An artist's impression of the gamma ray burst
An artist's impression of the gamma ray burstGemini Observatory/AURA, artwork by Lynette Cook

In the evening of 6 June, a signal from a distant and ancient galaxy reached Earth.

Nasa's Swift spacecraft, launched into orbit in 2004, detected the signal, a gamma ray burst originating from one of the Universe's earliest galaxies.

12 billion years ago, when the Universe was young, a massive star many times the size of our Sun exploded. The high-energy gamma rays thrown off by that event travelled for billions of years, closely followed by particles of light carrying information about the chemical composition of that star and its galaxy.

At precisely four minutes and 39 seconds past nine o'clock on 6 June, we picked up that message. The gamma ray burst alone lasted for four minutes.

Astronomers at the Harvard-Smithsonian Centre for Astrophysics had to act quickly. When a massive star explodes, the gamma-ray burst is followed by an after-glow caused when the exploding material collides with the surrounding gas and dust in space, heating it up. By examining that after-glow with spectroscopy, you can discover important details about the elements that existed at that time in the Universe.

This gamma-ray burst originated 12.7 billion years ago, when the Universe was only a billion years old. The after-glows of only three gamma-ray bursts of a similar age have ever been studied with spectroscopy before.

Within hours, the MMT Telescope in Arizona and the Gemini Telescope in Hawaii had been pointed towards the source of the gamma ray burst.

By examining the after-glow light, and detecting which wavelengths of light are missing, astronomers are able to detect which elements the light has interacted with but also at what part of its journey that interaction happened. This is called spectroscopy -- when the light hits an atom, it is absorbed, but each element absorbs a slightly different wavelength of light. By filling in the blanks, so to speak, you can detect oxygen atoms, for example, in galaxies billions of light years away.

On this occasion, the astronomers detected nitrogen, oxygen, silicon and carbon. In the early Universe, these heavier elements, which would eventually seed life on Earth, were only just beginning to accumulate as the first stars died and created them.

This period of the Universe is especially interesting for astronomers as it around the end of what is called the Dark Age. During the Dark Age, the Universe was entirely opaque, as a thick fog of ionised particles absorbed any light.

When this fog disappeared, becoming neutral hydrogen and helium gas, particles of light could move freely. Soon after, stars began to appear, forming the first distinct sources of light the Universe had ever known.

It's from that early period, when one of those early stars came to the end of its life, a gamma-ray burst exploded out into space, eventually landing here on Earth 12.7 billion years later.

The study can be read here on the arXiv pre-print site


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