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IN SPACE, time drags. Pick out any star in the Milky Way, and the chances are it has shone throughout the entire history of life on Earth. While people live and die like mayflies, the stars twinkle on, changeless, for millions or even billions of years.

So astronomers were astonished when in 1996, a sharp-eyed amateur spotted a star that, after first dying and shrinking to a glowing, Earth-sized ember, suddenly sprang back to life. In just a couple of years, it ballooned into a monster more than 100 times wider than the Sun. 鈥淎rguably, this was the fastest case of stellar evolution ever witnessed,鈥� says Martin Asplund of Australia鈥檚 Mount Stromlo Observatory near Canberra.

The star, known as Sakurai鈥檚 object, soon faded into darkness inside a cocoon of dust. But it is now staging another tantrum, blowing gases into space at hundreds of kilometres per second. As astronomers jostle for time on the world鈥檚 most powerful telescopes to watch it, they are also struggling to understand this born-again giant. They do know one thing, though: Sakurai鈥檚 object is no freak of nature. As many as 20 per cent of all lightweight stars might be reborn when they come to the end of their lives. And that includes our nearest star. 鈥淔rom what we understand today, the Sun is a prime candidate for going through a born-again phase,鈥� says Asplund.

Not everyone was caught off-guard by Sakurai鈥檚 discovery, however. In 1983, astronomer Icko Iben at the University of Illinois at Urbana Champaign and his colleagues proposed that dead stars could spring back to life. They were trying to explain the mysterious properties of some white dwarfs 鈥� dead stars that have burned nearly all their hydrogen and helium fuel, leaving mainly carbon and oxygen. Incapable of nuclear reactions, the white-hot starry embers shrink to a core about the size of the Earth.

According to models of stellar evolution, white dwarfs should have an outer coating of hydrogen gas that failed to ignite. For decades, though, astronomers had been puzzled to find elderly stars that didn鈥檛 show any of the spectral 鈥渇ingerprints鈥� of hydrogen. 鈥淭he Universe is absolutely dominated by hydrogen. So to find objects that appear to have hardly any hydrogen 鈥� in some cases none at all 鈥� that is just so bizarre,鈥� says Don Pollacco, an astronomer at Queen鈥檚 University Belfast.

To explain how these misfits came about, Iben and colleagues calculated what would happen if a star suddenly stopped burning helium before shrinking to a white dwarf. Their models showed that as the star becomes a tiny corpse the size of the Earth, instabilities could once again spark violent nuclear reactions in the remaining helium. These would be so fierce that the dense ember would turn itself inside out and swell back into an almighty giant, dragging the remaining hydrogen deep into the interior. According to Iben鈥檚 calculations, all this would last several decades, and after a century or so, the star would fade back into white-dwarfdom.

A trawl of old photographic plates suggested they might be right. In 1919 the German astronomer Max Wolf saw a star in the constellation Aquila suddenly flare up. Then in 1921, Swedish astronomer Knut Lundmark analysed the star鈥檚 spectrum and saw that it contained very little hydrogen. No one suspected the importance of his result until the 1970s, when astronomers in the US and Canada found the telltale glow of gas that had been blown off as the star entered its death throes. But because there were no detailed observations of the 1919 outburst, this didn鈥檛 count as definitive proof that white dwarfs really could be born again.

That proof came in 1996. One night in February, Japanese amateur astronomer Yukio Sakurai was out hunting for comets. Instead, he discovered a brand new bright star in the constellation Sagittarius. He thought it might be a nova 鈥� an explosion caused by material from a companion star falling onto a white dwarf and igniting suddenly 鈥� and he dutifully reported it to the International Astronomical Union, which spread the news.

Several teams of astronomers around the world decided to take a look. And as time went by, they realised it wasn鈥檛 a nova. Novae usually brighten rapidly over a few days, then fade over several weeks. But weeks later, Sakurai鈥檚 object was still getting brighter. It had all the hallmarks of something spectacular 鈥� the first born-again star that modern telescopes had had the luck to see.

At the time, Pollacco was working at the Isaac Newton Group of Telescopes at La Palma in the Canary Islands. 鈥淚t was just amazing to be there to see it. For me this was like a present from heaven,鈥� he says. Asplund led another team at the McDonald Observatory in Texas that took spectra of the star in May and October 1996. And watching the drama from Las Campanas Observatory in Chile was Florian Kerber, an expert on dying stars based at the Space Telescope European Coordinating Facility in Garching, Germany.

It turned out that Sakurai鈥檚 object also had a faint glow of gas around it called a planetary nebula 鈥� which for a star is a sure sign that it had once been close to death. All the observations suggested that the star had been a white dwarf the size of the Earth, with a temperature of about 100,000 kelvin. But it was rapidly cooling and swelling into a yellow-giant star roughly 50 times as wide as the Sun.

Astronomers watched amazed as Sakurai鈥檚 object changed from week to week. Chaotic convection was sucking its outer hydrogen layer down into the core, and at the same time dredging heavy elements up to the surface. During the six months following its outburst, the apparent quantity of hydrogen in the star fell by 80 per cent.

And on the star鈥檚 surface, heavy elements such as lithium, zinc, strontium and yttrium started to appear, as nuclear reactions ran wild. 鈥淲e were really seeing stellar nucleosynthesis live, which nobody had ever done before, except in complete stellar explosions like supernovae,鈥� says Asplund.

By 1998, Sakurai鈥檚 object was a bright but cool red supergiant about 150 times as wide as the Sun. Then it began to dim and disappear inside the cocoon of dust and debris that it had launched into space. The stellar core is now invisible to even the most powerful optical telescopes, although it is still bright in infrared radiation, which can penetrate the dust.

But the star has surprised astronomers again by beginning a new phase already, so soon after its rebirth. Late last year, Kerber, Asplund and their colleagues found signs that the gas around the star is starting to become ionised. One possible reason is that the star is shrinking again, heating up enough to knock electrons out of the gas atoms. If so, the observations suggest its temperature has already risen to about 20,000 kelvin. It should continue to heat up as it shrinks into a white dwarf once again. 鈥淭his time, there will be no turning back,鈥� says Asplund, who believes that the star has almost certainly used up its fuel.

Based on its previous record, astronomers think that Sakurai鈥檚 object is starting to form a second, inner planetary nebula, which will eventually become visible when the star gets hot enough to blow away its dust shroud. No one knows when this will happen, but as soon as it does, Asplund, Kerber and their colleagues hope to use the Very Large Telescope in Chile to monitor the star for any changes.

In the meantime, astronomers have plenty of questions. For instance, how far away is Sakurai鈥檚 object? There is no simple way to find out. Estimates based on the colour of the star and the streams of gas it has started shooting out, range from about 3000 to 25,000 light years. Astronomers can only be confident of other results, such as the object鈥檚 brightness and size, when they鈥檝e got the distance right.

But the most intriguing puzzle is why Sakurai鈥檚 object is evolving so fast. Its flare-up proceeded about 50 times faster than Iben鈥檚 theory predicted in the 1980s. 鈥淭here were predictions that born-again giants would evolve quickly, but most people thought the timescale would be 10 to 100 years, not a mere few months,鈥� says Asplund. One possible reason is that the star started off as a fairly massive star, at the top end of the mass range for these objects.

鈥淭he timescales are just crazy,鈥� agrees Pollacco. But he thinks that seeing the normally very slow gas-expulsion process speeded up so much could help solve one of the most nagging puzzles in astronomy today: the huge variety of patterns formed by the glowing planetary nebulae (see 鈥淲heels and butterflies鈥�).

To understand born-again stars, astronomers will also have to find out how common they are. There are at least five white dwarfs with hydrogen-deficient clouds of gas surrounding them in our Galaxy, and all these stars probably came back to life at some time in the past. Another kind of hydrogen-poor star, named 鈥淩 CrB鈥� stars after the prototype star R Coronae Borealis, might also have had a born-again phase, although that鈥檚 not clear yet. But tentatively including the R CrB stars as born-agains would mean there are about 50 or 60 candidates in our Galaxy and its satellite, the Large Magellanic Cloud.

Given that the Milky Way alone contains hundreds of billions of stars, the few born-agains among them seem like real oddballs. But they鈥檙e probably 鈥渞are鈥� only because the rebirth phase is very short-lived, says Asplund. Models of stellar evolution suggest that about a fifth of stars between roughly 1 and 8 times the mass of the Sun will have enough helium to undergo this final fling as they shrink to white dwarfs. Could this be the fate of our Sun when it fizzles out in about 7 billion years?

That probably depends on what happens when the Sun eventually run out of hydrogen in its core. Based on the behaviour of other stars with similar masses, it should next fuse hydrogen in a shell around the core and start to chew on the helium in the middle. The ignition of helium should puff the Sun up into a red giant, sending some of the solar atmosphere floating into space. Our star should then go through alternate cycles of burning hydrogen and helium in shells outside the core until the outer layers have blown away, leaving the core to shrink to a white dwarf. According to models of born-again stars, the shrinking white dwarf will only become hot enough for a violent rebirth if it ended its giant phase burning helium. We have no way of predicting whether that will be true for the Sun.

In the meantime, amateurs and professionals are keeping their eyes peeled for more born-again stars to find out how common they really are. Because they are so bright, it should be possible to see them as far away as the Andromeda Galaxy, over 2 million light years away. The armies of amateurs like Sakurai, who together scan vast areas of sky, could well be the ones to find them.

This time they鈥檒l be joined by the latest computerised astronomical detectors. One is SuperWASP, a wide-angle camera that can monitor brightness changes in 25,000 to 50,000 stars at a time. Although primarily designed to look for extrasolar planets passing in front of stars, as well as near-Earth asteroids, SuperWASP will blow the whistle on any nearby born-again stars. It will begin observing at La Palma early next year. 鈥淲e can do the work of amateur astronomers with SuperWASP. Things like born-again stars will be very easy meat for us,鈥� says Pollacco.

Born-again stars are simply too important to miss, and as soon as another star dares to grow old disgracefully, somebody somewhere will be on its case. 鈥淭here鈥檚 a certain notion in astronomy that stars are so well understood that they鈥檙e not the most exciting stuff to work on,鈥� says Kerber, 鈥渂ut Sakurai鈥檚 object proves there鈥檚 still a lot to learn.鈥�