– 3885 - MILKY WAY GALAXY - our journey around it? Earth's journey through the Milky Way may have had a profound impact on our planet's geology. Every 200 million years, when Earth passes through its galaxy's spiral arms, the planet is pummeled with high-energy comets, and this bombardment may even thicken Earth's continental crust.
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3885 - MILKY
WAY GALAXY - our
journey around it?
-
- The dense
clouds of gas in the spiral arms of the Milky Way galaxy interact with comets
at the edge of the solar system, sending them hurtling toward Earth. By examining zircon crystals from two of
Earth's oldest continents and regions, where the planet's earliest continental
history is preserved produced these conclusions.
-
- The decay of
uranium in zircon crystals in these regions has been used to create a
geological timeline spanning 1 billion years, from 2.8 billion to 3.8 billion
years ago, during the Archean eon. This timeline could help geologists discover
how Earth became the only planet known to have continents and active plate
tectonics.
-
- Isotopes of
the element hafnium in zircon enable scientists to spot periods in Earth's
history that experienced an influx of “juvenile magma”, magma containing
elements that have never reached the surface before which is a sign of crust
production.
-
- Over a long
timescale, patterns of crust production corresponded with galactic years . A galactic year is the time it takes the sun
to complete an orbit around the center of the Milky Way, 250,000,000
years.
-
- Not only does
the solar system travel around the Galactic Center, but the spiral arms that
radiate from it also turn, at a different rate.
The sun orbits the Galactic Center at around 536,000 mph, while the
spiral arms turn at approximately 47,000 mph. This means the sun and the solar
system, as well as many of the Milky Way's other stars, move in and out of the
spiral arms.
-
- When the
solar system moves into the spiral arms, icy planetesimals in the Oort cloud at
its outer edge (around 4.6 trillion miles from the sun) interact with dense
gas clouds of the whip-like arms, sending icy material hurtling toward the
inner solar system and our planet.
-
- These objects
arrive with more energy than the asteroids that regularly pelt Earth. Most of
those space rocks come from the main asteroid belt between Mars and Jupiter , a
region that is much closer to Earth than the Oort cloud.
-
- More energy
will result in more melting. When a meteor
hits, it causes larger amounts of decompression melting, creating a larger
uplift of material. The influence of impacts on rock formation and increased
crustal generation was also apparent in spherule beds, which are deposits of
small spheres created by ejected material that cools, condenses and falls back
to Earth after impacts.
-
- Spherule
beds were also correlated with Earth's passage into the Milky Way's dense
spiral arms between around 3.3 billion and 3.5 billion years ago, when the
planet was just over 1 billion years old.
-
- New data
from the star-mapping “Gaia satellite” are helping scientists unlock this
mystery of our Milky Way galaxy's spiral arms.
Since the 1950s, astronomers have known that our galaxy, the Milky Way,
looks like a spiral, with several dense streams of stars and dust emanating
from the galactic center, winding through the galactic disc and dissolving
around its edges. However, scientists have struggled to understand how many of
these streams there are and what created them.
-
- The problem
with our galaxy is that we are inside its disc and therefore it's very
difficult to understand the structure as a whole.
-
- The
European Space Agency's (ESA) Gaia mission has been mapping the Milky Way since
2014, measuring the precise positions and distances from Earth of nearly two
billion stars. The first two batches of data acquired by the spacecraft, which
were released to the scientific community in 2016 and 2018, have revolutionized
the study of our galaxy.
-
- In
addition to the fixed positions, the spacecraft also measures how fast stars
move in three-dimensional space, allowing astronomers to model the evolution of
the Milky Way in the past as well as into the future. This is enabling astronomers to disentangle
the spiral arms from the rest of the stars in the galactic disc with better
precision.
-
- Astronomers
derive the distance of the stars from a measure called the parallax. Parallax is a star's apparent movement
against the background of more distant stars as Earth revolves around the sun.
By measuring the change in the angle between the star and Earth from two
opposite points in the planet’s orbit, astronomers can calculate the distance
of the star using simple trigonometry.
-
- In areas
where they could see a higher-than-average concentration of certain stars,
astronomers could assume the existence of a spiral arm. They then compared
their analysis with previously developed models of the galaxy.
-
- The Milky
Way is known to have two main spiral arms, the Perseus arm and the
Scutum-Centaurus arm. Our galaxy also possesses two less pronounced arms, or
spurs, called the Sagittarius and the Local Arm ,which passes close to the sun.
-
- Young stars
are especially valuable when studying the spiral arms because with their dense concentration of dust and
gas, are believed to be where the majority of stars form.
-
- They
calculated, for each position in the disc, whether that region was more or less
populated with respect to the average.
They were able to construct a map of the spiral arms in the region that
Gaia maps, that is about 16,000
light-years around the sun.
-
- When the
researchers compared their galaxy map to previous models, they found that the
Perseus arm, one of the two dominant arms, lies further away from the center of
the galaxy in the studied region. The short Local arm appeared much longer than
the previous models expected.
-
- Astronomers
are also still speculating about the origin of those arms and their longevity.
Some earlier theories proposed that the shape of the arms is somehow fixed and
spins around the galactic center over a long period of time while individual
stars, orbiting at their own velocities, move in and out of this shape.
-
- This so-called
“density wave theory” , however, it is being disputed by the latest findings
enabled by the Gaia mission. Many scientists now think that the spiral arms
might not be fixed at all. Instead, they might form temporarily, as a result of
the rotation of the galactic disc, and later dissolve and reform again in a
different configuration.
-
- “Open
clusters” are groups of thousands of young stars born from the same cloud of
gas and dust. Due to their young age, these stars are still close to their birth
place, that is within the spiral arms. If the newer theories were correct, the
amount of younger open clusters in the spiral arms would be higher than the
amount of older open clusters. And that's exactly what the data showed.
-
- The spiral
structure appears to contain the younger population of stars but disappears if
you look at the older stars. The
rotation rate of the shape is more or less similar to the rotation rate of the
stars and varies with the radius to the galactic center.
-
- The shape and
the stars can't be decoupled, and that means we don't have a global shape,
which would be the spiral arms, and then the stars moving in and out of them as
the density wave theory suggests.
-
- The spiral
arms may exist for about 80 to 100 million years, a small fraction of time in
the 13-billion-year life of our galaxy.
While some theories expect this swirl of stellar streams may have been
born after another, smaller galaxy crashed into the Milky Way, others believe
it came to existence naturally as a result of the rotation of the galactic
disc.
-
- Future Gaia
data releases will give us more information about the motion of stars in a
greater portion of the galactic disc.
The next batch of Gaia data, the full Data Release 3, is expected to be
made available to scientists worldwide in about mid-2022.
-
- Gaia, one
of the most productive missions in history measured by the number of scientific
papers it produces, will continue scanning the sky until 2025. The vast
catalogues of stellar positions, motions and velocities it creates will keep
astronomers busy for decades to come.
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February 25, 2023 MILKY WAY
GALAXY - our journey aound it? 3885
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--- Saturday, February 25, 2023 ---------------------------
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