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University of Durham. Eurekalert. 13-Sep-2007

Was it warm or was it cold?

Scientists shed new light on how stars were born in the early universe, 13 billion years ago.


The birth of the first stars depends on the nature of ‘dark matter’, cosmologists at Durham University say. This is the mysterious material that contains most of the mass of the universe. Their results are published this week in the journal Science.


The discovery takes us another step towards understanding dark matter. This has been pretty much of a mystery since it was first detected more than 70 years ago. The new discovery also suggests that some of the very first stars that ever formed are still around today.


Even although nobody knows what dark matter is, the evidence that it exists is very strong. This evidence comes from observations of the motions of galaxies and clusters of galaxies, and of the behaviour of the universe as a whole.


Just after the Big Bang there were no stars or planets. The universe was mostly ‘smooth’, with small ripples where matter had gathered together just a little. Gradually gravitational forces on the dark matter in these ripples made them grow, slowly gathering more matter into great clumps that became the first stars.


This happened about 100 million years after the Big Bang.


In their research the team at Durham University’s Institute for Computational Cosmology carried out detailed computer simulations. They studied the birth of the first stars using accepted scientific models of ‘cold’ and of ‘warm’ dark matter.


The results showed that if dark matter is cold and slow-moving, the first stars must have formed by themselves. Each clump of dark matter gave birth to a single star with a very large mass.


On the other hand if dark matter is warmer and faster-moving, then a large number of stars of different sizes would have formed at the same time. There would have been a big burst of star births. These bursts would have created long, thin filaments of stars.


These filaments would have been around 9000 light years long, says Dr Liang Gao. This is about a quarter of the size of the Milky Way today. "The very luminous star burst would have lit-up the dark universe in spectacular fashion.”


Stars forming in cold dark matter are very massive. The larger a star is, the shorter is its lifetime. So these large stars would not still be around today.


But with warm dark matter, the new research shows, low mass stars would have been born as well as large ones. Low mass stars have very long lives. So some of these first stars in the universe would still be around today, the scientists say.


This result paves the way for research that could bring us closer to learning just what dark matter is.


A key question astronomers often ask is ‘where are the descendants of the first stars today’, says Dr Tom Theuns. "The answer is that, if the dark matter is warm, some of these primordial stars should be lurking around our galaxy.”


The Durham University scientists' work sheds new light too on how black holes could form. Monster black holes are at the centre of most galaxies. Some of these black holes have a mass more than a billion times the mass of our sun.


Seeds for such black holes are created by collisions between stars in the dense filament in the warm dark matter scenario.


These results raise the exciting possibility of learning about the nature of dark matter from studying the oldest stars, says Dr Theuns. "Another tell-tale sign could be the gigantic black holes that live in centres of galaxies like the Milky Way.


"They could have formed during the collapse of the first filaments in a universe dominated by warm dark matter.”



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What's it all about?


  1. What is dark matter?
  2. Scientists have now discovered that the birth of what depends on the nature of dark matter?
  3. The discovery does two things, the writer says. State one of them.
  4. Nobody knows what dark matter is. But most scientists believe it exists. Why?
  5. Just after the Big Bang matter was all spread out and not gathered together in the huge clumps we call stars. What force pulled matter together to form stars?
  6. What had dark matter to do with this?
  7. The scientists studied how two different types of dark matter would affect how stars form. What are these two types called?
  8. If dark matter is cold what did the first stars look like?
  9. If dark matter is warm what did the first stars look like?
  10. Which kind of star lives longer – a very massive star or a star that is relatively small and light?
  11. Which type of dark matter, according to the Durham research, gives rise to small, low-mass stars?
  12. Where would those stars be today?
  13. In one sentence why does this mean we might be able to learn about dark matter - which we can't see - by studying stars - which we can?
  14. What else would be different today, depending on whether dark matter is warm or cold?
  15. Science can be done in different ways, for example by setting up experiments, by observing what is out there, by doing calculations on a computer, by developing theories on paper. Which of these have the Durham scientists been doing?
  16. Other astronomers who make observations - through telescopes and other instruments - can now use the Durham scientists' work to try to learn if dark matter is warm or cold. What will they look for to do this?




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