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Proc. Natl. Acad. Sci. USA Vol. 74, No. 5, pp. 1767-1774, May 1977 Astronomy Galaxies: Outstanding problems and instrumental prospects for the coming decade (A Review)* t (cosmology/history of astronomy/quasars/space) JEREMIAH P. OSTRIKER Princeton University Observatory, Peyton Hall, Princeton, New Jersey 08540 Contributed by Jeremiah P. Ostriker, February 3, 1977 ABSTRACT After a review of the discovery of external It is more than a little astonishing that in the early part of this galaxies and the early classification of these enormous aggre- century it was the prevailing scientific view that man lived in gates of stars into visually recognizable types, a new classifi- a universe centered about himself; by 1920 the sun's newly cation scheme is suggested based on a measurable physical quantity, the luminosity of the spheroidal component. It is ar- discovered, slightly off-center position was still controversial. gued that the new one-parameter scheme may correlate well The discovery was, ironically, due to Shapley. both with existing descriptive labels and with underlying A little background to this debate may be useful. The Island physical reality. Universe hypothesis was not new, having been proposed 170 Two particular problems in extragalactic research are isolated years previously by the English astronomer, Thomas Wright as currently most fundamental. (i) A significant fraction of the (2), and developed with remarkably prescience by Emmanuel energy emitted by active galaxies (approximately 1% of all galaxies) is emitted by very small central regions largely in parts Kant, who wrote (see refs. 10 and 23 in ref. 3) in 1755 (in a of the spectrum (microwave, infrared, ultraviolet and x-ray translation from Edwin Hubble). wavelengths) that were previously inaccessible to observation. The physical processes by which regions with <10-9 the volume of the luminous stellar parts of galaxies produce such enormous I come now to another part of my system, and because it quantities of energy are currently the subject of much specu- suggests a lofty idea of the plan of creation, it appears to me as the lative debate. (il) It appears that most of the mass of ordinary most seductive. The sequence of ideas that led us to it is very sim- galaxies resides far from the central luminous region, with the ple and natural. They are as follows: let us imagine a system of volume containing most of this mass > 103 times the volume stars gathered together in a common plane, like those of the Milky containing most of the light-emitting stars; the nature, amount, Way, but situated so far away from us that even with the telescope and extent of this mass are quite unknown. we cannot distinguish the stars composing it; let us assume that its New instruments that will be operating in the next decade and distance, compared to that separating us from the stars of the that may be helpful in solving these two problems are briefly Milky Way, is in the same proportion as the distance of the Milky mentioned with particular emphasis on the advances expected Way is to the distance from the earth to the sun; such a stellar in angular resolution at wavelengths for which picture-taking world will appear to the observer, who contemplates it at so enor- ability has historically been poor or nonexistent. mous a distance, only as a little spot feebly illumined and subtend- ing a very small angle; its shape will be circular, if its plane is per- pendicular to the line of sight, elliptical if it is seen obliquely. The HISTORICAL INTRODUCTION faintness of its light, its form, and its appreciable diameter will ob- Exactly 56 years ago in this Academy, on April 26, 1920, there viously distinguish such a phenomenon from the isolated stars was an event, perhaps common in the public imagination, but around it. uncommon in real science: there was a great debate (1). It was between Heber Curtis from Lick Observatory and Harlow Shapley of Harvard, and was held in the hope of resolving, or But these ideas were almost pure speculation. They were at least illuminating, a scientific issue then of great moment. put on a quantitative foundation by William Herschel, a Ger- What was the real nature of the spiral nebulae (compare Fig. man-English musician, instrument builder, and astronomer, 1)? Although several ancillary issues were discussed as well who was delighted to find in 1784f with his own 1/2-m reflector (including, especially, the size of our own Milky Way), the that the Milky Way could be resolved into innumerable stars. conflict might be summarized in the following two quotes. "The glorious multitude of stars of all possible sizes was truly astonishing." We shall see again and again in extragalactic re- Shapley: The evidence is opposed to the view that the spirals are search an increase in angular resolving power, made possible galaxies comparable with our own. In fact there appears by technological progress, leading directly to a significant sci- as yet no reason for modifying the tentative hypothesis that entific breakthrough. the spirals are not composed of stars at all but are truly Herschel, the first systematic investigator of our stellar sys- nebulous objects." tem, conceived of a plan to count stars in certain selected 15° areas as a function of the apparent stellar brightness. From this, by assuming the inverse square law and that all stars were ap- Curtis: The spirals are not intragalactic objects but island universes proximately the same absolute brightness as the nearby star like our own galaxy. Sirius, he deduced the dimensions of our own system to be ap- X Abbreviation: pc, parsec = 3.26 light years = 3.09 X 1018 cm. proximately 300 parsec 1500 parsec (pc). (The numerical * From a lecture given at the National Academy of Sciences, Wash- values are derived with the now known distance to Sirius which ington, DC, Spring 1976. Herschel had used as a unit.) This was not a bad first estimate. t By invitation. From time to time, reviews on scientific and techno- logical matters of broad interest are published in the PROCEED- t This was the same year, incidentally, when Messier's famous catalog INGS. of nebulous objects to be avoided by comet hunters was published. 1767 Downloaded by guest on September 29, 2021 1768 Astronomy: Ostriker Proc. Natl. Acad. Sci. USA 74 (1977) FIG. 1. Spiral Galaxy NGC 5457 or M101, 101 in Messier's 1784 catalog of nebulous objects. A bright (M, = 8.2), relatively nearby (distance 4107 light years), giant, face-on spiral; an excellent example of Hubble type Sc (compare Fig. 3). Photograph courtesy of Hale Observatories. Believing the Island Universe hypothesis, he concluded that if brightest red stars in the central bulge of Andromeda, M32, and Andromeda (no. 31 in Messier's catalog) were the same size as NGC 205 (Astrophy. J. 100,137, 1944). Then Cepheid variable the Milky Way and made of the same kind of stars, it would be stars were also found (4) and the case was clinched. After that nebulous in appearance but that somewhat better techniques there was a great deal of hard work to be done but the outlook, should be able to resolve it also into individual stars. Herschel crystallized in Hubble's book, Realm ofthe Nebulae (5, 6), was clearly realized that the nebulae were not a homogeneous group fixed. We enter the modern era of extragalactic studies. of objects but that some, Orion for example, were comprised of "a shining fluid of nature totally unknown to us"; that is, he CLASSICAL VIEW OF GALAXIES recognized the division into gaseous and extragalactic nebu- Absolute, quantitative measurements had to await subsequent lae. decades when the photographic plate would be replaced by The analogous appearance between our own Milky Way and photoelectric detectors, but Hubble rapidly took the first es- certain edge-on systems made the Island Universe hypothesis sential steps. Following the early lead of the Irishman, Lord specially plausible (compare Fig. 2 upper and lower), but the Rosse, Hubble discovered certain regularities, the taxonomy concentration of spiral nebulae to the north and south galactic of extragalactic objects. He based his classification scheme on poles could not be reconciled with that picture before astron- dimensionless features which could be easily seen on optical omers knew that the apparent rifts in spiral galaxies, including photographic images. our own, were caused by dust obscuration rather than a defi- His classification scheme (compare Fig. 3) and its extensions ciency of stars. were dependent on three parameters: (i) for featureless (ellip- The Curtis-Shapley debate left the issue, as debates will, tical) systems, the ratio of axes of the flattened optical image; unresolved. But new instruments, particularly the 2.5-m Mt. (ii) for spirals, the character of the arms; and (iii) for spirals, the Wilson reflector, settled the question within 5 years. Edwin ratio of spheroidal to flat components. The most important Hubble was soon able to resolve the disc of M31 and its com- extensions and modifications were made individually by W. panion M33 into stars. These were of about the expected lu- W. Morgan, A. Sandage, G. de Vaucouleurs, and S. van den minosity. That is, if M31 is the same size and brightness as our Bergh (7-10). It is important to note that none of these systems galaxy and its apparent size gives its distance, then the resolved was based on basic physical properties of galaxies, although the stars are as bright as the brightest blue stars in our galaxy. classified properties might correlate with them. My own belief Somewhat (18 years) later, with new red-sensitive plates used is that the original system has been extraordinarily useful and on the 2.5-m Mt.

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