Bernie Fanaroff surveys a study that probes telescopes in history and across the electromagnetic spectrum.
Eyes on the Sky: A Spectrum of Telescopes
By Francis Graham-Smith
It is a little odd that many astronomers still call themselves optical, radio or X-ray astronomers. The major problems of astrophysics and cosmology, such as how stars form and the nature of active galactic nuclei, cannot be solved by observing in only one part of the electromagnetic spectrum. Thus we live in the era of multiwavelength and multimessenger astronomy, which demand different kinds of telescope and technology to observe different parts of the spectrum and even other particles and waves, such as neutrinos, cosmic rays and gravitational waves.
In Eyes on the Sky, British astronomer Francis Graham-Smith delivers a valuable survey of the history, technology and design of telescopes across the electromagnetic spectrum, starting with Galileo Galilei's pioneering seventeenth-century refracting telescope. Graham-Smith explains the principles of how telescopes, such as optical reflectors or X-ray telescopes in space, make images or spectra and how they detect waves and photons, using everything from radio receivers to solid-state mega-pixel charge-coupled devices. As he notes, the field has been transformed, especially in recent years, through a combination of technical advances and radical change in astronomy's organization and scale, with the advent of large international teams and multinational projects.
Astronomers are always pushing the boundaries of technology, out of the need to detect more and more of the spectrum from increasingly faint objects. Graham-Smith's account of that process is fascinating. Some of the groundbreaking technological advances have been in detectors, very fast electronics and computing, and space telescopes. As he explains, satellite observation is essential for the parts of the spectrum blocked by Earth's atmosphere, such as X-rays. New-generation telescopes of this kind currently include NASA's Kepler space observatory and the European Space Agency's Planck satellite telescope, while next-generation satellite instruments will include NASA's James Webb Space Telescope (JWST). And the JWST, along with ground-based instruments such as the radio telescopes of the Square Kilometre Array (SKA) in South Africa and Australia, will produce huge quantities of data from sky surveys of unimagined sensitivity and scope. With young researchers able to access a flood of wonderfully exciting data, this will be a new golden age. Meanwhile, the detection of gravitational waves with the Advanced Laser Interferometer Gravitational-Wave Observatory, announced this year (S. Rowan Nature 532, 28–29; 2016), heralds the beginning of gravitational-wave astronomy.
Graham-Smith gives a useful summary of what is to come from these telescopes and surveys. Huge data sets of galaxies and other objects are being produced by sky surveys at different wavelengths, and many astronomers spend a large part of their time cross-matching the objects found. For instance, objects found in radio surveys must be matched with those found in surveys at optical wavelengths, to learn about the source of the radiation (such as a galaxy) and its distance (measured using the Doppler shift, or 'red shift', of the spectrum, which is caused by the expansion of the Universe).
There are many challenging problems ripe for cracking. One is the structure of the Universe. How did a once-uniform ball of very hot gas and energy become a highly structured, complex Universe, evolving over the 13.8 billion years since the Big Bang? Optical surveys such as the Sloan Digital Sky Survey tell us about the distribution of galaxies, galactic clusters and super-clusters. Minute fluctuations in the cosmic microwave background radiation — measured by the Planck telescope, among others — tell us about conditions when the Universe was only 380,000 years old, before the first stars, galaxies and clusters formed. The SKA will probe this 'cosmological dawn' and track the development of structure by looking at the history of hydrogen gas in the Universe.
As Graham-Smith discusses, the structure and evolution of galaxies is another hot topic. Almost all galaxies have supermassive black holes spinning at their centres, with masses millions to billions of times that of the Sun. Vast amounts of energy are radiated from near the black hole, or carried off as kinetic energy by collimated (very narrow) jets squeezed out along the poles of rotation and extending, in some cases, for a megaparsec. This process is probably key to the formation of stars and the evolution of galaxies. Energy from the jets and radiation is dumped into the gas between the stars and galaxies, and is believed to significantly influence the rate of star formation and, as a result, galaxy evolution. The heating and stirring of the gas in turn affects the rate of accretion and energy generation around the black hole, in a powerful feedback mechanism.
To refine their picture of this activity, astronomers are marshalling findings from a range of telescopes to map the jets' radio emission and estimate their kinetic and magnetic energy, as well as the energy emitted at optical and ultraviolet wavelengths. They are using X-ray observations to determine how hot the gas is, and infrared observations to gauge how much dust there is in the interstellar medium. They observe spectral lines at millimetre wavelengths to map the outflow of molecular gas. X-rays and γ-rays also tell us about the gas dynamics close to the black hole or in the region where the jets are launched.
Eyes on the Sky does contain a few surprising errors. For example, the Karl G. Jansky Very Large Array radio telescope in New Mexico, for instance, still has 27 dishes after its upgrade, not 36. Nevertheless, Graham-Smith's book is a very interesting explanation of the multitude of telescopes and their history.
Telescope technology continues to develop at breakneck speed. The SKA, for instance, demands new technologies to increase sensitivity, process huge quantities of data very fast and keep costs in check. This and other planned great observatories — the JWST, as well as the γ-ray seeking Cherenkov Telescope Array and the optical/near-infrared European Extremely Large Telescope on the ground — are likely to produce major discoveries in areas such as transient sources of radiation, the understanding of planet formation, the nature of dark matter and the history of the Universe. They will undoubtedly also uncover unknown unknowns, those serendipitous discoveries that are the hallmark of the great telescopes of history.