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Astronomers
once pictured the cosmos as an arena of calm, as planets moved serenely
in their orbits around stars, stars orbited within galaxies, and
galaxies moved through galaxy clusters. The past four decades, however,
have revealed cosmic arenas of extreme violence. Crucial news about
these violent locales reaches Earth in the form of high-energy x-ray
and gamma-ray photons, typically created in enormous outbursts.
Earth's atmosphere blocks all x-rays and gamma rays, protecting
life on Earth's surface but seriously restricting our ability to
study the most violent cosmic phenomena.
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X-Ray
image of the Crab Nebula - the remnant of a supernova explosion
in 1054 A.D.
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The
Chandra X-ray Observatory, launched in 1999, has yielded superbly
detailed images of objects that generate copious amounts of x rays.
In many cases, these images reveal the sites of tremendous outbursts
such as supernovas, dying stars that blast hot gas into space at
velocities of many thousands of miles per second. More sensitive
x-ray observations will lead to understanding of the mechanisms
through which these explosions occur. Among these mechanisms are
the collapse of stellar cores, the sudden destruction of white dwarf
stars, and the creation of neutron stars. Neutron stars are made
entirely of neutrons and they pack into a region only a few miles
across a mass equal to the Sun's.
X-ray-observing
satellites have also found hot gas floating among the stars and
galaxies. Observations of the spectrum of the x rays from this gas
will make it possible to determine the temperature and relative
amounts of its chemical elements. This information will provide
crucial evidence for understanding the processes by which the elements
were formed. It will help to establish the history of the creation
of the elements throughout the universe. X-rays also arise from
high-temperature gas close to the surfaces of neutron stars, the
collapsed remnant cores of exploded stars.
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Optical
image of the Crab Nebula.
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A
new generation of x-ray satellites will be able to make superior
observations of another class of objects: black holes. Supermassive
black holes, each with millions of times the Sun’s mass, appear
at the centers of giant galaxies such as our Milky Way. Much closer
to our solar system, black holes with masses similar to the Sun’s
have been created by dying stars. By studying the motions of matter
close to these objects, astronomers have deduced the existence of
these star-mass black holes within our galaxy. X-ray telescopes
with high angular resolution and good spectral sensitivity can map
the motions of the extremely hot, x-ray-emitting gas that swirls
around and into the black holes. Such maps will verify that these
black holes do exist and will test theories of how black holes bend
nearby space.
Still
greater mysteries arise from observations made in the highest-energy
domain of the electromagnetic spectrum. During the past few years,
astronomers have identified a new class of objects, gamma-ray bursts,
that suddenly release enormous outflows of gamma rays. The nature
and origin of these objects remain largely a mystery. We know that
they lie far outside the Milky Way, but in all but a very few cases,
astronomers have not yet been able to match the locations of these
outbursts with any object visible in the sky. Because gamma-ray
astronomy remains in relative infancy, we must look to the next
generation of gamma- ray-detecting satellites to determine the nature
of the gamma-ray bursts.
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