First Light Switching on Stars at the Dawn of Time

In the early days of the universe, there was darkness. Until somebody said, "let there be light"? Not quite. In First Light, astrophysicist Emma Chapman introduces you to ongoing research into the first billion years of our Universe and the birth of the first stars. Popular science at its finest, this book challenged me pleasantly but was above all – with apologies for the terrible pun – enlightening.

Though I have never given it much thought, the idea of there being a first generation of stars seems logical once you mention it. As Chapman shows, there is a gap in our understanding of what, exactly, happened during this time. Ongoing research shows this to be an unusual period, going by such evocative names as the Dark Ages, the Cosmic Dawn, and the Epoch of Reionisation.

Before we get to these, Chapman introduces you to the basics in the first four chapters. To grasp the science, you will need to understand the properties of light, stellar classification, the Big Bang and the expansion of the universe, and how stars are born. With that sorted, she then walks the reader through those first billion years.

An awful lot happened in the first three minutes after the Big Bang. Protons and neutrons were able to combine in a process known as nucleosynthesis within ~14 seconds. Within minutes, this process ended and the early Universe was full of hydrogen and helium isotopes. Electrons were still too energetic to bind to them to form atoms proper. It would take another 380,000 years of the Universe expanding and cooling down for so-called recombination to take place and hydrogen atoms to form. Something else happened at this point: the Universe became transparent to radiation. Before this time, "we cannot see anything with light [...] because the environment is too volatile to allow photons to travel on unimpeded paths to our telescope" (p. 84). Although photons were released during these steps, the continued expansion of the universe meant their wavelengths increased (a process known as redshifting) until they left the visible part of the electromagnetic spectrum. With stars not yet born, the Universe entered its Dark Ages.

And yet, something was stirring in the darkness. Hydrogen gas coalesced into clouds until some became dense enough to ignite fusion, the first stars flickering into life. This was the Cosmic Dawn, some 180 million years after the Big Bang. Chapman talks at length about these Population III stars as they were unique. Consisting of nothing but hydrogen, they were huge (hundreds of solar masses) and short-lived (millions of years). As fusion consumes a star's hydrogen, they go through several cycles: "with heavier and heavier metals created in onion-like shells" (p. 153). (To astronomers, all elements heavier than helium are simply "metals".) This continues until you hit iron at which point the whole thing goes supernova. The first Population III stars thus seeded the cosmos with heavier elements, paving the way for new iterations of star formation.

There are two big narrative threads regarding Population III stars. The first was, for me, rather technical and challenging. This has Chapman talk about how to find the first stars using the radiation emitted by hydrogen gas that is cooling down, which has a very particular wavelength. This involves the abstract property of an atom's spin state and the emission of a 21 cm wavelength photon when a hydrogen atom goes through a spin-flip transition to settle in its energetically least excited state, its ground state.

The second point is that Population III stars are still theoretical entities: we have never observed any. This leads Chapman down two very interesting avenues of research. The first is known as stellar archaeology and tries to find quiet parts of the Universe, so-called stellar tombs, where rare, light-weight Population III stars might have survived until now. The other, complementary approach searches for ancient dwarf galaxies that have escaped the galactic cannibalism through which galaxies grow by gobbling up smaller ones. This would provide information about the environment in which these first stars formed.

Lastly, there is the Epoch of Reionisation. The Big Bang initially resulted in a Universe with nuclei that did not yet have electrons bound to them; it was ionized. Recombination resulted in a Universe filled with neutral hydrogen. But something happened to reionise the Universe's hydrogen, a state that persists to this day. Research is ongoing to find out what that something was. Early quasars emitting X-rays have been implicated, but photonic emissions by Population II stars are another likely contributor. Similarly, astronomers are trying to constrain the when, with current estimates suggesting that the process took some 500 million years and was complete 1 billion years after the Big Bang.

For a biologist such as myself – admittedly one fascinated by astronomy – this subject could have ended up being impenetrable in the hands of a lesser science communicator. Chapman has received various commendations and prizes and is a well-known public speaker. She excels at explaining the astrophysics and uses some imaginative metaphors. Why do some particle interactions require a low-energy environment? "Try hugging someone sprinting in the opposite direction and you'll understand why sometimes slower is better when it comes to interactions" (p 130). How do you find a black hole? Through its gravitational effect: "Like spotting someone well known in a shopping arcade, you are unlikely to see that person, but you know that something is happening from the mass of people heading to one point from all directions" (p. 221). Furthermore, she makes good use of subheadings in the text, includes diagrams to visualize abstract concepts, and ends each chapter with a helpful recap.

Although the focus of her writing remains firmly on the science, she delivers it in a conversational style. There are some nerdy jokes, but never too many. There are lyrical passages, but used in moderation. For example, the Big Bang "[...] is a theory that has been forced on our uncomprehending three-dimensional brains, incapable of visualising infinity but able to understand the overwhelming evidence" (p. 70), while stellar archaeology is described as "[...] a field that has moved from seeking the first stars to conversing with the second stars and hearing tales of their ancestors" (p. 178). Peppered throughout are historical episodes and Chapman highlights some of the underacknowledged female pioneers.

First Light represents everything I like about the Bloomsbury Sigma imprint: intellectually challenging and well-crafted popular science, written by knowledgeable experts. Next to a wildly fascinating book, this is another shining addition to their roster.