环状星云的真实形状

The Ring Nebula¹'s distinctive² shape makes it a popular illustration for astronomy books. But new observations by NASA's Hubble Space Telescope of the glowing gas shroud³（覆盖物） around an old, dying, sun-like star reveal a new twist. "The nebula is not like a bagel, but rather, it's like a jelly doughnut, because it's filled with material in the middle," said C. Robert O'Dell of Vanderbilt University in Nashville, Tenn. He leads a research team that used Hubble and several ground-based telescopes to obtain the best view yet of the iconic nebula. The images show a more complex structure than astronomers⁴ once thought and have allowed them to construct the most precise 3-D model of the nebula.   "With Hubble's detail, we see a completely different shape than what's been thought about historically for this classic nebula," O'Dell said. "The new Hubble observations show the nebula in much clearer detail, and we see things are not as simple as we previously⁵ thought."   The Ring Nebula is about 2,000 light-years from Earth and measures roughly 1 light-year across. Located in the constellation⁶ Lyra, the nebula is a popular target for amateur（业余的） astronomers.   Previous observations by several telescopes had detected the gaseous⁷ material in the ring's central region. But the new view by Hubble's sharp-eyed Wide Field Camera 3 shows the nebula's structure in more detail. O'Dell's team suggests the ring wraps around a blue, football-shaped structure. Each end of the structure protrudes⁸ out of opposite sides of the ring.   The nebula is tilted⁹ toward Earth so that astronomers see the ring face-on. In the Hubble image, the blue structure is the glow of helium. Radiation from the white dwarf¹⁰ star, the white dot in the center of the ring, is exciting the helium to glow. The white dwarf is the stellar remnant of a sun-like star that has exhausted¹¹ its hydrogen fuel and has shed its outer layers of gas to gravitationally collapse¹² to a compact object.   O'Dell's team was surprised at the detailed¹³ Hubble views of the dark, irregular knots of dense¹⁴ gas embedded¹⁵ along the inner rim¹⁶ of the ring, which look like spokes¹⁷ in a bicycle wheel. These gaseous tentacles¹⁸ formed when expanding hot gas pushed into cool gas ejected previously by the doomed¹⁹ star. The knots are more resistant#p#分页标题#e#²⁰ to erosion by the wave of ultraviolet light unleashed²¹ by the star. The Hubble images have allowed the team to match up the knots with the spikes²² of light around the bright, main ring, which are a shadow effect. Astronomers have found similar knots in other planetary nebulae.   All of this gas was expelled by the central star about 4,000 years ago. The original star was several times more massive than our sun. After billions of years converting hydrogen to helium in its core, the star began to run out of fuel. It then ballooned in size, becoming a red giant. During this phase, the star shed its outer gaseous layers into space and began to collapse as fusion²³ reactions began to die out. A gusher²⁴ of ultraviolet light from the dying star energized²⁵ the gas, making it glow.   The outer rings were formed when faster-moving gas slammed into slower-moving material. The nebula is expanding at more than 43,000 miles an hour, but the center is moving faster than the expansion of the main ring. O'Dell's team measured the nebula's expansion by comparing the new Hubble observations with Hubble studies made in 1998.   The Ring Nebula will continue to expand for another 10,000 years, a short phase in the lifetime of the star. The nebula will become fainter and fainter until it merges²⁶ with the interstellar medium.   Studying the Ring Nebula's fate will provide insight into the sun's demise²⁷ in another 6 billion years. The sun is less massive than the Ring Nebula's progenitor²⁸（祖先，起源） star, so it will not have an opulent ending.   "When the sun becomes a white dwarf, it will heat more slowly after it ejects its outer gaseous layers," O'Dell said. "The material will be farther away once it becomes hot enough to illuminate²⁹ the gas. This larger distance means the sun's nebula will be fainter because it is more extended."   In the analysis, the research team also obtained images from the Large Binocular Telescope at the Mount Graham International Observatory³⁰ in Arizona and spectroscopic data from the San Pedro Martir Observatory in Baja California, Mexico.