galaxy in binary star systems, each one and farther out and consequent ly look back with a mass roughly ten times that of the to earlier and earlier times due to the ½nite sun. When quasars–enormously lumi- speed of light. The most distant galaxies nous objects at the centers of distant that we can ½nd are thus seen as they ex - galaxies–were discovered, their variability isted several billion years ago. With major and incredible luminosity immediately telescopes, we can use the universe as a led to speculations that they were much time machine and directly study the evolu- more massive black holes. We now know tion–and perhaps even the formation–of that the centers of most massive galaxies galaxies in the distant past. Pieter van in fact harbor these enigmatic beasts Dokkum has done just that; in his essay,

143 (4) Fall 2014 69

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00306 by guest on 28 September 2021 From the “The Formation and Evolution of Galax- over from the Big Bang. The angular fluc- Atom to the ies,” he will limn out what we have learned tuations seen in the sky by ever more sen- Universe through the use of powerful telescopes and sitive and precise satellites studying the giant computers that simulate the physics cbr (“cobe,” launched in 1989; “wmap,” of galaxy evolution. The discoveries are launched in 2001; and now “Planck,” piling up at a great rate in the last decade launched in 2009) have essentially ban- and we now know that for the most mas- ished doubt about the essential correct- sive galaxies, a two-phase evolution seems ness of the standard model of cosmology. to occur: when the galaxy ½rst forms, cold David Spergel, a leader of the wmap satel- streams of gas converge, flowing into deep lite team, authors the ½nal essay in this wells where the cosmic gravity is greatest volume, “Cosmo logy Today,” which tells due to dark matter ac cumulations, and the story of how these results came huge numbers of stars are formed in rela- about, what they tell us about the universe, tively small regions. Then, at a later epoch, and what (large) puz zles still remain. these monster systems eat their neighbor - These six essays lay out for the interested ing smaller galaxies (massive black holes reader the extraordinary renaissance that and all) and grow further–perhaps by a astronomy has undergone in the last de- factor of two in mass and four in size with- cades. We are fortunate indeed to have out much additional star formation. Can- ½rsthand discoverers’ tales of these ad- nibalism among the gal axies! The evolu- ven tures to entertain and inform us. tion for lower-mass galaxies like the Milky Way and our compan ion Andromeda is less well un der stood. Stay tuned. Finally, climbing up the ladder of dis- tance and time, we approach the largest known scale: the universe itself. During the last two decades, the knowledge of the universe accumulated over the last century has been synthesized into a well-de½ned global model that ½ts all cosmic observa- tions to an uncanny degree. While the na - ture of the two chief ingredients in this model–dark matter and dark energy– re mains a mystery to us, the model has passed all tests given to it so far. The exis- tence of dark matter has been con½rmed by both gravitational lensing and the growth patterns of various cosmic struc- tures. Simi larly, dark energy seems to have produced the amply observed accelera- tion of the uni verse (it is expanding faster and faster rather than slower and slower, as had been ex pected). But the primary tool for re½ning and precisely testing the new model has been the analysis of the mi- crowave background (Cosmic Background Radiation, or cbr): the relic radiation left

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