Podcaster: Stuart Clark
This podcast originally aired on July 17th, 2010
Description: Were the first celestial objects giagantic stars or powerful black holes?
Bio: Dr Stuart Clark is an award-winning astronomy author and journalist. His books include The Sun Kings, and the highly illustrated Deep Space, and Galaxy. His next book is Big Questions: Universe, from which this podcast is adapted. Stuart is a Fellow of the Royal Astronomical Society, a Visiting Fellow of the University of Hertfordshire, UK, and senior editor for space science at the European Space Agency. He is also a frequent contributor to newspapers, magazines, radio and television programmes. His website is www.stuartclark.com and his Twitter account is @DrStuClark.
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WHAT WERE THE FIRST CELESTIAL OBJECTS?
Hello I’m Dr Stuart Clark, astronomy author and journalist. Today I’d like to explore the question: What were the first celestial objects?
Beginning around 380,000 years after the big bang, when atoms had just formed and X-rays permeated space, there were no galaxies, no stars, and no planets. Astronomers call it the dark ages. Gradually, gravitational attraction drew together clouds of matter and eventually the first celestial objects were born.
Whatever they were, they pumped so much energy into space that they ripped apart almost every atom in the Universe by blasting away the electrons from their atomic nuclei. In the process, they must have created giant clouds of glowing gas. Those first luminous sources, more than 13 billion light years away, should still be visible to sufficiently large telescopes as tiny pinpricks of light.
The first attempt to see into these furthest reaches was in 1995, when the orbiting Hubble Space Telescope pioneered the technique of ‘deep field’ astronomy. A ten-day observation looked at a single patch of sky, no larger than a tennis ball placed 100 metres away, near the constellation of Ursa Major. After ten days of collecting light, the telescope revealed 3000 celestial objects: mostly small galaxies more than 10 billion light years away.
The Hubble Deep Field offered astronomers their first real look at extremely distant realms. Previous observations, with ground-based telescopes, had only detected galaxies about halfway across the Universe, and they seemed indistinguishable from present-day ones. Galaxies are classified according to their shape: elliptical galaxies are elongated clouds of stars; spirals are flat with arms that spiral around a central hub; barred-spiral galaxies have an elongated central hub connecting to the spiral arms; and there are irregularly shaped galaxies too.
The large numbers of small, distant galaxies in the Hubble Deep Field confirmed that today’s large galaxies began as small collections of a few million stars, either irregularly shaped or elliptical, colliding and merging to build into larger galaxies. As they grew they developed appreciable gravitational fields that pulled in gas from intergalactic space until star formation spontaneously began and surrounded the galaxy with sweeping arms of new stars.
A spiral galaxy will gather gas and may occasionally cannibalize a smaller galaxy. However, should it veer too close to a similarly sized galaxy, they will both lose all structure, resulting in a fuzzy cloud of stars: an elliptical galaxy. The collision will trigger a sudden explosion of star formation, a starburst in the gas. The two supermassive black holes will sink towards the centre of the merged galaxy, drawing each other into spiralling orbits. Their huge gravitational fields will interact, swallowing stars and throwing others into eccentric orbits. Plunging together, they will release a torrent of radiation. Then the newly enlarged black hole will continue to consume clouds of gas, stars or planets that haplessly stray into its gravitational reach. This can be a massive amount of material, and the merging galaxies will most likely become a quasar: a highly active, tremendously luminous galaxy. Quasars once populated the Universe in great numbers but have dwindled to extinction, doubtlessly because the black holes have devoured everything within their grasp. Once the quasar dies down, it becomes an ordinary galaxy with a dormant black hole.
Cosmologists believe that in this way the Universe built its current quota of galaxies. But the nature of the first step of the sequence – the origin of the collections of a few million stars – remains elusive.
With the Hubble Space Telescope’s new camera, astronomers took the Hubble Ultra Deep Field. Covering an area of about one-tenth that of the full Moon it revealed 10,000 small galaxies, showing them as they looked around 800 million years after the big bang. But still there was no sign of the very first, individual celestial objects, more distant still, and too faint to be seen by the Hubble.
Theorists use computers to model what they were likely to have been. There are two possibilities: either they were gigantic stars, or they were black holes, already greedily sucking in radiating gas. As stars and black holes would develop in different ways, it is crucial for cosmologists to determine which they were.
Stars exert the biggest influence over the Universe’s chemical composition and none have affected it more that the earliest stars. During the ‘dark ages’ before the first luminous objects, all that existed was a diffuse sea of atoms: roughly three-quarters of it hydrogen, one quarter helium, with a seasoning of lithium. Computer models suggest that the lack of variety had a tremendous effect on the first generation of stars. As gravity pulls gas together, it heats up resisting further compression. The heat must be radiated so the star can complete pulling itself together.
Calculations show that heavy chemical elements are highly efficient radiators, whereas the light gaseous elements are not. In star formation today, the presence of elements heavier than lithium speeds up the collapse, allowing stars to form from relatively compact pockets of gas. As a result most stars contain less mass than the Sun. Back in the dark ages, the forming stars didn’t have heavy elements to help lose heat, so more gas had to build before gravity overwhelmed it, resulting in masses from several hundred to a thousand times that of the Sun.
It seems likely a mixture of stars and black holes constituted the first celestial objects. But the only way to know is to find a way of seeing all the way back to the dark ages.
In an attempt to detect the heat from the first stars, astronomers launched a high-altitude balloon experiment in 2006, intended to measure the infrared radiation, which had been redshifted into radio waves by the expansion of the Universe. Instead, the experimenters found that a mysterious wall of radio noise, six times louder than anything the astronomers were expecting, deafened their detectors.
It may be coming from the death throes of the earliest stars. Indeed, when massive stars explode, they become billions of times brighter than normal. So perhaps the first glimpse of something from just after the dark ages will be the brilliant explosion marking the death of a monstrous star.
There is a type of celestial explosion called a gamma ray burst, apparently more energetic than any supernova in the nearby Universe. One can be seen from Earth every day or two, but from a completely unpredictable direction. Astronomers have to be really quick to spot them because having taken billions of years to travel to us, they arrive and pass by in just a few seconds. Highly sophisticated spacecraft now lie in wait ready to guide other orbiting and ground-based telescopes into the correct direction.
Another strategy for investigating the dark ages is to look for the signal from the hydrogen gas that existed throughout space at that time. As the electrons were stripped away by the radiation from the first objects, so the hydrogen would lose its way of giving out radio waves. A radio-quiet ‘bubble’ will have formed around each new celestial object and should now appear as a collection of dark holes in the hydrogen maps. Once astronomers find these holes, the theory goes that at the centre of each will be either a megastar or a black hole and computer models predict that megastars will ‘blow bubbles’ in subtly different ways from black holes.
Astronomers dream that eventually they will be able to actually see and take pictures of the first objects, and analyse each object’s chemical composition and physical conditions; to tell them definitively what the first celestial objects were, and how the first wave of heavy chemical elements was generated. Then they can investigate how these individual objects gathered together to become the small early galaxies seen in the Hubble Deep and Ultra Deep Fields.
End of podcast:
365 Days of Astronomy
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