ASTRONOMY can boast a new superstar. It is known as Sakurai鈥檚 object and, in the past few years, it has swollen from a ball roughly the size of the Earth to become a monstrous globe 80 times wider than the Sun. 鈥淭his is quite likely the fastest case of stellar evolution ever seen,鈥� says Martin Asplund of Uppsala Observatory in Sweden. 鈥淣ormal stars like the Sun evolve over millions or billions of years.鈥�
Sakurai鈥檚 object lies in the constellation Sagittarius. It is named after a Japanese amateur astronomer who discovered it in February 1996, when it was rapidly brightening. A team led by Asplund鈥檚 colleague, Bengt Gustafsson, used a telescope at the McDonald Observatory in Texas to observe the star for the six months following its discovery.
They conclude that the star has swelled from being a hot dwarf with a surface temperature of about 50 000 掳C into a bright yellow supergiant no hotter than 6000 掳C at the surface. 鈥淭he brightening is because the increase in surface area has more than compensated for the drop in temperature,鈥� says Asplund.
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During their observations, the researchers found that there was a fivefold decrease in the amount of hydrogen in the star and a fourfold increase in other elements such as zinc, strontium and yttrium. 鈥淭hese rapid changes are key to understanding Sakurai鈥檚 object,鈥� says Asplund (Astronomy & Astrophysics, vol 321, p L17).
According to Asplund, the star must be a 鈥渂orn-again giant鈥�-in other words, it was once a red giant star that started to shrink after running out of fuel. Eventually, it would have been on the brink of becoming a white dwarf. This is the remnant of a dying star that is incapable of generating its own heat through nuclear reactions, and eventually shrinks under the force of its own gravity.
For Sakurai鈥檚 object, however, such a fate was postponed when the inner core of the star-containing helium, carbon and oxygen-contracted, and generated enough heat to spark helium 鈥渂urning鈥� in nuclear reactions. The onset of helium burning generates vast amounts of heat, which in turn creates convection that moves hot material up to the star鈥檚 surface. 鈥淚n Sakurai鈥檚 object, this circulation also brings hydrogen from the surface down into the core, where it burns vigorously,鈥� says Asplund. 鈥淭his is why we see a dramatic drop in the star鈥檚 hydrogen content.鈥�
The onset of nuclear reactions, which would explain the inflation of the star, also explains why the amounts of many heavy elements have increased in Sakurai鈥檚 object. At the core, where temperatures reach 100 million 掳C, some reactions associated with hydrogen burning produce free neutrons. These neutrons can stick to nuclei and make heavy elements via what is called the s-process. 鈥淭he s-process is responsible for many elements important to our technology, such as the yttrium of some high-temperature superconductors and the rubidium of some lasers,鈥� says Asplund. 鈥淚t鈥檚 remarkable to see it in action.鈥�
Asplund says there are about half a dozen known born-again giants, but that Sakurai鈥檚 object is undergoing the fastest evolution ever seen in a star, apart from in stellar explosions. He adds that theory suggests that about 10 per cent of all stars like the Sun go through this phase at the end of their lives: 鈥淚n Sakurai鈥檚 object, we may therefore be seeing a premonition of the final death of the Sun.鈥�
