- 4364 - MILKY WAY GALAXY ? - We are living in a golden age of Milky Way research and exploration. The Milky Way is a “barred spiral galaxy” around 13.6 billion years old with large pivoting arms stretching out across the universe.
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4364 - MILKY
WAY GALAXY ?
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– ----------------------------------- Galaxy type: Barred spiral
–----------------------------------- Age: 13.6 billion years (and counting)
–----------------------------------- Size: 100,000 light-years across
–----------------------------------- Number of stars: about 200 billion
–----------------------------------- Rotation time: 250 million years
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- Our home galaxy's
disk is about 100,000 light-years in diameter and just 1000 light-years
thick. Just as Earth orbits the sun,
the solar system orbits the center of the Milky Way. Despite hurtling through
space at speeds of around 515,000 mph our solar system takes approximately 250
million years to complete a single revolution. The last time our planet was in
this position, dinosaurs were just emerging and mammals were yet to evolve.
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- If the center of
the Milky Way were a city, we would be living in suburbia, about 25,000 to
30,000 light-years from the city center.
We find ourselves nestled in one of the smaller neighborhoods, the
Orion-Cygnus Arm, sandwiched between larger Perseus and Carina-Sagittarius arms
of stars. If we were to travel inwards
towards the city center, we would find the Scutum-Centaurus and Norma arms.
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- Lying at the very
heart of the Milky Way is a supermassive black hole, Sagittarius A*. About 4 million times the mass of the sun,
this beast consumes anything that strays too close, gorging on an ample supply
of stellar material enabling it to grow into a giant. In 2022, we imaged this
glutton at the core of our galaxy for the very first time, through an
innovative technique allowing us to view the shadow of the black hole.
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- Our galactic home
is called the Milky Way after its apparent milky white appearance as it
stretches across the night sky. In Greek mythology, this milky band appeared
because the goddess Hera sprayed milk across the sky.
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- Earth is located
roughly halfway to the edge of the Milky Way, at a distance of about 26,000
light years from the center. We reside in a feature known as the Orion Spur,
which is an offshoot between the larger Sagittarius and Perseus Arms that lie
inwards and outwards of our location.
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- The Milky Way is a
large barred spiral galaxy, with a relatively small bar compared to most
galaxies of a similar size. A central bar (or central bulge) is a circular to
oval shaped structure of old stars which lies at the center of spiral galaxies.
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- The biggest
known planet in the Milky Way may be “HD 100546 b”, which is a very large gas
giant in the process of forming with a diameter roughly 6.9 times that of
Jupiter, or 77 times that of Earth.
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- The radius
measurement is highly uncertain, as some of the material surrounding the planet
may be masquerading as being part of the planet itself. The largest planets
whose sizes are known for certain are “HAT-P-67 b and XO-6b”, both with
diameters around 2.1 times that of Jupiter. Both of these planets have had
their diameters measured directly as they transit their parent star.
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- Dutch astronomer
Jan Oort was the first to realize that the Milky Way isn't motionless but
rotates, and he calculated speeds at which stars at various distances orbit
around the galactic center. It also was Oort who determined the position of our
sun in the vast galaxy. The Oort Cloud, a repository of trillions of comets far
from the sun, was named after him.
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- At the center of
the Milky Way sits a supermassive black hole called Sagittarius A*. With a mass
equal to that of four million suns, the black hole, discovered in 1974, can be
observed in the sky with radio telescopes close to the constellation
Sagittarius.
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- In its immediate
surroundings is a tightly packed region of dust, gas and stars called the
“galactic bulge”. In the case of the Milky Way, this bulge is peanut-shaped,
measuring 10,000 light-years across. It harbors 10 billion stars out of the
Milky Way's total of about 200 billion, mostly old red giants, which formed in
the early stages of the galaxy's evolution.
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- Beyond the bulge
extends the “galactic disk”. This feature is 100,000 light-years across and
1,000 light-years thick, and it's home to the majority of the galaxy's stars,
including our sun. Stars in the disk are dispersed in clouds of stellar dust
and gas. When we look up to the sky at night, it's the edge-on view of this
disk extending toward the galactic center that takes our breath away.
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- Stars in the disk
orbit around the galactic center, forming swirling streams that appear to
emanate like arms from the galactic bulge. Research into the mechanisms that
drive the creation of spiral arms is still in its infancy, but the latest
studies suggest that these arms form and disperse within relatively short
periods of up to 100 million years , out of the galaxy's 13 billion years of
evolution.
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- Inside those arms,
stars, dust and gas are more tightly packed than in the more loosely filled
areas of the galactic disk, and this increased density triggers more intense
star formation. As a result, stars in the galactic disk tend to be much younger
than those in the bulge.
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- Spiral arms are
like traffic jams in that the gas and stars crowd together and move more slowly
in the arms. As material passes through the dense spiral arms, it is compressed
and this triggers more star formation.
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- The Milky Way has four spiral arms. There are two main
arms, Perseus and Scutum-Centaurus, and the Sagittarius and Local Arm.
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- The Milky Way disk
is not flat but warped. As it rotates, it precesses like a wobbling spinning
top. This wobble, essentially a giant ripple, circles the galactic center much
more slowly than the stars in the disk, completing a full rotation in about 600
to 700 million years. Astronomers think this ripple may be a result of a past
collision with another galaxy.
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- Sprinkled around
the disk and the bulge are globular clusters, collections of ancient stars, as
well as approximately 40 dwarf galaxies that are either orbiting or colliding
with the larger Milky Way.
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- All of that is
surrounded by a spherical halo of dust and gas, which is twice as wide as the
disk. Astronomers believe that the entire galaxy is embedded in an even larger
halo of invisible “dark matter”. Since dark matter doesn't emit any light, its
presence can only be inferred indirectly by its gravitational effects on the
motions of stars in the galaxy. Calculations suggest that this puzzling stuff
makes up to 90% of the galaxy's mass.
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- The mass of the
Milky Way, dark matter included, equals 1.5 trillion solar masses. The galaxy's
visible matter is distributed between its 200 billion stars, their planets and
the massive clouds of dust and gas that fill the interstellar space.
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- Astronomers aren't
quite sure how many planets are in the Milky Way, given we have only found
5,000 all told, but one NASA estimate suggests it's more than 100 billion
planets. How many solar systems there are in the Milky Way is also a mystery,
as we are still looking for the planets.
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- The sun orbits
about 26,000 light-years from the black hole Sagittarius A*, roughly in the
middle of the galactic disk. Traveling at the speed of 515,000 mph, the sun
takes 230 million years to complete a full orbit around the galactic center.
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- The sun sits near
the edge of the Local Arm of the Milky Way, one of the two smaller spiral arms
of the galaxy. In 2019, using data from the Gaia mission, astronomers found
that the sun is essentially surfing a wave of interstellar gas that's 9,000
light-years long, 400 light-years wide and undulates 500 light-years above and
below the galactic disk.
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- Planets of the
solar system do not orbit in the plane of the galaxy but are tipped by about 63
degrees.
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- The black hole in
the Milky Way is called Sagittarius A*. The black hole is mostly dormant, which
makes it very challenging to observe. Sagittarius A* has a mass 4.3 million
times that of the sun. The approximate diameter is 14.6 million miles. By comparison, the Milky Way itself is
roughly 100,000 light-years wide and 1,000 light-years thick.
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- A huge disk of gas
around Sagittarius A* billows out as far as 5 to 30 light-years from the
supermassive black hole. It is this huge, but tenuous, area of gas that gives a
bit of material for Sagittarius A* activity. The region is known to emit X-rays
due to feeding on the gas, or because of friction within the disk as
temperatures soar to as much as 18 million degrees Fahrenheit.
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- Stellar-mass
black holes form when huge stars, many times the mass of our sun, collapse
after stopping nuclear fusion. Since they are no longer able to stop the
gravitational collapse, they shrink to a gravitationally powerful object that
can warp time and space around it so much that light no longer can escape.
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- Edwin Hubble's
measurements of Cepheid variable stars proved Andromeda was located far outside
the Milky Way. Modern estimates suggest the Andromeda galaxy, our nearest
galaxy neighbor is 2.5 million light-years away.
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- We now know that
the Milky Way resides within the Local Group of galaxies, made up of over 30
galaxies including Andromeda, Triangulum and Leo I to name but a few. The Milky Way is currently hurtling towards
Andromeda at 250,000 mph. Though there is no need to worry just yet, this crash
of cosmic proportions is not due for another 4 billion years.
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- Observations of a
three-way galactic collision in 2022 using the Hubble Space Telescope gave some
intriguing insights. The largest of the group, as it got into a tight orbit
with the other two, snagged some material with its relatively stronger gravity.
This created an intriguing streak of gas, dust and other materials flowing into
the larger galaxy, visible even from Earth.
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- While the arms of
the Milky Way will surely be ripped up by this process, individual stars are
relatively safe as the spaces between them are quite large. In other words,
don't look for star collisions, as they will be practically non-existent.
Starbirth will accelerate due to the amount of gas being pumped into our
galaxy, causing our galaxy to brighten and for its population to expand in the
coming millions of years following the collision.
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- Our own solar
system should be relatively safe due to the low risk of star collision. That
said, we may find ourselves thrown into a completely different path around the
new galactic center as the merger pushes through.
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- The evolution of
the Milky Way began when clouds of gas and dust started collapsing, pushed
together by gravity. First stars sprung up from the collapsed clouds, those
that we see today in the globular clusters. The spherical halo emerged soon
after, followed by the flat galactic disk. The galaxy started small and grew as
the inescapable force of gravity pulled everything together.
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- “Gaia Space telescope”
measures the exact positions and distances of more than 1 billion stars, as well as their light
spectra, which enables scientists to understand the stars' composition and age.
The position data allow astronomers to determine the speeds and directions in
which the stars move in space.
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- As things in
space follow predictable trajectories, astronomers can reconstruct the paths of
the stars billions of years into the past and future. Combining these
reconstructed trajectories into one stellar movie captures the evolution of the
galaxy over eons.
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- There is also
evidence that the Milky Way collided with several smaller galaxies during its
evolution. In 2018, a team of Dutch astronomers found a group of 30,000 stars
moving in sync through the sun's neighborhood in the opposite direction to the
rest of the stars in the data set. The motion pattern matched what scientists
had previously seen in computer simulations of galactic collisions. These stars
also differed in color and brightness, which suggested they came from a
different galaxy.
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- Remnants of
another, slightly younger, collision were spotted in 2019. The Milky Way
continues devouring smaller galaxies to this day. A galaxy called Sagittarius
(not to be mistaken with the black hole) currently orbits close to the Milky
Way and has likely smashed through its disk several times in the past 7 billion
years.
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- Using Gaia data,
scientists found that these collisions triggered periods of intense star
formation in the Milky Way and may even have something to do with the galaxy's
trademark spiral shape. The study suggests that our sun was born during one of
those periods some 4.6 billion years ago.
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- Since the
beginning of its operations, the Gaia mission has provided three updates to its
massive stellar catalog. Astronomers from all over the world continue analyzing
the data in search of new patterns and revelations. Gaia will continue charting the galaxy until
at least 2025, as long as the spacecraft remains in good health, and the
catalog it has compiled will keep astronomers busy for decades to come.
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- Even though Gaia
sees less than 1% of stars in the galaxy, astronomers can expand their findings
and model the behavior of the entire Milky Way. Thank God we still have a lot more to
learn. Listen up grand children.
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February 20, 2024 MILKY WAY
GALAXY ? 4364
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Saturday, February 24, 2024 ---------------------------------
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