- 4359 - GOLD IN SUPERNOVAE? - The universe is pretty good at smashing things together. All kinds of stuff collides, stars, black holes and ultradense objects called neutron stars. When neutron stars do it, the collisions release a flood of elements necessary for life.
------------------- 4367 - GOLD IN SUPERNOVAE? -
- ( See Review
4366 “Closest Supernova” )
- Scientists spot
kilonova explosion from an epic 2016 crash.
Just about everything has collided at one point or another in the
history of the universe, so astronomers had long figured that neutron stars,
superdense objects born in the explosive deaths of large stars, smashed
together, too.
-
- A neutron star
collision would go out with a flash. It wouldn't be as bright as a typical
supernova, which happens when large stars explode. But astronomers predicted
that an explosion generated from a neutron star collision would be roughly a
thousand times brighter than a typical nova,
a “kilonova”.
-
- As the name
suggests, neutron stars are made of a lot of neutrons. And when you put a bunch
of neutrons in a high-energy environment, they start to combine, transform,
splinter off and do all sorts of other wild nuclear reaction things. With all the neutrons flying around and
combining with each other, and all the energy needed to power the nuclear
reactions, kilonovas are responsible for producing enormous amounts of heavy
elements, including gold, silver and xenon.
-
- Together with
their cousins, supernovas, kilonovas fill out the periodic table and generate
all the elements necessary to make rocky planets ready to host living organisms
like us.
-
- In 2017,
astronomers witnessed their first kilonova. The event occurred about 140
million light-years from Earth and was first heralded by the appearance of a
certain pattern of gravitational waves, or ripples in space-time, washing over
Earth.
-
- These
gravitational waves were detected by the Laser Interferometer
Gravitational-Wave Observatory (LIGO) and the Virgo observatory, which
immediately notified the astronomical community that they had seen the distinct
ripple in space-time that could only mean that two neutron stars had collided.
Less than 2 seconds later, the Fermi Gamma-ray Space Telescope detected a
gamma-ray burst, a brief, bright flash of gamma-rays.
-
- Astronomers around
the world trained their telescopes, antennas and orbiting observatories at the
kilonova event, scanning it in every wavelength of the electromagnetic
spectrum. 0ne-third of the entire
astronomical community around the globe participated in the effort. It was
perhaps the most widely described astronomical event in human history, with
over 100 papers on the subject appearing within the first two months.
-
- Kilonovas had long
been predicted, but with an occurrence rate of 1 every 100,000 years per
galaxy, astronomers weren't really expecting to see one so soon. In comparison,
supernovas occur once every few decades in each galaxy.
-
- The addition of
gravitational wave signals provided an unprecedented glimpse inside the event
itself. Between gravitational waves and traditional electromagnetic
observations, astronomers got a complete picture from the moment the merger
began.
-
- That kilonova alone
produced more than 100 Earths' worth of pure, solid precious metals, confirming
that these explosions are fantastic at creating heavy elements. The gold in jewelry was forged from two
neutron stars that collided long before the birth of the solar system.
-
- But that wasn't
the only reason the kilonova observations were so fascinating. Albert
Einstein's theory of general relativity predicted that gravitational waves
travel at the speed of light. But astronomers have long been trying to develop
extensions and modifications to general relativity, and the vast majority of
those extensions and modifications predicted different speeds for gravitational
waves.
-
- With that single
kilonova event, the universe gave us the perfect place to test this. The
gravitational wave signal and the gamma-ray burst signal from the kilonova
arrived within 1.7 seconds of each other. But that was after traveling over 140
million light-years. To arrive at Earth that close to each other over such a
long journey, the gravitational waves and electromagnetic waves would have had
to travel at the same speed to one part in a million billion.
-
- That single
measurement was a billion times more precise than any previous observation, and
thus wiped out the vast majority of modified theories of gravity. No wonder a third of astronomers worldwide
found it interesting.
-
- Researchers know
that stars fuse light atomic nuclei to create heavier nuclei. Elements in the
universe heavier than hydrogen (but lighter than iron) are created by a process
known as stellar nucleosynthesis. These
are nuclear reactions that occur deep inside stars' cores. But it has been a
long-standing mystery as to where in the universe elements heavier than iron
are synthesized.
-
- The origin of the
really heaviest chemical elements in the universe has baffled the scientific
community for quite a long time. Now, we
have the first observational proof for neutron star mergers as sources. They could well be the main source of
the “ r-process
elements," which are elements heavier than iron, like gold and platinum.
-
- After black holes,
neutron stars are the densest known objects in the universe. Each is the size
of a city, with a mass greater than that of Earth's sun; a teaspoon of this
dense material would therefore weigh a billion tons.
-
- Neutron stars are
created after stars more massive than Earth's sun explode as supernovas,
leaving behind superdense magnetized balls of spinning matter composed mainly
of neutrons, neutral particles that, along with protons, are found inside
atomic nuclei.
-
- Neutron stars
therefore contain some of the building blocks of atomic nuclei. If these
neutrons are somehow released from a neutron star, they might undergo reactions
that allow them to stick together, creating elements heavier than iron.
-
- Newly formed
particles will be highly unstable and will lose neutrons, radioactively
decaying into lighter particles. But if the surrounding environment is dense in
free neutrons, more neutrons can be captured before the nuclei will decay, so
heavier and heavier elements can be formed.
-
- If a neutron star
smashes into another neutron star, clumps of neutrons are blasted into space
and can rapidly synthesize heavy elements like gold via a mechanism called
“rapid neutron capture process”, or "r-process."
-
- So, when
astronomers confirmed the detection of the gravitational wave signal “GW170817”
that emanated from the site of a gamma-ray burst in a galaxy 130 million
light-years away, they realized they were looking at an intense cosmic
collision called a "kilonova." This was a ripe environment for the
r-process to take place. Kilonovas are
powerful explosions that unleash gamma-rays and have been long theorized to occur
when neutron stars collide.
-
- By comparing
observations made using the Hubble Space Telescope and Gemini Observatory with
theoretical models, astronomers have now confirmed that the r-process occurs in kilonovas,
observing the spectroscopic fingerprint of heavy elements being created in the
explosion's afterglow.
-
- We are witnessing a
distant heavy-element factory synthesizing maybe hundreds of Earth masses'
[worth] of gold and … maybe 500 Earth masses' worth of platinum.
-
-
February 26, 2024
GOLD IN SUPERNOVAE? 4367
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Monday, February 26, 2024 ---------------------------------
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