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سحابی Helix ...
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[تصویر:  13497060851.jpg]







Helix Nebula - Unraveling at the Seams
A
dying star is throwing a cosmic tantrum in this combined image from
NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer
(GALEX), which NASA has lent to the California Institute of Technology
in Pasadena. In death, the star's dusty outer layers are unraveling into
space, glowing from the intense ultraviolet radiation being pumped out
by the hot stellar core.

This object, called the Helix nebula,
lies 650 light-years away, in the constellation of Aquarius. Also known
by the catalog number NGC 7293, it is a typical example of a class of
objects called planetary nebulae. Discovered in the 18th century, these
cosmic works of art were erroneously named for their resemblance to
gas-giant planets.

Planetary nebulae are actually the remains
of stars that once looked a lot like our sun. These stars spend most of
their lives turning hydrogen into helium in massive runaway nuclear
fusion reactions in their cores. In fact, this process of fusion
provides all the light and heat that we get from our sun. Our sun will
blossom into a planetary nebula when it dies in about five billion
years.

When the hydrogen fuel for the fusion reaction runs out,
the star turns to helium for a fuel source, burning it into an even
heavier mix of carbon, nitrogen and oxygen. Eventually, the helium will
also be exhausted, and the star dies, puffing off its outer gaseous
layers and leaving behind the tiny, hot, dense core, called a white
dwarf. The white dwarf is about the size of Earth, but has a mass very
close to that of the original star; in fact, a teaspoon of a white dwarf
would weigh as much as a few elephants!

The glow from
planetary nebulae is particularly intriguing as it appears surprisingly
similar across a broad swath of the spectrum, from ultraviolet to
infrared. The Helix remains recognizable at any of these wavelengths,
but the combination shown here highlights some subtle differences.


The intense ultraviolet radiation from the white dwarf heats up the
expelled layers of gas, which shine brightly in the infrared. GALEX has
picked out the ultraviolet light pouring out of this system, shown
throughout the nebula in blue, while Spitzer has snagged the detailed
infrared signature of the dust and gas in yellow A portion of the
extended field beyond the nebula, which was not observed by Spitzer, is
from NASA's all-sky Wide-field Infrared Survey Explorer (WISE). The
white dwarf star itself is a tiny white pinprick right at the center of
the nebula.

The brighter purple circle in the very center is
the combined ultraviolet and infrared glow of a dusty disk circling the
white dwarf (the disk itself is too small to be resolved). This dust was
most likely kicked up by comets that survived the death of their star.


Before the star died, its comets, and possibly planets, would have
orbited the star in an orderly fashion. When the star ran out of
hydrogen to burn, and blew off its outer layers, the icy bodies and
outer planets would have been tossed about and into each other, kicking
up an ongoing cosmic dust storm. Any inner planets in the system would
have burned up or been swallowed as their dying star expanded.


Infrared data from Spitzer for the central nebula is rendered in green
(wavelengths of 3.6 to 4.5 microns) and red (8 to 24 microns), with WISE
data covering the outer areas in green (3.4 to 4.5 microns) and red (12
to 22 microns). Ultraviolet data from GALEX appears as blue (0.15 to
2.3 microns).

Image Credit: NASA/JPL-Caltech
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پاسخ
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  سحابی اوریون amir astronomer 0 203 04-02-2013، 10:36 PM
آخرین ارسال: amir astronomer

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