A stellar wind speeding from one of the hottest-known stars in the Universe has sculpted ripples resembling mother-of-pearl in the butterfly-shaped Red Spider Nebula.
The wavy texture, shown in a Hubble space telescope image released this week by the European Space Agency, is the result of a mysterious interaction between the nebula, a glowing ember of an exploded star, and a white dwarf, or collapsed star.
The exposed, hot core of the white dwarf wins the battle in this case, spewing a stellar wind blowing with a speed of 2,000-4,500 kilometers per second (about 7 to 16 million km/hr, or 4 to 10 million mph) and creating waves that tower as high as 100 billion kilometers (62 billion miles).
The result is disturbances in the texture of the nebula, just as an Earthly wind passing over a lake can generate waves on the water.
For starters, the butterfly shape is fairly easy to explain, says Garrelt Mellema of Leiden University, who worked on the analysis.
The fast wind from the white dwarf ran into the material thrown out by the star when it was still a red giant -- an expanding phase in a star's life before it becomes a white dwarf, Mellema says. The material here then stretched into a thin and dense disk around the star, possibly due to the influence of a nearby star or massive planet, he said.
"When the star then evolves into a hot white dwarf, its fast stellar wind is stopped by this dense disk, but in the other directions the wind is more successful in pushing up the material thrown out by the red giant," Mellema told SPACE.com. "Hence the bipolar shape."
Think of an inflating balloon with an elastic band around its middle, he said.
But the waves are harder to explain. It is possible that during "inflation," the lobes expand very quickly, which can lead to instabilities in their shape, he said. Or there could be density variations in the material thrown out by the red giant which distort the nebula as it braces against the solar wind.
The nebula is located about 3,000 light years away in the constellation Sagittarius.
Fracturing wave crests
Detailed study of this Hubble image have shown that the central white dwarf must have a temperature of at least half a million degrees, making it one of the hottest stars known.
The waves are generated by supersonic shocks formed when the local gas is compressed and heated in front of the rapidly expanding lobes.
Atoms caught in the shocks radiate the visible light seen in this image, Mellema said. The process appears to have been under way long enough to make the edges of the lobe walls look as if they have started to fracture into wave crests.
The nebula itself is a windy and hot place with gas temperatures reaching a scorching 10,000 K. Planetary nebulas form when stars explode and expel their material into space, occasionally forming two lobes.
Symmetry in the two lobes of the Red Spider nebula comes from the gravitational pull of the white dwarf and a companion star, both of which are invisible in the Hubble image, Mellema said. They are emitting much more powerful X-ray radiation which Hubble cannot detect.
The Hubble images were taken on September 12, 1997, with the Wide Field and Planetary Camera 2 in five different filters. Light from sulphur ions is displayed in red, nitrogen ions in orange and ionized hydrogen (H-alpha) in green, while atomic oxygen is in light blue and ionized oxygen in dark blue.