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William Atkins
Friday, 23 February 2007 20:53
At that time, the Sun will expand greatly, consume many of its planets including the Earth, and then dramatically blow off its outer layers of gases—leaving only a burnt out core called a white dwarf.
Infrared observations by the Spitzer Space Telescope have shown dust around a white dwarf in the Helix nebula (also called NGC 7293), which is located about 650 to 700 light-years away from the Earth in the constellation Aquarius. (One light-year is the distance that light travels in one year while in vacuum.) Scientists think that the dust is left over from collisions between comets in the vicinity of the dead star.
The Helix nebula looks similar to a giant eye (sometimes called The Eye of God) when viewed by Spitzer, with lividly glowing dust and fluorescent gases hiding the white dwarf. Many planetary nebulae are found throughout our Milky Way galaxy, showing the end stage of stars of low to medium mass, which are not massive enough and hot enough to fuse carbon in nucleosynthesis reactions. About six percent of all stars in the Milky Way galaxy are or will eventually become white dwarfs.
Just like the comets in the Kuiper Belt that orbit the Sun beyond the orbit of the planet Neptune, these Helix comets once orbited its host star. They were lucky enough to survive their star’s death due to their great distance away. According to Kate Su and her colleagues at the University of Arizona (Tucson), these comets were originally in stable orbits about the aging star. However, the explosion that caused the star to become a white dwarf scattered the comets into irregular orbit. This action caused them to collide with each other—thus producing the dust seen by Spitzer. The scientific results from Su and her colleagues appear in the March 1, 2007 issue of Astrophysical Journal Letters.
Photographs of the Helix nebula can be found at http://hubblesite.org/newscenter/archive/releases/2003/11/ (HubbleSite, Space Telescope Science Institute) and http://www.seds.org/messier/xtra/ngc/n7293.html (SEDS, Students for the Exploration and Development of Space).
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