Universe is fading away, say astronomers
David Adam, science correspondent Monday August 18, 2003 The Guardian
The lamps are going out all over the universe. Astronomers have found that not enough bright young stars are emerging to take the place of the old stars burning out, so, in the ultimate retirement crisis, the cosmos is simply fading away. And we shall not see them lit again in our lifetime - by the time our swollen sun is expected to swallow the Earth in the dim and distant future, the light from the stars could be down to around half of what it is now. In time, there will be no stars left shining at all.
"The age of star formation is drawing to a close," said the appropriately named Professor Alan Heavens from Edinburgh University's Institute for Astronomy, who helped to carry out the new study.
"It's not suddenly going to get very dark, but it's been getting dimmer over the last few thousand million years and that will continue."
The great galactic dimmer switch is being turned down because the universe just can't make stars like it used to.
"Stars are formed in galaxies and there was a peak in the rate at which galaxies formed, and that time has passed and been and gone," Prof Heavens said.
The number of stars being created has been in decline for about the last 6bn years, and there is little chance of a surge in productivity in the future. The conditions for making stars are less favourable now and most of the ingredients required, including hydrogen and helium, have already been used up.
Although some stars hurl material back into space for recycling as they age and begin to expand, much of it remains locked away. All of this means that, as the stars formed billions of years ago begin to lose their twinkle one by one, the universe will eventually be made up of nothing but the dark and cold corpses of white dwarfs, neutron stars and black holes.
Professor Tony Hewish, who won the Nobel prize in 1974 for his work in discovering quasars at Cambridge University, said the dimming effect would be made worse by those bright stars that did remain being spread further apart as the universe expanded.
"We live in an accelerating universe now and so, as time goes on, the density of galaxies is going to thin out," he said. "As far as we know the universe as it is now is going to expand for ever, so ultimately it must do something like that."
To judge how many recently formed, young stars there were at different times during the universe's 14bn year history, Prof Heavens and his team looked at the light from those making up some of the 40,000 galaxies nearest to Earth.
Although still a billion light years away or so, these galaxies are much closer than the stars typically used for this kind of analysis. This means their light takes much less time to arrive and so the astronomers get a more up to date picture.
"What we do is look at the spectra of light that comes out of the galaxies and try to decode it to work out when the stars were formed," Prof Heavens said.
The key to this is the colour of light. Most of the light from young stars is blue, coming from very hot, massive stars. These blue stars live fast and die young, ending their lives in supernova explosions. When they have gone, they no longer outshine the smaller red stars, which are more long-lived.
Using computer analysis that tied up over a dozen computers for several weeks, the astronomers built up a detailed picture of the age of the stars in each galaxy. Their findings will appear in the Monthly Notices of the Royal Astronomical Society this month.
This is not the first time astronomers have predicted the dimming of the universe, but the new study is the most detailed so far because it could take advantage of new information from a huge digital survey of the night sky.
Previously, astronomers had to rely on light billions of years old arriving from some of the most ancient parts of the universe.
Many of those earliest regions will be the first to have their lights extinguished. Although none of us will be around to see it, our own corner of space could be one of the last as our galaxy will be among the few bright spots left as darkness falls across the universe.
"[The milky way] has a fair amount of gas still in it, so it is still showing signs of star formation now," Prof Heavens said. "It's certainly going to be one of the later ones to carry on shining."
Rise and fall of the cosmos
14bn years ago The universe, space and time are generated in one Big Bang. All of the matter and energy the universe will ever hold are hurled outwards in one gigantic explosion. Galaxies, stars and planets begin to form.
5bn years ago A cloud of gas in a spiral arm of the Milky Way galaxy spawns our sun. A disc of gas and debris around it cools and condenses to form the planets. Earth is the third planet from the sun.
Present day New calculations show that star formation has peaked. Not enough new stars are being born to replace those dying. Astronomers say the universe is getting dimmer.
5bn years ahead So many stars have gone out that scientists predict the universe will be half as bright as it is now. The sun will swell to become a red giant until it engulfs Earth.
? billion years ahead All the stars have long burned out and the cosmos is a cold and dark place. Dead stars and black holes are all that remain.
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Net notes 12.04.2001: Yuri Gagarin 22.03.2001: Mir space station
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