- 4416 - SUPERNOVA EXPLOSION - we live on what's left behind? - The threat posed by stellar cataclysms such as supernovae and related phenomena such as gamma-ray bursts are cataclysms are remote, but when they occur closer to home they can pose a threat to life on Earth.
----------- 4416
- SUPERNOVA EXPLOSION
- we live on what's left behind?
-
- Supernovae are exploding stars and they are
rare. They emit torrents of
radiation. One close enough to Earth
could threaten life on the planet.
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- Stars like the sun are remarkably constant.
They vary in brightness by only 0.1% over years and decades, thanks to the
fusion of hydrogen into helium that powers them. This process will keep the sun
shining steadily for about 5 billion more years, but when stars exhaust their
nuclear fuel, their deaths can lead to pyrotechnics.
-
- The sun will eventually die by growing
large and then condensing into a type of star called a “white dwarf”. But stars
more than eight times more massive than the sun die violently in an explosion
called a “supernova”.
-
- Supernovae happen across the Milky Way only
a few times a century, and these violent explosions are usually remote enough
that people here on Earth don't notice. For a dying star to have any effect on
life on our planet, it would have to go supernova within 100 light years from
Earth.
-
- Very few stars are massive enough to die in
a supernova. But when one does, it briefly rivals the brightness of billions of
stars. At one supernova per 50 years, and with 100 billion galaxies in the
universe, somewhere in the universe a supernova explodes every hundredth of a
second.
-
- The dying star emits high energy radiation
as gamma rays. Gamma rays are a form of electromagnetic radiation with
wavelengths much shorter than light waves. They're invisible to the human eye.
The dying star also releases a torrent of high-energy particles in the form of
cosmic rays which are subatomic particles moving at close to the speed of
light.
-
- Supernovae in the Milky Way Galaxy are rare,
but a few have been close enough to Earth that historical records discuss them.
In 185 A.D., a star appeared in a place where no star had previously been seen.
It was probably a supernova.
-
- Observers around the world saw a bright
star suddenly appear in 1006 A.D. Astronomers later matched it to a supernova
7,200 light years away. Then, in 1054 A.D., Chinese astronomers recorded a star
visible in the daytime sky that astronomers subsequently identified as a
supernova 6,500 light years away.
-
- Johannes Kepler observed the last supernova
in the Milky Way in 1604, so in a statistical sense, the next one is overdue.
-
- At 600 light years away, the red supergiant
Betelgeuse in the constellation of Orion is the nearest massive star getting
close to the end of its life. When it goes supernova, it will shine as bright
as the full moon for those watching from Earth, without causing any damage to
life on our planet.
-
- If a star goes supernova close enough to
Earth, the gamma-ray radiation could damage some of the planetary protection
that allows life to thrive on Earth. There's a time delay due to the finite
speed of light. If a supernova goes off 100 light years away, it takes 100
years for us to see it.
-
- Astronomers have found evidence of a
supernova 300 light years away that exploded 2.5 million years ago. Radioactive
atoms trapped in seafloor sediments are the telltale signs of this event.
Radiation from gamma rays eroded the ozone layer, which protects life on Earth
from the sun's harmful radiation. This event would have cooled the climate,
leading to the extinction of some ancient species.
-
- Safety from a supernova comes with greater
distance. Gamma rays and cosmic rays spread out in all directions once emitted
from a supernova, so the fraction that reach the Earth decreases with greater
distance. For example, imagine two identical supernovae, with one 10 times
closer to Earth than the other. Earth would receive radiation that's about a
hundred times stronger from the closer event.
-
- A supernova within 30 light years would be
catastrophic, severely depleting the ozone layer, disrupting the marine food
chain and likely causing mass extinction. Some astronomers guess that nearby
supernovae triggered a series of mass extinctions 360 to 375 million years ago.
Luckily, these events happen within 30 light years only every few hundred
million years.
-
- Neutron stars merge when gravity pulls them
together, which releases intense radiation when neutron stars collide. So, supernovae aren't the only events that
emit gamma rays. Neutron star collisions cause high-energy phenomena ranging
from gamma rays to gravitational waves.
-
- Left behind after a supernova explosion,
neutron stars are city-size balls of matter with the density of an atomic
nucleus, 300 trillion times denser than the sun. These collisions created many
of the gold and precious metals on Earth. The intense pressure caused by two
ultradense objects colliding forces neutrons into atomic nuclei creates heavier
elements such as gold and platinum.
-
- A neutron star collision generates an
intense burst of gamma rays. These gamma rays are concentrated into a narrow
jet of radiation that packs a big punch.
If the Earth were in the line of fire of a gamma-ray burst within 10,000
light years, or 10% of the diameter of the galaxy, the burst would severely
damage the ozone layer. It would also damage the DNA inside organisms' cells,
at a level that would kill many simple life forms like bacteria.
-
- That sounds ominous, but neutron stars do
not typically form in pairs, so there is only one collision in the Milky Way
about every 10,000 years. They are 100 times rarer than supernova explosions. But, across the entire universe, there is a
neutron star collision every few minutes.
-
- Gamma-ray bursts may not hold an imminent
threat to life on Earth, but over very long time scales, bursts will inevitably
hit the Earth. The odds of a gamma-ray burst triggering a mass extinction are
50% in the past 500 million years and 90% in the 4 billion years since there
has been life on Earth.
-
- By that math, it's quite likely that a
gamma-ray burst caused one of the five mass extinctions in the past 500 million
years. Astronomers have argued that a gamma-ray burst caused the first mass
extinction 440 million years ago, when 60% of all marine creatures disappeared.
-
- The most extreme astrophysical events have
a long reach. Astronomers were reminded
of this in October 2022, when a pulse of radiation swept through the solar
system and overloaded all of the gamma-ray telescopes in space.
-
- It was the brightest gamma-ray burst to
occur since human civilization began. The radiation caused a sudden disturbance
to the Earth's ionosphere, even though the source was an explosion nearly 2
billion light years away. Life on Earth was unaffected, but the fact that it
altered the ionosphere is sobering, a similar burst in the Milky Way would be a
million times brighter.
-
- Astronomers have found evidence that the
bright gamma-ray burst “GRB 230307A” observed last year was caused by two
neutron stars merging, not from a collapsing massive star. The study analyzed
data from both the Hubble Space Telescope and the James Webb Space Telescope.
-
- Prior research has shown that the strongest
occasional flashes of light in the night sky are made by gamma-ray bursts.
There are two basic kinds of GRBs: those that last longer than two seconds and
those that are shorter. Study of these bursts has shown that the shorter bursts
are typically the result of merging neutron stars. Longer bursts were believed
to occur when a massive star collapses.
-
- Neutron stars are created when massive
supergiant stars collapse during a supernova. Once created, they can wander
aimlessly alone through space. Sometimes, though, they travel close to another
neutron star, forming a neutron binary system. As they orbit one another, they
release gravitational waves, which can be measured here on Earth.
-
- As they spiral, they are also pulled more
tightly to one another until they eventually merge, emitting a massive burst of
gamma rays, which on Earth looks like a bright burst of light, called “kilonovae”. In studying GRB 230307A,
researchers found that not only had it had been the source of the
second-largest gamma-ray burst ever recorded, but it was also due to a
kilonova, confounding theories regarding how GRBs are created.
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- The merger itself, and the material left
after the collision was the first study of its kind. In focusing on the atomic
nuclei left behind after the collision, the researchers found evidence of the
creation of several heavy elements, including gold and silver. Further study of
how such elements were formed, they suggest, could help to better understand
how the universe as a whole was formed.
-
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April 2, 2023 SUPERNOVA
EXPLOSION - we live on what's left behind? 4416
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