Questions in astronomy are invariably “big.” Even the simplest question can lead down a winding path of investigation that results in a profound answer. This answer may well be accompanied by a mind-blowing revelation, and this is surely one of the subject’s greatest attractions. The overwhelming size of the Universe, stretching across billions of light years of space and billions of years in time, and the unimaginable numbers involved in its description, provide a sense of awe in themselves.
When you stand at a truly dark site—in a desert or some other wilderness where the only light to be seen is coming from the stars above—the stars fill the sky in such profusion that even the most familiar of the constellations is difficult to pick out. Although there may seem to be countless stars, in fact the human eye can resolve about 3000 under pristine conditions. This is but the tiniest fraction of the total number of stars in the Universe. It has long been a cliché to say that the number of stars in the Universe is the same as the number of grains of sand on all the beaches in the world, but while there is indeed a staggering quantity of sand grains on Earth, this total is not nearly large enough. According to the latest estimates there are some 70 sextillion stars in the entire Universe; that is 70 thousand million million million, or a seven followed by 22 zeros. To pursue the comparison, this roughly equates to the number of grains of sand to be found on the beaches of 10,000 Earth-like planets.
This book attempts to answer questions that spring to people’s minds about the wonders of the Universe. There are discussions of the exotic, half-glimpsed celestial objects such as quasars and pulsars, and the glorious close-up investigations of the nearby planets, such as Mars and Jupiter. There is a chapter devoted to those renowned celestial “superstars” that retain their mystery no matter how many years pass by: the black holes. The question “What is a black hole?” persists as the inevitable one as soon as anyone learns about my background in astronomy. Do not expect a complete answer here because even the experts don’t have that yet. The study of black holes exerts a powerful allure; when a full understanding is attained, that knowledge will almost certainly propel astrophysicists to a new comprehension of the Universe as a whole.
Of the 19 other big questions discussed here, some have definitive answers after centuries of scientific effort; others are tantalizingly close to a solution; and some still remain utterly without resolution. These unsolved ones are perhaps the most captivating because they set the agenda for modern astronomy and cosmology. Regardless of our ability, or not, to fully answer it, each question tackles an important foundation stone in both our perception of the Universe and our efforts to appreciate our own place in its vastness; each question also delves a little into that special magic that we all feel a touch of when contemplating the Universe.