Night of exploding stars

Astronomer Royal Martin Rees throws some light on the snapshots of distant galaxies seen by Hubble

On 24 April 1992, the Independent filled its front page with an elaborate "time chart", tracing the course of cosmic evolution back to the first microsecond of a hot and colossally dense "cosmic fireball" in which our expanding universe began. Cosmology was deemed newsworthy because Nasa had announced the first results from a satellite called Cobe, which made very precise measurements of the radiation that cosmologists interpret as a relic of the initial "big bang".

Cobe's success was a relief to Nasa because its highest profile scientific project, the Hubble Space Telescope (HST), was attracting embarrassing attention. Its launch, scheduled for the early Eighties, had been delayed until 1990, so someinstruments seemed outdated. Worse, its images were badly out of focus because the mirror had been incorrectly set up.

A team of astronauts in 1994 carried out a "refurbishment" and the HST, albeit after delays and cost overruns, is now fulfilling its promise to bring the cosmos into sharper focus. Its latest results will surely give spectacular insights into the key stage in cosmic history when the first stars formed.

Competition for access to HST is so keen that even those who succeed are generally granted only a few hours of observing time. However, its director, Bob Williams, is allocated a quota to use at his own discretion. He seized this enviable opportunity and pointed the telescope for 10 whole days towards the same small patch of sky, yielding the most detailed image of the distant universe yet seen. Close-packed all over the sky are hundreds of objects - each seeming so small that it would be an almost imperceptible smudge on a picture taken from a ground-based telescope. These objects, with a wide variety of shapes, are a thousand million times fainter than any star we can see with the unaided eye. But they are not single stars: each is an entire galaxy, tens of thousands of light years in size, which appears so small and faint because its distance is measured in billions of light years.

What is fascinating about these pictures is not the record-breaking distance in itself, but the huge span in time that separates us from these remote galaxies. They look different from their nearby counterparts because they are being viewed at an early phase of their evolution. Their light set out when our expanding universe was much more compressed and everything in it was younger. These galaxies would have only recently formed from the expanding debris of the "cosmic fireball". They have not settled down into steadily spinning "flywheels", like the beautiful nearby spiral galaxies depicted in astronomy books. Some consist mainly of glowing diffuse gas, not yet fragmented into individual "droplets", each of which will become a star.

Astronomers have two ways of inferring our origins. One is to understand how the stars around us form, evolve and die. This is the same method whereby geologists or fossil-hunters infer the history of Earth. But astronomers have the advantage that they can observe the past. This is not, of course, a "time machine" of the kind that leads to paradoxes ("killing your grandmother in her cradle" and suchlike). We are not seeing the remote past of our own locality. But we are seeing snapshots of many distant galaxies which should look similar to the way our Milky Way and other nearby systems would have looked when newly formed, about 10 billion years ago.

A newly formed galaxy is made of pristine material from the initial cosmic fireball. This is essentially just hydrogen and helium - the simplest kinds of atom. All the other elements of the periodic table - carbon, oxygen, iron and so forth - were transmuted inside stars as a by-product of the nuclear fusion process that keeps them shining (a controlled version of an H-bomb). The young Milky Way would have contained many bright blue stars. The atoms that we and our Earth are made of are the "nuclear waste" from those ancient stars.

The HST is now showing us what our Milky Way would have been like when its first stars were shining brightly. There would have been no complex chemistry, no planets and (presumably) no life. But these marvellous images offer new insight into our cosmic environment in an immensely remote era when the basic building blocks of our solar system were being laid down.

Fantastic Voyage, Section Two