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3823 - MILKY WAY FACTS - We
are still learning?
--------- 3823 - MILKY WAY FACTS - We are still learning?
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---------------------------- Galaxy type: Barred spiral
---------------------------- Age: 13.6 billion years
---------------------------- 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 1,000 light-years thick.
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- 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. Life in the outskirts is good; we find ourselves nestled in one of
the smaller neighborhoods, the Orion-Cygnus Arm, sandwiched between larger
Perseus and Carina-Sagittarius arms. If we were to travel inwards towards the
city center, we would find the Scutum-Centaurus and Norma arms.
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- On a clear night, void of light pollution,
we can catch a glimpse of the bright lights of the galactic city streaking
across the night sky. Our window into the universe, this milky white band of
stars, dust and gas is where our galaxy gets its name.
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- Lying at the very heart of the Milky Way is
a supermassive black hole called Sagittarius A*. About 4,000,000 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.
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- In 2022, we imaged this blackhole 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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- Around the world, the Milky Way is known by
different names. In China it is called
"Silver River" and in the Kalahari Desert in South Africa it's called
the "Backbone of Night".
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- Modern estimates suggest the Andromeda
galaxy, our nearest galaxy neighbor is 2.5 million light-years away. Astronomers have been trying to figure out
what type of galaxy the Milky Way is. Our best estimates these days suggest that
it is a barred spiral, meaning that there is a bar structure across the center.
Astronomers can estimate the shape of the Milky Way by looking at its
population of stars, as well as their movements across the sky.
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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.
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- 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. Star collisions will be practically
non-existent. Starbirth, however, 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.
One practical effect is that the constellations we observe from Earth
may change as star orbits alter or new stars are added into the mix; the
collision is happening so far in the future that the constellations we see
today may be altered in any case, due to natural starbirth and star death
outside of the collision.
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- Major strides have been made, especially
since the 2013 launch of the European Space Agency's (ESA) Gaia mission in
reconstructing the shape and structure of our own galactic home. The process
involved building catalogs of stars, charting their positions in the sky and
determining how far from Earth they are. Gradually, a complex picture emerged
of a spiral galaxy that appears quite ordinary.
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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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- Everything else in the galaxy revolves
around this powerful gateway to nothingness. 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, according to ESA. 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. 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 currently has four spiral
arms. There are two main arms, Perseus and
Scutum-Centaurus, and the Sagittarius and Local Arm, which are less pronounced.
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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.
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- 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 a few thousand 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 230,000,000 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,
Sagittarius A* 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 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 and
intermediate-mass black holes form when huge stars, many times the mass of our
sun that 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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- 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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- The Gaia mision 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 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 a year later. The Milky Way continues devouring smaller
galaxies to this day. A galaxy called Sagittarius 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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- Gaia data currently generates more research
papers than even the famous Hubble Space Telescope. 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.
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January 22, 2022 MILKY WAY
FACTS -
We are still learning? 3837
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--------------------- --- Monday, January 23, 2023 ---------------------------
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