Wonders of the Universe

THE LIFE OF THE UNIVERSE

Just as human beings, planets and stars are born, live their lives and die, so the Universe also lives its life in distinct stages. It began 13.75 billion years ago with the Big Bang, and in this embryonic period, known as the Primordial Era, the Universe was a place without the light from the stars, although in its early years the swirling hot matter would have glowed as brightly as a sun. For the first 100 million years, the conditions were far too violent for stars to form. This changed when the Universe had expanded and cooled sufficiently for the weak force of gravity to begin to clump the primordial dust, gas and dark matter into galaxies. With this came the dawning of the second great epoch in the life of our universe: the Stelliferous Era, the age of stars.

The moment the first stars were born is one of the most evocative milestones in the evolution of the cosmos. It signals the end of an alien time when the Universe was without structure – a formless void. The beginning of the Stelliferous Era marks the beginning of the age of light, the moment when the Universe would have become recognisable to us. The sky would have become black, punctuated with the glowing mist of the galaxies and the sharp silver of the stars. This is our universe today – a place where starlight decorates our nights and illuminates our days.

The Sun is one of at least two hundred billion stars in our galaxy, and it, along with countless others, shine brightly over Earth, night and day, in an ever-changing, ever-evolving cosmos.

Our sun is one of at least two hundred billion stars in our galaxy; one of a hundred billion galaxies in the observable universe. We live in a cosmos of countless islands of countless stars which bathe the Universe in light. Yet despite the fact that the Universe is over 13 billion years old, we are still just at the beginning. Although the cosmos is awash with stars, is populated with vast nebulae and systems of planets and countless billions of worlds that we’ve yet to explore, we are living close to the beginning of the Stelliferous Era, an era of astonishing beauty and complexity. But the cosmos isn’t static and unchanging; it won’t always be this way because as the arrow of time plays out, it produces a cosmos that is as dynamic as it is beautiful.

The moment the first stars were born is one of the most evocative milestones in the evolution of the cosmos…it marks the beginning of the age of light, the moment when the Universe would have become recognisable to us.

In our age of stars, the Milky Way Galaxy is filled with stars igniting and scattering their light across the night sky.

A gamma-ray burst is one of the Universe’s most spectacular and luminous explosions. As the core of a dying star collapses into a black hole, gas jets blast out from it into space.

This dramatic image shows the gamma-ray burst from GRB 090423, combining data from the Ultraviolet/Optical (blue, green) and X-ray (orange, red) telescopes of NASA’s Swift satellite.

NASA

THE FIRST STAR

On 23 April 2009 at 07.55 GMT, NASA’s Swift detected one of the most distant cosmic explosions ever seen – a gamma-ray burst that lasted ten seconds. The Swift satellite was designed and built with the intention that it would aid the study of a rare type of event known as a gamma-ray burst. These events, which last only a few seconds, are the most energetic and powerful emitters of radiation in the known universe. It is thought that gamma-ray bursts occur in supernova explosions – as the dying act of the most massive stars as they collapse to form black holes. By 08.16 GMT, minutes after the burst had faded away, the UK’s Infrared Telescope (UKIRT) in Hawaii saw the glowing ember of the explosion. As the day wore on, the largest telescopes across the world focused on the event as it appeared above their horizon. The afterglow was observed for several hours, but by 28 April the event had faded completely from view.

When these stars run out of nuclear fuel…they die in a dramatic fashion, collapsing in an instant and releasing more energy in one second than our sun will produce in its entire 10-billion-year lifetime.

The picture shown here merges data from two of Swift’s telescopes, and the important feature of this composite image is the rather unremarkable-looking red blob at the centre. This blob is the fading remains of GRB 090423 – once one of the brightest stars in the Universe. The poetically named GRB 090423 was once a Wolf-Rayet star. Named after the two French astronomers who discovered the first one in 1867, Wolf-Rayet stars are massive – over twenty times the mass of our sun – and because they are so massive, and burn so brightly, they are also extremely short-lived. When these stars run out of nuclear fuel after only a few hundred thousand years, they die in a dramatic fashion, collapsing in an instant and releasing more energy in one second than our sun will produce in its entire 10-billion-year lifetime.

GRB 090423 was a big Wolf-Rayet star – perhaps 40 or 50 times the mass of the Sun – however, this is not the only thing that is interesting about it. It’s not just the story of the death of this star, revealed by the brief appearance of the pale red dot, that has captivated astronomers, it’s the age of it. The light from this dot has travelled a very long way across the Universe to reach us, and has taken a very long time to do it. When we look at the afterglow of this explosion, we are looking at an event that happened a long time ago, in a galaxy far, far away. In fact, this light has been travelling towards us for almost the entire history of the Universe. GRB 090423 died over thirteen billion years ago, just over 600 million years after the Universe began. This is incredibly early in the Universe’s history. At the time of filming Wonders of the Universe, in autumn 2010, GRB 090423 was the oldest single object ever seen, although just after filming a galaxy was discovered in the Hubble Space Telescope’s Ultra Deep Field Image (see Chapter 3) that is slightly older than GRB 090423. Even more poetically named UDFy-38135539, this galaxy currently holds the distance and age record with a light travel time of slightly over 13 billion years. Allowing for the expansion of the Universe, the (so-called co-moving) distance of UDFy-38135539 is currently 30 billion light years away from Earth.

However, it is the discovery of GRB 090423, this ghostly pale red dot, and the sight of the explosive death of one of the first stars in the Universe, that gives us a glimpse of the grandest timescale of them all