Sakurai's object is a star, the first star ever seen to die and then come back to life.
In 1996 an amateur astronomer named Yukio Sakurai noticed that an extremely dim star (a 'white dwarf' the size of the earth) suddenly blazed with brightness. It was not an explosion -- not a nova or supernova, and the white dwarfs don't explode like that anyway. It swelled over the next couple of years to become a red giant 100 times the size of our sun. "Arguably, this was the fastest case of stellar evolution ever witnessed" says astronomer Martin Asplund of the Mount Stromlo Observatory in Australia.
White dwarfs are dead stars; they have burned (fused, technically) nearly all their hydrogen and helium, leaving mostly carbon and oxygen, which they are not massive enough to fuse any further. This remnant, hot enough to glow white, shrinks to the size of a small planet. Simple theories of stellar evolution end the story there, with fusion burning off the star's remaining hydrogen until the star dies altogether.
However, Sakurai's object didn't play along. It had died, then blossomed back to life at great speed. After a couple of years of growth, the star became shrouded with interstellar dust and faded from view, but recently has begun spewing hot gasses at hundreds of kilometers per second, providing another show for the astronomers who are tracking it.
A few astronomers had been working on the problem of stellar resurrection, and were not taken totally by surprise when Sakurai made his find. This group included astronomer Icko Iben at the University of Illinois at Urbana-Champaign. His team had identified white dwarfs that lacked any detectable coating of unfused hydrogen. Some of them had no detectable hydrogen at all. This was very strange, as almost everything in the universe is full of the stuff. (In the words of Harlan Ellison, "the two most common things in the universe at Hydrogen and stupidity.")
The UIUC team developed a model for the declining years of a white dwarf that showed in some cases these stars effectively turning inside out, dragging the remaining hydrogen into the interior and briefly blazing as a full-sized sun. They calculated that the growth phase would last a couple of decades, and that the star would be back to white dwarfdom in about a century.
Sakurai's object has been playing this out much faster than Iben had anticipated, but otherwise the story seems to fit.
Astronomers expect that in time the dust cloud currently shrouding the star will be blown away, and the star will shrink back to white dwarf size. Since Sakurai's Object had a glowing planetary nebula to start with, and will probably form another one during its second collapse, the result will be a white dwarf surrounded by two concentric planetary nebulae.
New Scientist, March 1, 2003