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In physics and astronomy symmetry and simplicity are the
guiding precepts for the description of the universe. Few things
exemplify this idea better than planetary nebula. This small and
nearly perfectly spherical shell of gas glows due to the intense
ultra-violet radiation of the central star. Our own Sun will end
its life in this way by casting off its outer gases and revealing
its bare nucleus- a white dwarf. So intense will be the direct radiation
from the naked core that the Earth's atmosphere will be vaporized and
its surface will be sterilized. Not to worry, we have about 5 billion
years to be anxious about this particular problem. However, an observer
on Earth would see a brilliant white point of light with the rest of
the sky glowing in pastel greens and reds. From our far removed vantage
point of this nebula, BV 5-3, we see how the sphere of gas glows. Greens
and blues are more prominent in the interiors of planetary nebulae because
the gas here is more strongly ionized and stripped of electrons. Further
out the nebula glows more reddish because this same gas is less ionized
and energized.
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