The Hubble Space Telescope has imaged the death throes of a searingly hot, Sun-like star that has cast off its outer layers in a form resembling the opalescent wings of a giant butterfly.
Gas in the Bug Nebula, officially called NGC 6302, is being ionised by an unseen star 鈥� one of the hottest known 鈥� at the intersection of the wings.
Hubble鈥檚 Wide Field and Planetary Camera 2 made this composite image from exposures of hydrogen and ionised nitrogen in the nebula. The gas is expanding outward, rippling into finger shapes where it collides with slower-moving gas.
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But what excites astronomers most is not the shimmer of the wings but a dark band that bisects them. A dense ring of gas and dust 鈥� called a torus 鈥� girdles and obscures the dying star and contains most of the star鈥檚 ejected gas.
鈥淲e really don鈥檛 know what causes the material to be ejected primarily in one plane,鈥� says Albert Zijlstra, an astronomer at the University of Manchester Institute of Science and Technology, UK. His analysis of the Hubble image and others will appear in Astronomy & Astrophysics.
The Bug Nebula is one of about 1600 known planetary nebula. These form when stars up to eight times the mass of the Sun begin to die. They bloat into red giants before shedding as much as half their mass as gas and dust nebulae, which often take on the pinched appearance of the Bug Nebula.
Secret companion
Astronomers hope to understand why the stars expel their gas in these shapes as opposed to simple spheres. The star鈥檚 own rotation may cause the gas to accumulate in a belt around the star.
鈥淏ut it鈥檚 not clear why that would cause such a strong effect,鈥� says Zijlstra. Alternatively, the unseen central star may have a companion that draws the ejected gas into the plane of the stars鈥� mutual orbit.
A third possibility is that the star swallowed a massive planet when it expanded into a red giant. The slowly rotating red giant would gain the planet鈥檚 angular momentum and spin up to 100 times more quickly, causing the gas to eject into a flat disc.
鈥淚f you threw Jupiter into the Sun, the Sun would start to spin like mad,鈥� Zijlstra explained. In fact, the Sun will expand only to Earth鈥檚 orbit when it becomes a red giant about five billion years from now. But other stars are orbited by large planets that lie at about Mercury鈥檚 distance from the Sun.
Future observations with radio telescopes could help map out the torus and explain the nebula鈥檚 shape. The star鈥檚 high temperature 鈥� about 250,000掳 Celsius 鈥� also fascinates astronomers. The Sun鈥檚 surface is about 5000掳 C, and white dwarfs, the remains of Sun-like stars that have puffed out their gas in planetary nebulae, reach temperatures of about 100,000掳C.
鈥淚 can鈥檛 think of any known star hotter than this,鈥� says Zijlstra. The hottest phase in a star鈥檚 life comes just before it runs out of hydrogen fuel and then fades quickly into a white dwarf, and it appears that Hubble has captured this fleeting phase.