This is the story of a gold rush in the sky.
Astronomers
have now seen and heard a pair of dead stars collide, giving them the
first glimpse of what they call a “cosmic forge,” where the world’s
jewels were minted billions of years ago.
The
collision rattled space-time and sent a wave of fireworks across the
universe, setting off sensors in space and on Earth on Aug. 17 as well
as producing a long loud chirp in antennas designed to study the
Einsteinian ripples in the cosmic fabric known as gravitational waves.
It set off a stampede around the world as astronomers scrambled to turn
their telescopes in search of a mysterious and long-sought kind of
explosion called a kilonova.
After
two months of underground and social media rumblings, the first wave of
news is being reported Monday about one of the least studied of cosmic
phenomena: the merger of dense remnants known as neutron stars, the
shrunken cores of stars that have collapsed and burst.
Such
collisions are thought to have profoundly influenced the chemistry of
the universe, creating many of the heavier elements in the universe,
including almost all the precious metals like gold, silver, platinum and
uranium. Which is to say that the atoms in your wedding band, in the
pharaoh’s jewels and the bombs that destroyed Hiroshima and still
threaten us all were formed in a cosmic gong show that reverberated
across the heavens billions of years ago.
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As
astronomers gather for news conferences in several cities around the
world, a blizzard of papers are being published, including one in The Astrophysical Journal Letters
that has 4,500 authors — a third of all the professional astronomers in
the world — from 910 institutions. “That paper almost killed the
paperwriting team,” said Vicky Kalogera, a Northwestern University
astrophysicist who was one of 10 people who did the actual writing.
More papers are appearing in Nature and in Science, on topics including nuclear physics and cosmology.
“It’s
the greatest fireworks show in the universe,” said David Reitze of the
California Institute of Technology and the executive director of the
Laser Interferometer Gravitational-Wave Observatory, or LIGO.
Daniel
Holz, an astrophysicist at the University of Chicago and a member of
the LIGO Scientific Collaboration, a larger group that studies
gravitational waves, said, “I can’t think of a similar situation in the
field of science in my lifetime, where a single event provides so many
staggering insights about our universe.”
It
was a century ago that Albert Einstein predicted that space and time
could shake like a bowl of jelly when massive things like black holes
moved around. But such waves were finally confirmed only in 2016, when
LIGO recorded the sound of two giant black holes colliding, causing a
sensation that eventually led this month to a Nobel Prize.
For
the LIGO researchers, this is in some ways an even bigger bonanza than
the original discovery. This is the first time they have discovered
anything that regular astronomers could see and study. All of LIGO’s
previous discoveries have involved colliding black holes, which are
composed of empty tortured space-time — there is nothing for the eye or
the telescope to see.
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But
neutron stars are full of stuff, matter packed at the density of Mount
Everest in a teacup. When neutron stars slam together, all kinds of
things burst out: gamma rays, X-rays, radio waves. Something for
everyone who has a window on the sky.
“Joy
for all,” said David Shoemaker, a physicist at the Massachusetts
Institute of Technology who is the spokesman for the LIGO Scientific
Collaboration.
It
began on the morning of Aug. 17, Eastern time. Dr. Shoemaker was on a
Skype call when alarms went off. One of the LIGO antennas, in Hanford,
Wash., had recorded an auspicious signal and sent out an automatic
alert. Twin antennas, in Washington and Louisiana, monitor the distance
between a pair of mirrors to detect the submicroscopic stretching and
squeezing of space caused by a passing gravitational wave. Transformed
into sound, the Hanford signal was a long 100-second chirp, that ended
in a sudden whoop to 1000 cycles per second, two octaves above middle C.
Such a high frequency indicated that whatever was zooming around was
lighter than a black hole.
Checking
the data from Livingston to find out why it had not also phoned in an
alert, Dr. Shoemaker and his colleagues found a big glitch partly
obscuring the same chirp.
Meanwhile,
the Fermi Gamma-Ray Space Telescope, which orbits Earth looking at the
highest-energy radiation in the universe, recorded a brief flash of
gamma rays just two seconds after the LIGO chirp. Fermi sent out its own
alert. The gamma-ray burst lasted about two seconds, which put it in a
category of short gamma ray bursts associated with the formation of
black holes perhaps as a result of neutron stars colliding.
“When we saw that,” Dr. Shoemaker said, “the adrenaline hit.”
Dr. Kalogera, who was in Utah hiking and getting ready for August’s total solar eclipse when she got the alarm, recalled thinking: “Oh my God, this is it. This 50-year-old mystery, the holy grail, is solved.”
Together
the two signals told a tale of a pair of neutron stars — dense balls
about as massive as the sun but only about the size of Manhattan —
spiraling around each other like the blades of a kitchen blender.
The
stars were each the battered survivors of cosmic violence: All that was
left of a pair of stars whose explosions had once lit up their galaxy,
circling each other and merging in a cataclysm never before seen by
human eyes.
And
it was loud, meaning that it was relatively close to Earth, said Zsuzsa
Marka, a Columbia astrophysicist, showing off the chirp on a laptop in
her office recently. But where?
Luckily
the European Virgo antenna had joined the gravitational wave network
only two weeks before, and it also showed a faint chirp at the same
time. The fact that it was so weak allowed the group to localize the
signal to a small region of the sky in the southern constellation Hydra
that was in Virgo’s blind spot.
An Earthling’s Guide to Black Holes
Welcome to the place of no return — a region in space
where the gravitational pull is so strong that not even light can escape
it. This is a black hole.
The
hunt was on. By then Hydra was setting in the southern sky. It would be
11 hours before astronomers in Chile could take up the chase.
One
of them was Ryan Foley, who was working with a team on the Swope
telescope run by the Carnegie Institution on Cerro Las Campanas in
Chile. Figuring the burst had come from a galaxy, they made a list of
the biggest galaxies in that region and set off to photograph them all
systematically, the biggest ones first. “Just mow the lawn,” as Dr.
Foley, a professor at the University of California, Santa Cruz, put it
in a phone interview.
The
fireball showed up in the ninth galaxy photographed, as a new bluish
pinprick of light in the outer regions of NGC 4993, a swirl of stars
about 130 million light years from here. “These are the first optical
photons from a kilonova humankind has ever collected,” Dr. Foley said.
Within
10 minutes, another group of astronomers, led by Marcelle Soares-Santos
of Brandeis University and using the Dark Energy Camera, which could
photograph large parts of the sky with a telescope at the nearby Cerro
Tololo Interamerican Observatory, had also spotted the same speck of
light.
Emails went flying about in the Chilean night. Within hours four more groups had found the fireball.
When
the Hubble Space Telescope swung over to the galaxy, it was
inadvertently announced on Twitter, which led J. Craig Wheeler, an
astronomer at the University of Texas, to respond with his own tweet:
“New LIGO. Source with optical counterpart. Blow your sox off!”
Graphic: What Is General Relativity?
Dr. Wheeler quickly deleted his tweet, but the discovery was creating a social media buzz among astronomers and stargazers.
When
it was first identified, the fireball of 8,000-degree gas was about the
size of Neptune’s orbit and radiating about 100 million times as much
energy as the sun.
Within
a few days, the orbiting Chandra X-ray Observatory and the Swift
satellite both detected X-rays coming from the location of the burst,
and radio telescopes like the Very Large Array in New Mexico recorded
radio emissions.
Over the course of a few days, meanwhile, the visible
fireball faded and went from blue to red.
From all this, scientists have begun patching together a tentative story of what happened in the NGC 4993 galaxy.
“It’s
actually surprising how well we were able to anticipate what we’re
seeing,” said Brian David Metzger, a theorist at Columbia University who
coined the term kilonova back in 2010. But Dr. Kalogera cautioned that
this was not a “vanilla gamma ray burst, being way fainter and closer
than any previously observed.”
As
they tell it, the merging objects were probably survivors of stars that
had been orbiting each other and had each puffed up and then died in
the spectacular supernova explosions in which massive stars end their
luminous lives. Making reasonable assumptions about their spins, these
neutron stars were about 1.1 and 1.6 times as massive as the sun, smack
in the known range of neutron stars.
As
they approached each other swirling a thousand times a second, tidal
forces bulged their surfaces outward. Quite a bit of what Dr. Metzger
called “neutron star guts” were ejected and formed a fat doughnut around
the merging stars.
At
the moment they touched each other, a shock wave squeezed more material
out of their polar regions, but the doughnut and extreme magnetic
fields confined the material into an ultra-high-speed jet emitting a
blitzkrieg of radiation. Those were the gamma rays, carrying news of the
catastrophe to the outside universe.
As
the jet slowed down and widened, encountering interstellar gas in the
galaxy, it began to glow in X-rays and then radio waves.
The
subatomic nuggets known as neutrons meanwhile were working their cosmic
alchemy. The atoms in normal matter are mostly empty space: a teeny
tiny nucleus of positively charged protons and electrically neutral
neutrons enveloped in a fluffy ephemeral cloud of negatively charged
electrons. Under the enormous pressures of a supernova explosion,
however, the electrons get squeezed back into the protons turning them
into neutrons packed into a ball as dense as an atomic nucleus.
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The
big splat liberates these neutrons into space where they inundate the
surrounding atoms, transmuting them into heavy elements. The
radioactivity of these newly created elements keeps the fireball hot and
glowing.
Dr.
Metzger estimated that an amount of gold equal to 40 to 100 times the
mass of the Earth could have been produced over a few days and blown
into space. For uranium the number is 10 to 30 times the mass of the
Earth. In the coming eons, those metals could be incorporated into new
stars and planets and in some far, far day become the material for an
alien generation’s jewels or weapons.
The
discovery filled a long-known chink in the accepted explanation of how
the chemistry of the universe evolved from pure hydrogen and helium into
the diverse place it is today. Stars and supernovas could manufacture
the elements up to iron or so, according to classic papers in the 1950s
by Margaret and Geoffrey Burbidge, Fred Hoyle and William A. Fowler, but
heavier elements required a different thermonuclear chemistry called
r-process and lots of free neutrons floating around. Where would they
have come from?
One
idea was neutron star collisions, or kilonovas, which now seem destined
to take their place on the laundry list of cosmic catastrophes along
with the supernova explosions and black hole collisions that have shaped
the history of the universe.
Until
now there was only indirect evidence of kilonovas. Astronomers found a
fireball from a gamma-ray burst in 2013, but there was no proof that
neutron stars were involved. At least some of the mysterious flashes in
the sky known as short gamma-ray bursts, astronomers now know, are
caused by mating neutron stars. Dr. Kalogera said this had been expected
for decades: “For the first time ever, we have proof.”
One
burning question is what happened to the remnant of this collision.
According to the LIGO measurements, it was about as massive as 2.6 suns.
Scientists say that for now they are unable to tell whether it
collapsed straight into a black hole, formed a fat neutron star that
hung around in this universe for a few seconds before vanishing, or
remained as a neutron star. They may never know, Dr. Kalogera said.
Neutron
stars are the densest form of stable matter known. Adding any more mass
over a certain limit will cause one to collapse into a black hole, but
nobody knows what that limit is.
Future observations of more kilonovas could help physicists understand where the line of no return actually is.
Back
when LIGO was being designed, in the 1970s, few astronomers knew there
would be black hole collisions to see, but everybody knew there were
binary star systems containing neutron stars, that should collide. And
so LIGO was designed and sold to see these. And now it has.
Dr.
Holz, the University of Chicago astrophysicist, said, “I still can’t
believe how lucky we all are,” reciting a list of fortuitous
circumstances. They had three detectors running for only a few weeks,
and it was the closest gamma-ray burst ever recorded and the loudest
gravitational wave yet recorded. “It’s all just too good to be true. But
as far as we can tell it’s really true. We’re living the dream.”
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