Triumphs of 20th-Century Astrophysics I: Telescopes and Observations

By Virginia Trimble

At the April APS meeting in St. Louis, the Forum and the Division of Astrophysics co-sponsored two invited sessions on “Triumphs of 20th-Century Astrophysics.” Organized by Virginia Trimble, they were each meant to feature three speakers focusing on the past, present and future of stellar astrophysics. Both sessions were expertly chaired by Ramanath Cowsik (Washington University, St. Louis), who set a standard of dress matched by few audience members or speakers.

For the first session, three knowledgeable speakers had agreed to discuss one important observatory from the recent past (Lick Observatory), one currently at peak productivity (Hubble Space Telescope), and a suite of future plans for 30-meter class telescopes and associated surveys. Unfortunately, the first speaker, Joseph Miller, a former Lick director, was forced to cancel on about 12 hours notice, due to a death in his family. His abstract indicates that he intended to address three significant aspects involved in the development of Lick and other observatories (Mt. Wilson, Palomar Mountain) that shifted the focus of world-wide observational astronomy to California between about 1900 and 1965. The first is location. Relatively dry mountain tops are much better telescope sites than the low-lying, near-urban environments that had been common in the past. Second is the improved technology of the telescopes themselves and of their focal-plane instrumentation (cameras and spectrographs). Third is a particular sort of community of users, relatively small (permitting both extended surveys and speculative projects) and closely coupled to development of instrumentation (permitting more effective use of it). In his stead, Trimble spoke for a slightly shorter time on these and other aspects of telescope history, focusing on the growth of what was the biggest in each generation, changes in basic designs, and some of the science enabled thereby.

Mario Livio (Space Telescope Science Institute) showed some of the glorious HST images that we have all gradually come to expect and described what he regards as Hubble’s top seven scientific achievements, many done in partnership with ground-based and non-optical facilities, and—he was quick to say—not necessarily in order of importance. These were: (1) evidence for the acceleration of cosmic expansion and existence of dark energy; (2) an accurate measurement of the cosmic distance scale and the Hubble constant (currently with 10 percent error bars, like Edwin Hubble’s, but 72 ± 8 rather than 536 ± 50 (km/sec)/Mpc); (3) the evolution of galaxies and the history of star-formation rates as revealed by sources in the Hubble Deep Field, whose shapes are more like train wrecks than like the classic sequence of ellipticals and spirals; (4) the existence and characterization of extra-Solar-System planets, including the presence of sodium, carbon, oxygen, and hydrogen in one planetary atmosphere and methane in another (from absorption features as the planet passes in front of its host star); (5) the existence and non-dissipative nature of dark matter in the so-called “bullet cluster,” where the gas has been left behind while two clusters of galaxies and dark matter (revealed by gravitational lensing) passed through each other; (6) the stellar populations in M31 and other galaxies, and how they differ from Milky Way populations; and (7) the presence of supermassive black holes in the nuclei of virtually every large galaxy.

Elizabeth (Betsy) Barton (University of California, Irvine) summarized plans for some of the large telescopes of the future, especially the Thirty-Meter Telescope (a collaboration of Caltech, the University of California, and Canadian universities), the Giant Magellan Telescope (involving about ten institutions and to be sited at Las Campanas Observatory in Chile), and the Large Synoptic Survey Telescope (a still larger collaboration including UC Irvine). The first two have roughly equivalent collecting areas (seven 8-meter circular mirrors for GMT versus a Keck-style filled aperture of many hexagons for TMT, 492 at last count) and relatively small fields of view. By contrast, the LSST has a single 8-meter mirror with a much larger field of view that will survey the entire visible sky every three nights. TMT and GMT will both make use of adaptive optics, initially in the near infrared, and their science goals are somewhat similar: to see stellar and planetary systems in formation; to detect the “first lights” (probably stars but perhaps black hole accretion) that began to reionize the Universe between redshifts of about 20 and 6; to characterize the stellar populations in galaxies from the earliest times to the present; and, just possibly, to learn more about more-nearly-earthlike planets. The amount of information available from a mirror of diameter D scales at least as D2, as D4 when sky noise is comparable with signal from target sources, and even as D6 with adaptive optics in crowded fields. There is also a European Extremely Large Telescope, intended to be 42 meters in diameter and having similar science goals as these two American projects. The LSST will map out dark matter via gravitational lensing and identify a wide range of transient sources ranging from near-earth asteroids to supernovae, which can be used to study dark energy. The TMT site is still to be determined, with candidates in Hawaii and Chile.

Note Added: This article represents the views of the author, which are not necessarily those of the FHP or APS.