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This mysterious object that seems to defy classification was found by astronomers using NASA's Hubble Space Telescope. The object has been classified as a planetary nebula, the glowing remains of a Sun-like star in its death throes, although the Hubble observations suggest it may not fit that classification, either. A quick glance at the Hubble picture at top shows that the object, He2-90, looks like a young, dust-enshrouded star with narrow jets of material resembling strings of beads streaming from each side. The other light streaks running diagonally from He2-90 are artificial effects of the telescope's optical system. Each jet possesses at least six bright clumps of gas speeding along at rates estimated to be at least 600,000 kilometers an hour (375,000 miles an hour). These gaseous clumps are ejected into space about every 100 years and may be caused by periodic instabilities in He2-90's accretion disk. Jets from very young stars behave in a similar way. Deep images taken from a terrestrial observatory show each jet extending at least 100,000 astronomical units (one astronomical unit equals the Earth-Sun distance, 150 million kilometers or 93 million miles). The Hubble astronomers, Dr. Raghvendra Sahai of NASA's Jet Propulsion Laboratory, Pasadena, Calif., and Lars-Ake Nyman of the European Southern Observatory, Chile, and Onsala Space Observatory, Sweden, suspect that He2-9 is a pair of aging stars masquerading as a single youngster. One member of the duo is a bloated red giant star shedding matter from its outer layers. This matter is then captured by gravity in a rotating accretion disk around a compact partner, most likely a young white dwarf (the collapsed remnant of a Sun-like star). The stars are not visible in the Hubble images because they're obscured by a disk of dust. The jets' relatively modest speed implies that one member of the duo is a white dwarf. An accretion disk needs gravity to form. For gravity to create He2-90's disk, the two stars must reside at a cozy distance from each other: within about 10 astronomical units. Astronomers are uncertain about the details, but they believe that magnetic fields associated with accretion disks produce and constrict the pencil-thin jets seen in the Hubble image. The close-up Hubble photo at bottom shows a dark, flaring, disk-like structure (off-center) bisecting the bright light from the object. The disk is seen edge-on. Although this disk is too large to be an accretion disk, it may provide indirect proof of the disk's existence. Most theories for producing jets require the presence of an accretion disk. The round, white objects at the lower left and upper right corners are two bright clumps of gas in the jets, which are close to the companion star. The astronomers traced the jets to within 1,000 astronomical units of the central obscured star. The star ejected this material about 30 years ago. This oddball star was discovered during an imaging survey of planetary nebulae. The images were taken Sept. 28, 1999 with Hubble's Wide Field and Planetary Camera 2. The images and results appear in the Aug. 1 issue of the Astrophysical Journal Letters. JPL designed and built the Wide Field and Planetary Camera 2. The Space Telescope Science Institute, Baltimore, Md, manages space operations for the Hubble Space Telescope for NASA's Office of Space Science, Washington, D.C. The Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, Md. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. #####
Description
This mysterious object that seems to defy classification was found by astronomers using NASA's Hubble Space Telescope. The object has been classified as a planetary nebula, the glowing remains of a Sun-like star in its death throes, although the Hubble observations suggest it may not fit that classification, either. A quick glance at the Hubble picture at top shows that the object, He2-90, looks like a young, dust-enshrouded star with narrow jets of material resembling strings of beads streaming from each side. The other light streaks running diagonally from He2-90 are artificial effects of the telescope's optical system. Each jet possesses at least six bright clumps of gas speeding along at rates estimated to be at least 600,000 kilometers an hour (375,000 miles an hour). These gaseous clumps are ejected into space about every 100 years and may be caused by periodic instabilities in He2-90's accretion disk. Jets from very young stars behave in a similar way. Deep images taken from a terrestrial observatory show each jet extending at least 100,000 astronomical units (one astronomical unit equals the Earth-Sun distance, 150 million kilometers or 93 million miles). The Hubble astronomers, Dr. Raghvendra Sahai of NASA's Jet Propulsion Laboratory, Pasadena, Calif., and Lars-Ake Nyman of the European Southern Observatory, Chile, and Onsala Space Observatory, Sweden, suspect that He2-9 is a pair of aging stars masquerading as a single youngster. One member of the duo is a bloated red giant star shedding matter from its outer layers. This matter is then captured by gravity in a rotating accretion disk around a compact partner, most likely a young white dwarf (the collapsed remnant of a Sun-like star). The stars are not visible in the Hubble images because they're obscured by a disk of dust. The jets' relatively modest speed implies that one member of the duo is a white dwarf. An accretion disk needs gravity to form. For gravity to create He2-90's disk, the two stars must reside at a cozy distance from each other: within about 10 astronomical units. Astronomers are uncertain about the details, but they believe that magnetic fields associated with accretion disks produce and constrict the pencil-thin jets seen in the Hubble image. The close-up Hubble photo at bottom shows a dark, flaring, disk-like structure (off-center) bisecting the bright light from the object. The disk is seen edge-on. Although this disk is too large to be an accretion disk, it may provide indirect proof of the disk's existence. Most theories for producing jets require the presence of an accretion disk. The round, white objects at the lower left and upper right corners are two bright clumps of gas in the jets, which are close to the companion star. The astronomers traced the jets to within 1,000 astronomical units of the central obscured star. The star ejected this material about 30 years ago. This oddball star was discovered during an imaging survey of planetary nebulae. The images were taken Sept. 28, 1999 with Hubble's Wide Field and Planetary Camera 2. The images and results appear in the Aug. 1 issue of the Astrophysical Journal Letters. JPL designed and built the Wide Field and Planetary Camera 2. The Space Telescope Science Institute, Baltimore, Md, manages space operations for the Hubble Space Telescope for NASA's Office of Space Science, Washington, D.C. The Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, Md. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. #####
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