Astronomers have discovered an object whose make-up gives an insight into the formation of the Universe. Marcus Chown reports on the discovery of the 'genesis star'
17 September 2003, Independent UK
In archaeological terms, it's like finding a Pharaoh surviving in modern-day London. Astronomers have discovered a star hiding in the Milky Way that has survived from a time before the advent of galaxies and even before many of the atoms making up today's universe. Crucially, this "genesis star" may have a "memory" of the very first objects to form from the debris of the Big Bang - super-massive stars, thought to have blazed briefly in the dawn of time. "The lost generation of stars blew up and vanished around 13 billion years ago," says Timothy Beers of Michigan State University in East Lansing. "They're the missing jigsaw piece in cosmic history."
Astronomers believe the universe's earliest stars formed in small gas clouds, containing only a few dozen or a few hundred stars. Later, these clusters coalesced to form giant galaxies like the Milky Way which dominate today's universe. So some ancient stars should be mixed in with the galaxy's other stars.
Such ancient stars stand out because of their extremely low levels of heavy atoms like calcium and iron. Whereas nature's two lightest atoms, hydrogen and helium, were forged in the fireball of the Big Bang, heavier atoms have been cooked up since that time by nuclear reactions inside stars. When very massive stars explode, they enrich the gas of the interstellar medium with such atoms. Since this gas provides the raw material for new stars, each successive generation of stars has a higher average abundance of heavy atoms. The oldest stars will therefore have extremely low abundance of heavy atoms. This should be apparent in the light they emit since each atom in nature has a unique spectral "fingerprint", showing or absorbing light as characteristic colours, or wavelengths.
Finding stars with the all-important spectral fingerprint among the 200 billion or so in our Milky Way involves sifting painstakingly through millions upon millions of individual stars. In the early 1990s, astronomers from the University of Hamburg and the European Southern Observatory studied 4 million stars across the sky. They then looked at promising candidates in detail using some of the world's largest telescopes. "One star jumped out at us," says Beers. "It went by the rather uninspiring name of HE 0107-5240."
At the end of 2001, Beers and his colleagues spent a mammoth six hours collecting light from the star with the Very Large Telescope on Cerro Paranal in Chile. "What we got stunned us," says Beers.
HE 0107-5240 had 200,000 times less iron than the Sun - 200 times less than any known star. What's more, it had hardly any heavy atoms - just seven different kinds compared with 25 to 30 in old stars with 1000 times less iron than the Sun. It had to be the most ancient star ever found. According to the best estimates, it was 13.5 billion years old, meaning it was around only 200 million years after the Big Bang. "HE 0107-5240 was truly the genesis star," says Beers.
One of the most significant features of the star's spectrum is the total lack of any atoms heavier than nickel or iron. Atoms heavier than iron are built up by the repeated capture of particles called neutrons, either during the late stages of stars' lives or during the catastrophic explosions in which massive stars die. "The genesis star must therefore have come from a time before the onset of significant neutron-capture processes, which produce many of the heavier atoms in today's Universe," says Beers.
By contrast, many ancient stars with 1000 times less iron - estimated to be about 13 billion years older than the Sun - do contain atoms heavier than iron. "We can therefore say that neutron-capture processes, which made many of the atoms in the periodic table, probably got going some time between 13.5 and 13 billion years ago," says Beers.
Not everything about the genesis star makes sense. "The star has 10,000 times as much carbon relative to iron as the Sun," says Beers. "In fact, it has the highest carbon abundance of any known star."
Finding a star with such low levels of most heavy atoms and an enhanced level of carbon is like finding a 120-year-old with skin as soft as six-month-old baby.
Beers believes the enhanced carbon reflects atom-building processes going on in the very first stars, whose explosions provided the raw material for the genesis star. If he is right, the heavy atom abundances in the genesis star provide a "window" on the very first stars - short-lived, super-massive stars that blew up and disappeared 13.5 billion years ago. The genesis star is therefore not only like a Pharaoh that has survived to modern times but one who also remembers the last Neanderthal.
Stars formed from the helium and hydrogen of the Big Bang and were born with no heavy atoms at all. "The atoms they produced during their short lifetimes would be preserved in stars like the genesis star," says Beers.
If Beers is right, the genesis stars may tell us about the lost generation of stars. "It's been known for a decade or so that the more deficient a star is in heavy atoms the more carbon it has. My bet is that the very first carbon in the Universe was made by some nuclear process different from the one that makes it in today's universe and it's this carbon we see in these primitive stars."
Beers believes we may even be able to learn about the masses of the lost generation of stars from whether or not the genesis star has certain heavy atoms in it - such as europium, thorium and uranium, made in the inferno of a supernova explosion. If the first stars were as massive as some theorists suspect - hundreds of times bigger than the Sun - these atoms would be sucked back into the supernova relic - possibly a black hole - and never escape. The genesis star should therefore show no sign of them. If, on the other hand, the first stars were less massive, these atoms would have escaped and the genesis star will show evidence of their presence.
The key to utilising information from the genesis star is to observe more stars. "Discussions about a single star have a limited scope," says Abraham Loeb of Harvard University. "One would prefer to have many more examples before drawing statistical conclusions."
Only a few tens of thousands of stars have been examined in detail. Of all these, only one star - the genesis star - has been found with an iron abundance of 200,000 times less than the Sun. "The statistics are small but, even if only one in 50,000 stars in our Milky Way has an ultra-low abundance of heavy atoms like the genesis star, we are talking about as many as a million stars of a similar nature yet to be found," says Beers.
The key to finding them is going to be observing tens of millions of stars. A great place to look is the American-Japanese Sloan Digital Sky Survey, in New Mexico. "We're confident of finding stars with 10,000 to 100,000 times less iron than the Sun, or even lower," says Beers.
For most astronomers, the sexy objects being found by the Sloan survey are quasars - the ultra-bright cause of newborn galaxies - and stars are the poor relatives. However, quasars look like stars and, as a result, can be mistaken for stars with hardly any heavy atoms. "About 10 per cent of the quasars candidates are not quasars but the kind of stars were looking for," says Beers. "We just have to sit back and let the quasar hunters do the a job for us!"
Marcus Chown is the author of 'The Universe Next Door', published by Headline, £15.99
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