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It was a faint
signal, but it told of one of the most violent acts in the universe, and it
would soon reveal secrets of the cosmos, including how gold was created. Astronomers
around the world reacted to the signal quickly, focusing telescopes located on every
continent and even in orbit to a distant spot in the sky.
What they
witnessed in mid-August and revealed Monday was the long-ago collision of two
neutron stars - a phenomenon California Institute of Technology's David H.
Reitze called "the most spectacular fireworks in the universe."
"When these
things collide, all hell breaks loose," he said. Measurements of the light
and other energy emanating from the crash have helped scientists explain how
planet-killing gamma ray bursts are born, how fast the universe is expanding,
and where heavy elements like platinum and gold come from.
"This is
getting everything you wish for," said Syracuse University physics
professor Duncan Brown, one of more than 4,000 scientists involved in the blitz
of science that the crash kicked off. "This is our fantasy
observation."
It started in a
galaxy called NGC 4993, seen from Earth in the Hydra constellation. Two neutron
stars, collapsed cores of stars so dense that a teaspoon of their matter would
weigh 1 billion tons, danced ever faster and closer together until they
collided, said Carnegie Institution astronomer Maria Drout.
The crash,
called a kilonova, generated a fierce burst of gamma rays and a gravitational
wave, a faint ripple in the fabric of space and time, first theorized by Albert
Einstein. The signal arrived on Earth on Aug. 17 after traveling 130 million
light-years. A light-year is 5.88 trillion miles.
NASA's Fermi
telescope, which detects gamma rays, sent out the first alarm. Then, 1.7
seconds later, gravity wave detectors in Louisiana and Washington state that
are a part of the LIGO Laboratory , whose founders won a Nobel Prize earlier
this month, detected the crash. It issued a worldwide alert to focus telescopes
on what became the most well-observed astronomical event in history.
Before August,
the only other gravity waves detected by LIGO were generated by colliding black
holes. But black holes let no light escape, so astronomers could see nothing. This
time there was plenty to see, measure and analyze: matter, light, and other
radiation. The Hubble Space Telescope even got a snapshot of the afterglow.
"The
completeness of this picture from the beginning to the end is
unprecedented," said Columbia University physics professor Szabolcs Marka.
"There are many, many extraordinary discoveries within the
discovery."
The colliding
stars spewed bright blue, super-hot debris that was dense and unstable. Some of
it coalesced into heavy elements, like gold, platinum and uranium. Scientists
had suspected neutron star collisions had enough power to create heavier
elements, but weren't certain until they witnessed it.
"We see the
gold being formed," said Syracuse's Brown. Calculations from a telescope
measuring ultraviolet light showed that the combined mass of the heavy elements
from this explosion is 1,300 times the mass of Earth. And all that stuff -
including lighter elements - was thrown out in all different directions and is
now speeding across the universe.
Perhaps one day
the material will clump together into planets the way ours was formed, Reitze
said - maybe ones with rich veins of precious metals. "We already knew
that iron came from a stellar explosion, the calcium in your bones came from
stars and now we know the gold in your wedding ring came from merging neutron
stars," said University of California Santa Cruz's Ryan Foley.
The crash also
helped explain the origins of one of the most dangerous forces of the cosmos -
short gamma ray bursts, focused beams of radiation that could erase life on any
planet that happened to get in the way. These bursts shoot out in two different
directions perpendicular to where the two neutron stars first crash, Reitze
said.
Luckily for us,
the beams of gamma rays were not focused on Earth and were generated too far
away to be a threat, he said. Scientists knew that the universe has been
expanding since the Big Bang. By using LIGO to measure gravitational waves
while watching this event unfold, researchers came up with a new estimate for
how fast that is happening, the so-called Hubble Constant. Before this,
scientists came up with two slightly different answers using different
techniques. The rough figure that came out of this event is between the
original two, Reitze said.
The first
optical images showed a bright blue dot that was very hot, which was likely the
start of the heavy element creation process amid the neutron star debris, Drout
said. After a day or two that blue faded, becoming much fainter and redder. And
after three weeks it was completely gone, she said.
Scientists
involved with the search for gravitational waves said this was the event they
had prepared for over more than 20 years. The findings are "of spectacular
importance," said Penn State physicist Abhay Ashtekar, who wasn't part of
the research. "This is really brand new."