A long, long time ago when the first stars and galaxies were forming, the Universe was filled with electrically neutral hydrogen. This thin soup of gas absorbed ultraviolet light and all of space would have seemed filled with softly glowing fog. The light of the first generation of stars was blocked by this all-pervading medium and as a result they are lost to our sight, hence cosmologists call this lost aeon the “Dark Ages”.

Image of early galaxies

An artist’s impression shows galaxies at a time less than a billion years after the Big Bang, when the Universe was still partially filled with hydrogen fog that absorbed ultraviolet light. (Image credit: ESO/M. Kornmesser)

It is a shame that we cannot see these first stars, the so-called Population III stars, as they were vast short-lived leviathans fascinating to theorists. They were composed of nothing but hydrogen and helium as there was nothing else to be made of. Only these two elements were created in the Big Bang. All other elements, including all that makes up you and me was formed in exploding stars, beginning with the Population III stars.

As time passed the ultraviolet radiation from the early galaxies energised the gas, tearing electrons from atoms, causing it to become electrically charged or ionised, and it rapidly ( in 200 million years or so) became transparent to ultraviolet light.

Astronomers call this lost period less than a billion years after the Big Bang the Age of  Reionisation, as there is thought to have been a brief period even earlier (within the first 100 000 years after the Big Bang) in which the Universe’s hydrogen was also ionised.

image of galaxies from VLT

The real thing. The red speck at the centre of this image (look very carefully) from the ESO Very Large Telescope shows the galaxy NTTDF-474, one of the most distant ever to have had its distance measured accurately. This extremely faint object is one of five that have been used to investigate the reionisation of the Universe about 13 billion years ago. (Image credit: ESO/ L. Pentericci)

New observations with the European Southern Observatory (ESO)’s Very Large Telescope of some of the most distant galaxies ever detected are revealing the mysteries of this important phase of the early Universe. Astronomers are exploring the rate at which this primordial hydrogen fog cleared and isolating the most likely source of the ultraviolet light which made space see-through. The most two popular candidates are the Pop III stars and the intense radiation emitted by matter as it spirals furiously into black holes. The research at the ESO favours the former candidate. It seems it was light from stars that cleared space. But what stars!

“These would have been very young and massive stars, about five thousand times younger and one hundred times more massive than the Sun, and they may have been able to dissolve the primordial fog and make it transparent.” says Eros Vanzella of the INAF Trieste Observatory, a member of the research team at ESO.

To look back at this lost era, the team used the mighty 8.2m Very Large Telescope  (VLT) to carry out spectroscopic observations, selecting galaxies first seen in deep sky images created with both the Hubble Space Telescope and  the VLT. These powerful instruments are essential for exploring this early period in cosmic history as they alone can enable the accurate observations of  these extremely distant and faint galaxies. Yet magnificent as the VLT and HST are, astronomers are looking beyond them. This research pushes even these telescopes to their design limits, The highly precise measurements required to confirm or disprove whether or not it was the energy of starlight that made space transparent require observations from space perhaps from the HST’s troubled successor, the James Webb Telescope, or from ESO’s cyclopean European Extremely Large Telescope (a 40m telescope!) which a decade from now ought be the world’s largest eye looking into the sky.

Astronomy never fails to inspire awe as reveals how all we see around us came to be.


Julie · June 8, 2014 at 19:58

I have always been curious about our Galaxy and now that I have learned so much about our galaxy I am very happy… Thank you…

History of the Universe in a Year | Astronotes · October 15, 2013 at 03:02

[…] this point on, space becomes transparent and starlight can travel freely through space.The most distant galaxies in the celebrated Hubble Ultra Deep Field existed at the end of this […]

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