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Eager to probe the X-ray background
further, Giacconi once again
proposed an advanced telescope.
In 1999, this became the Chandra
X-Ray Observatory, the third of
the orbiting Great Observatories.
Chandra is the most powerful X-ray
telescope ever built, tens of billions
of times more sensitive than the
early detectors. Its phenomenal
performance outstripped all
expectations and its mission
lifetime was tripled from five to
15 years. As of 2016, however,
its mission is ongoing. Chandra’s
outstanding technical firsts include
detecting sound waves coming
from a supermassive black hole.
The X-ray data, when combined
with optical observations from
the Hubble Space Telescope and
infrared data from the Spitzer
Space Telescope, have provided
stunning images of the cosmos.
Realm of the X-rays
X-ray astronomy observes the
highest-energy objects in space:
colliding galaxies, black holes,
neutron stars, and supernovae.
The energy source behind this
NEW WINDOWS ON THE UNIVERSE
activity is gravity. As matter falls
toward a massive concentration
of material, particles collide and
accumulate. They give up their
energy by emitting photons,
which at these speeds have X-ray
wavelengths (0.01–10 nanometers,
or billionths of a meter)—equivalent
to temperatures of tens of million
of degrees. The same mechanism,
is at work in a wide range of
dramatic phenomena: active stars
more massive than the sun, for
example, produce strong solar
winds and significant amounts of
X-rays. “X-ray binary star” systems,
in which mass transfers from one
star to its partner, also produce
intense radiation.Seeing black holes
When stars explode at the end of
their lives, the blast waves from the
supernova compress the interstellar
medium, causing the gas to release
X-rays. Left within what remains
of the supernova, the massive starcontinues life as a neutron star or
a black hole. Turbulence generated
by material being torn apart as it
is sucked into a black hole will also
produce X-rays. The radiation being
pumped out causes the outer layers
of the supernova remnant to fluoresce
in a range of colors.
Certain galaxies have centers
that outshine all the billions of stars
in the galaxy itself, with emissions
that are bright at all wavelengths.
The center of such an “active
galactic nucleus” is assumed to
contain a supermassive black hole.
Material falling toward the centers
of galaxy clusters—the largest
structures in the universe—also
shines in X-rays, and is not visible in
other light frequencies. Chandra has
now taken two “deep field” images
of the X-ray background—23- and
11-day exposures of the northern
and southern hemispheres of the
sky. X-ray instruments of the future
may help scientists see how black
holes are distributed. ■Observations in the X-ray spectrum
reveal hidden structures. The larger
blobs in this patch of sky from an ESA
X-ray survey are galaxy clusters;
smaller dots are black holes.
The universe is popping
all over the place.
Riccardo Giacconi