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Scattered amongst the billions of stars in our galaxy alone, a sizeable number of them are thought to explode, collapse or collide each year. Some estimates suggest that the Milky Way may contain as many as 30,000 pairs of orbiting neutron stars, many of which eventually collide with each other. Beyond our home galaxy are myriads of other galaxies, each harboring potential spacetime warpers. With such odds, it's no surprise that some physicists are confident of detecting a variety of gravitational wave emitters.
What are the prime suspects? Where are the best places to look?
The suspects come in many different sizes and answer to a variety of descriptions. Some are "merely" stellar, others truly supermassive. Some reside in our own galaxy -- The Milky Way. Others lurk beyond, even at the very edges of spacetime itself.
Supermassive Black Holes
The Ultimate Singularity
Putting It All Together
Supernova 1987A Before, After, and Close-up
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In 1987, for the first time in 400 years, a brilliant supernova lit up the sky when a star known as Sanduleak 69 202 exploded in a neighboring galaxy.
This star, located in the Large Magellenic Cloud some 160,000 light years away, had originally weighed about 20 solar masses. However, at the time of explosion, its iron core's mass was only 1.5 that of the Sun. Astronomers expect that the core had contra cted to form a neutron star, but they're still searching for it.
The Crab Nebula
Another supernova, observed by Chinese astronomers in 1054, left behind this nebula. It surrounds a neutron star whose radio emissions pulsate 30 times per second. Both the shock waves and the accompanying implosions from this event might have spawned a very short pulse of gravitational waves that might be detectable here on Earth.
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A rotating pulsar emits powerful beams of radiowaves. As the pulsar rotates, the beams are observed to fluctuate between an "on" and "off" position. These fluctuations may be so accurate that the pulsar can be used to measure time with pinpoint accuracy.
Pulsar's may also emit X rays. Here's an X-ray view of the pulsar in the Crab Nebula in its "on" and "off" states.
Pulsar "On", Pulsar "Off"
Judging from the powerful magnetic field of the pulsar left behind in the Crab Nebula, the explosion resulted in the formation of a neutron star. During the past three decades, astronomers have pinpointed over 300 pulsars scattered across the heavens. Spinning rapidly at first, the pulsars eventually rotate more slowly, perhaps because part of their rotational energy gets converted into gravitational radiation.
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Here's an optical image of the powerful X-ray source Cygnus X-1.
Located but 8000 light years from Earth, Cygnus X-1 is a suspected black hole with a mass exceeding 6 suns. Every 5.9 days,
the invisible object orbits a hot, blue supergiant companion whose bulk approaches that of 30 suns.
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The black hole's powerful gravitational pull sucks gas away from the giant star. As infalling gas strikes an "accretion disk" spinning around the edge of the black hole, the gas becomes superhot and emits short, powerful bursts of X rays. The spiralling motions of this deadly duet could prove to be a strong source of gravity waves, as might other binary systems containing white drawfs or neutron stars.
For example, the orbital motions of the neutron binary pulsar, PSR 1913+16, have been observed to decline over several years. As the paired stars draw closer, they may eventually collide.
General Relativity implies that some of the stars' orbital energy is being dissipated as gravitational radiation; the theory predicts precisely the observed change in orbit. This system is now considered an important test of General Relativity, for which its discoverers, Hulse and Taylor, were awarded the 1983 Nobel Prize in Physics.
Neutron Binary in Simulation
A supercomputer simulation portrays two neutron stars locked together gravitationally. Eventually they may collide as their orbits
around each other decay.
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Neutron Stars Collide Head-on
The moment of impact between two colliding neutron stars is depicted in this supercomputer simulation. The collision produces a shock wave that ejects matter outward and back into the stars' cores, causing them to expand and contract in powerful oscillations.
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A cataclysmic event such as this would likely generate a burst of gravitational waves as the stars' cores spiral inwards and collide.
Elsewhere in the universe, perhaps nearer than we realize, two orbiting black holes may be drawing closer, spiralling towards each other and eventually coalescing to form a larger black hole. Each stage of this process is expected to generate distinct displacements in spacetime. Such interactions and the patterns of gravitational radiation they generate can now be studied computationally.
Once ground-based gravitational wave observatories are built and operational, scientists expect that it will be only a matter of time before the calculated gravitational wave signatures are matched against the "real" thing.
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M87: A jet is revealed
This giant elliptical galaxy named M87 lies 50 million light years away in the constellation Virgo.
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M87: Image Processed Jet
Upon closer examination, a luminous jet some 6500 light years long shoots out from the galaxy's core. Computer-enhancement more clearly reveals the jet, shown here in blue.
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What awesome "central engine" powers the jet? Many astrophysicists believe that a supermassive black hole lurks at center of M87. Owing to its immense gravitational field, the gigantic black hole pulls in any matter within its vicinity. But before falling inwards, stars, gas and dust orbit the black hole and form an accretion disk.
Recent observations of M87 by the Hubble Space Telescope show that near the galaxy's center, its accretion disk spins at 1.2 million miles per hour! Only the gravity of a gigantic object with a mass of two, maybe three million suns could prevent the disk from flying apart. As the swirling material spirals inward, it generates gravitational energy that drives the jets. Some of this energy might also be released as gravitational waves.
Some astronomers speculate that M87's accretion disk was formed when a small galaxy approached M87 too closely and was pulled into its central region. Look again at M87. Dwarf galaxies can be seen near this huge galaxy.
Centaurus A: Optical Image
When viewed optically, a dust lane obscures part of the central region of this active galaxy.
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Centaurus A: Radio Image
A radio image of the same galaxy reveals a dramatically different view: giant radio lobes are emerging perpendicular to the dust lane.
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Cygnus A: Radio Image
This radio image of another active galaxy, Cygnus A, depicts twin radio lobes stretching 160,000 light years from the center. Consisting of beams of electrons travelling at near the speed of light, the jets travel tens or even hundreds of thousands of light years, slam into the intergalactic medium and spread out to form radio lobes.
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Many astrophysicists believe that a powerful "central engine" drives the radio jets; in other words, like M87, active galaxies probably contain supermassive black holes at their cores.
Collapsing 500 million years into seconds, this supercomputer simulation shows what happens when two spiral galaxies are pulled towards each other gravitationally and collide. In some cases, the galaxy cores, or the black holes they might contain, could coalesce to form a massive, central black hole -- an event that would spawn powerful gravitational waves. However, compared to the millions of years it would take for the galaxies to merge, the final cataclysm at the cores would be very brief. So the odds of detecting such an event are small.
During the next decade, the astronomer Edwin Hubble announced that observations showed that, in fact, the universe was expanding and many galaxies appear to be moving away from us at mind-boggling speeds. This finding validated Einstein's original theory -- minus the cosmological constant. Einstein later remarked that his attempt to modify his theory was the "... biggest blunder of my life ..."
Nevertheless, the expansion of the universe begged the question, "what is it expanding from?" The currently accepted theory, popularly known as the Big Bang theory of creation, has it that the fabric of the entire universe -- space, time, matter and radiation -- was born from an unimaginably hot and dense state, a cosmic singularity, some 13-20 billion years ago.
There are numerous astronomical observations that support the Big Bang theory. For example, radiation thought to originate from moments after the Big Bang has been observed. Known as the c osmic microwave background, its almost perfect "smoothness" in all directions is consistent with a gigantic "explosion" of spacetime that cosmologists have termed inflation.
Gravitational waves may have also survived from this explosion. Unlike electromagnetic radiation, their passage through the vastness of space will have remained undisturbed by intervening matter. If such gravitational ripples exist and can be detected, th ey could reveal much about the physics of how the universe came into being.
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