Mapping the Universe: Surveys of the Sky as Discovery Engines in Astronomy Michael A. Strauss Abstract: Astronomers can map the sky in many ways: observing in different regions of the electromagnetic spectrum, obtaining spectra of stars and galaxies to determine their physical properties and distances, and repeatedly observing to measure the variability, explosions, and motions of celestial objects. In this review I describe recent surveys of the sky astronomers have carried out, focusing on those in the visible part of the spectrum. I describe in detail the Sloan Digital Sky Survey, an ongoing imaging and spectroscopic survey of over one quarter of the celestial sphere. I also discuss some of the major surveys planned for the next decade, using telescopes both on the ground and in space. Astronomy is an observational science. Unlike chemistry or biology, the objects of study in astron- omy are far removed, at distances to which we will not have the capability to travel using even the most advanced foreseeable technology. This means that we cannot carry out experiments on the stars and galaxies that are the bread and butter of our disci- pline; all the information we can glean about them is the result of measuring the tiny fraction of the light that they emit that happens to fall on our eyes and our telescopes. We then interpret these data in the context of the laws of physics to draw conclu- sions about the nature of these distant bodies, allow - MICHAEL A. STRAUSS is Professor ing us to infer, for example, the conditions in the of Astrophysical Sciences and As - cores of stars, or the existence of new forms of matter sociate Chair of the Department of that are unknown from our experience and experi- Astrophysical Sciences at Princeton ments here on Earth. University. His research concerns The range of phenomena in the universe is vast, all aspects of extragalactic astron- and the rate of astronomical discovery today tells omy and observational cosmology. us that we are far indeed from a complete under- He has published over two hundred refereed papers on subjects ranging standing of all that the universe has to teach us. This from the large-scale distribution of essay describes one of the most productive approach- galaxies to the discovery of the most es we have toward astronomical discovery; namely, distant quasars known. using our telescopes to map the heavens and create © 2014 by the American Academy of Arts & Sciences doi:10.1162/DAED_a_00309 93 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00309 by guest on 28 September 2021 Surveys of a census of the objects we ½nd. Astronomi- ent regimes is a powerful tool for explor- the Sky as cal surveys have always been a key aspect ing these phenomena. However, normal Discovery Engines in of our ½eld: such surveys have much to stars (and thus galaxies, which are made Astronomy teach us about the formation and struc- up of stars) emit most of their radiation ture of the Milky Way galaxy in which the at visible and near-infrared wavelengths, sun sits; the expansion, future fate, and making this the most effective regime in origin of the universe as a whole; and the which to survey the sky for these objects. nature of stars and the planets that orbit them. Indeed, astronomy is the study of Go outside on a clear moonless night, origins; we ask (and occasionally answer!) far from the lights of civilization. Once the most fundamental questions about your eyes have adapted to the darkness, where planets, stars, galaxies, and the uni - between two and three thousand stars are verse as a whole come from. With each discernable to the naked eye at any given advance in technology and new way to time. You will also see a silvery band cross - survey the heavens, we uncover new phe- ing the sky: the Milky Way. Galileo Galilei nomena that we did not anticipate, and was the ½rst to point a telescope to the we ½nd ourselves addressing questions heavens, and he discovered that the light that we previously did not have the imag- of the Milky Way comes from countless ination to ask. stars. Since that time, astronomers have With one hundred billion stars in our used ever larger telescopes to map objects Milky Way, and one hundred billion gal - in the sky. axies in the observable universe, our sur- However, the images we make with cam- veys have entered the realm of big data. eras placed at the back of our telescopes The biggest survey telescopes today can are two-dimensional. We have no depth gather terabytes of data in a single night, perception, and the stars look all to be and our catalogs of galaxies and stars in - equidistant, with no sense of which are clude over a billion objects, a number that closer and which are farther away. In fact, will increase by a factor of ten over the the nearest star (other than the sun) is next decade or so. The discovery potential about four light years (or about forty tril- of our surveys is limited by a combina- lion kilometers) from us, a distance that tion of raw computer processing power, is completely outside our everyday expe- the cleverness of our algorithms, and our rience. It would take thirty thousand years imagination. Just as Google allows us to to cover that distance traveling at the speed query human databases to uncover facts of our fastest spacecraft (forty-½ve kilome- and the relations between them, astron - ters per second). While most of the indi- omers have been developing similar tech- vidual stars visible to the naked eye are nology to query the database of the uni- within a few hundred light years of us, the verse. bulk of stars in the Milky Way are much This essay will focus on surveys in visi- farther away, arrayed in a vast flattened ble light, which of course represents only spiral structure some one hundred thou- a small sliver of the full range of electro- sand light years across, containing roughly magnetic waves, from high-energy gamma one hundred billion stars. rays to long-wavelength radio waves. Very One hundred years ago, astronomers different physical phenomena are respon- un derstood the Milky Way galaxy to be sible for emission at different regions of the full extent of the universe. However, in the electromagnetic spectrum, and com- addition to the myriad stars apparent in parison of maps of the sky in these differ- astronomical images, one also sees fuzzy 94 Dædalus, the Journal ofthe American Academy of Arts & Sciences Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00309 by guest on 28 September 2021 extended objects, termed nebulae (the Lat in tells us that galaxies are made of stars. Michael A. word for cloud). Edwin Hubble demon- However, there is an important difference: Strauss strated in the 1920s that these nebulae were the absorption lines in galaxy spectra are other “island universes,” as large as our shifted systematically to longer (redder) own Milky Way but at much greater dis- wavelengths. Hubble found that the spec- tances. This discovery enormously expand - tra of essentially all galaxies are redshifted, ed our understanding of the size of the and the degree of redshift is proportional uni verse. The nearest big gal axy to our own to the distance of the galaxy. is about two million light years distant, This relationship between redshift and and the number of galaxies seen in the distance is a consequence of the expan- deep est images with the epon ymous Hub- sion of the universe and, as explained in ble Space Telescope imply that there are David Spergel’s companion article in this about one hundred billion of them in the volume, it leads directly to our modern observable universe. understanding of the Big Bang. For our purposes, however, this becomes a valu- Surveys of galaxies based on photo- able tool for mapping the universe in three graphic plates show that they are far from dimensions: measuring the spectrum of uniformly distributed in the sky: clusters a galaxy allows us to determine its redshift, a few million light years across containing and thus, by Hubble’s law, its distance. hundreds of galaxies are apparent, with Photographic ½lm records the presence hints of larger structures yet. But to really of only (at best) a few percent of the pho- map the distribution of galaxies in three tons that fall upon it. Thus, even with the dimensions, we need an unambiguous way largest telescopes available in Hubble’s to measure their distances. Hubble’s sec- time, measuring the spectrum of a faint ond great discovery–that the universe is galaxy in order to determine its redshift expanding–gives us the way to do so. was enormously time-consuming, requir- Consider the spectrum of an astronomical ing exposure times of many hours for even object, which measures the intensity of the nearest galaxies. Modern electronic its light as a function of wavelength. This detectors, such as those in your digital spectrum gives much more detailed infor - cam era, are far more sensitive, detecting mation about the physical nature of the close to 100 percent of the photons that object than the properties (size, brightness, fall on them. Their development and adop - and color) measureable from an astronom- tion by the astronomical community ical image. For example, the wavelength starting in the late 1970s meant that ap - at which the spectrum of a star peaks is a preciable numbers of galaxy spectra, and measure of its surface temperature, which thus redshifts and distances, could be mea - for most stars in turn indicates their mass.