Dark heart of the cannibal universe

We live in a universe of galaxies. These basic building blocks come in many types, from grand-design spirals such as the often-imaged Great Galaxy in Andromeda, to featureless giant ellipticals, and irregular shapes such as the Magellanic clouds visible from Earth's southern hemisphere. Some galaxies erupt in starbursts; most contain giant black holes at their cores, each containing several million or billion times as much mass as our sun. William Waller and Paul Hodge give us a magisterial tour of these galaxies and their environment in space.

Waller is an astronomer at Tufts University involved in astronomy education. Hodge, emeritus professor at the University of Washington in Seattle, is a leading expert on galaxies and long-term editor of the Astronomical Journal . For decades, he has been the one person I have consulted for information about galaxies for my own textbooks. Many years ago he wrote the book on galaxies in the Harvard University Press popular astronomy series, of which the current volume is a greatly expanded version.

Waller and Hodge tell the story of our understanding of galaxies, ranging back as far as a wonderful carved sarcophagus lid, with stars arcing across it, showing the Egyptian sky goddess. As they write, and as the photographs they reproduce show, "Galaxies are among nature's most beautifully formed objects. Deep photographs of galaxies, taken with the most advanced telescopes, reveal enthralling structures on almost every scale. Even when glimpsed through a small telescope, these ghostly realms can mystify and inspire."

We learn about the advances made by astronomers using the Hubble space telescope to look at galaxies as far back in time as possible, including the Hubble deep field, in which the telescope found thousands of galaxies in an unremarkable bit of sky by concentrating on observing it for ten days. Interested in galactic cannibalism? Waller and Hodge describe how some galaxies eat others and how the results appear. We also learn about the supermassive black hole at the heart of our galaxy, and how it is studied in the radio and X-ray parts of the spectrum. "If, in our imaginings, a giant moth could hover above the Milky Way, it would most likely head toward our galaxy's brilliant nucleus," they write. Astronomers are now waiting to find out more about this object from detailed infrared studies planned for this year's Spitzer space telescope.

They also await the release of the Hubble ultradeep field, made with the advanced camera for surveys installed at the last Hubble servicing mission.

We astronomers strongly regret the cancellation of the next Hubble servicing mission on the pretext of shuttle safety, and hope that the decision can be reversed. If not, Hubble will die in approximately 2007 as its gyros give out, or from some other malfunction. It certainly seems as though its life should be extended by not only this next mission, which was to install the $200 million (£110 million) instruments already built, but by another, three or four years later, to bring Hubble's lifetime well into the next decade. I do not know of anybody who believes in the accuracy of the prospective 2011 launch date of the James Webb space telescope, billed as Hubble's successor but actually an infrared-optimised telescope that will not duplicate the high optical resolution or other features of Hubble.

The authors also describe how so much of today's universe is invisible to us. They tell how spectra obtained by Vera Rubin and Kent Ford three decades ago revealed that outer parts of our own spiral Milky Way galaxy are not rotating as slowly as expected, showing the presence of dark matter. One figure even shows a projection of a three-dimensional map of the distribution of dark matter in a cluster of galaxies, based on observations using Einstein's idea that gravity can act as a lens to focus background light. Later in the book, they bring the story up to date, describing how recent work, especially from studying supernovas, has led astronomers to realise that the universe's expansion is not slowing down but rather accelerating. This discovery has led to an increasing consensus that most of the stuff in the universe is dark energy, something whose composition we do not understand at all. "As a consequence, geometry no longer dictates destiny." Reading Galaxies and the Cosmic Frontier prepares us for understanding the discoveries now coming thick and fast about the composition and future of the universe.

The book contains not only black-and-white diagrams and photographs scattered through the text, but also an extensive 32-page colour section that shows images from major ground-based telescopes as well as from the Hubble space telescope. I was surprised that the Palomar Observatory was mistakenly said to be on Mount Palomar instead of Palomar Mountain, a common misstatement perhaps stemming from the one-time combination of the Palomar Observatory with the Mount Wilson observatory. The book went to press before last year's release of detailed observations of the cosmic microwave background made with the Wilkinson microwave anisotropy probe (WMAP) though it prepares us for the results based on the earlier Cosmic Background Explorer spacecraft and updates us on its website.

The authors hold out the possibility that the Hubble space telescope's key project's value for the rate of the universe's expansion is not the final answer, though WMAP's agreement with this value seems to end the controversy. But with such experts as these authors, we remain respectful of their evaluation.

Waller and Hodge provide a website at http://cosmos.phy.tufts.edu/cosmicfrontier  that contains more detailed and more mathematical information about galaxies. The published book contains not only a list of selected readings and a glossary but also suggestions for useful websites.

Jay Pasachoff is professor of astronomy, Williams College, Massachusetts, US.