- 2854 - MILKY WAY GALAXY - learn more about our home? The satellite “Gaia” has truly revolutionized the study of the Milky Way. It ushered in “galactic archaeology“, a discipline that searches for evidence of past galactic events in the characteristics and behavior of the stars and stellar populations.
------------------ 2854 - MILKY WAY GALAXY - learn more about our home?
- On a clear night you can see the Milky Way Galaxy stretching across the night sky. The center of the galaxy is located jut above the spout of the teapot, the constellation Sagistarius on the southern horizon.
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- The mission for Gaia is reconstructing the Milky Way’s evolution to the past and future over billions of years. The Milky Way is a galactic cannibal. Astronomers have suspected since the 1990s that the Milky Way was born out of collisions between smaller galaxies that had taken place over the billions of years of its history.
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- Earth-based telescopes, such as the Sloan Digital Sky Survey, provided first hints of the galaxy’s violent past, but it wasn’t until Gaia that astronomers could really deconstruct the processes that led to the creation of the Universe that surrounds us.
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- In 2018 astronomers discovered that a group of 30,000 stars moves in a synchronized way through the neighborhood of the Sun in the opposite direction to the rest of a sample of seven million stars. This atypical motion pattern matched what the scientists had previously observed in computer simulations modeling galactic collisions and mergers.
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- These stars also stood out in the “Hertzsprung-Russell diagram“, which compares the color and brightness of stars, indicating that they come from a different stellar population, that is from another galaxy.
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- The stars, which now form part of the Milky Way’s inner halo and the outer layer of the galactic disc, must have originated from another galaxy. This galaxy, since nicknamed Gaia-Enceladus, must have collided with the Milky Way some 10 billion years ago.
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- About the size of one of the Magellanic Clouds which are two satellite galaxies roughly ten times smaller than the current size of the Milky Way, Gaia-Enceladus smashed into and was gradually devoured by the Milky Way, which was at that time only four times bigger than Gaia-Enceladus.
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- In 2019, astronomers discovered in the Gaia data a signature of another collision with a smaller galaxy, since then nicknamed “Sequoia“, which had hit the Milky Way’s galactic disc not long after Gaia-Enceladus.
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- Gaia has also shed light on the interactions with the dwarf galaxy “Sagittarius“, which has been orbiting around the Milky Way’s core for billions of years. Discovered in the 1990s, Sagittarius contains only a few tens of millions of stars, compared to the Milky Way’s hundreds of billions, which makes it 10,000 times less massive than the Milky Way.
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- As the Milky Way’s gravity pulled Sagittarius closer, the smaller galaxy started smashing through the Milky Way’s disc. That happened at least three times in the past, some five or six billion years ago, two billion years ago, and one billion years ago.
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- With each collision, the Milky Way stripped stars from Sagittarius, leaving the dwarf galaxy smaller and smaller. Eventually, Sagittarius will be completely devoured by the Milky Way. Yet, the dwarf has a profound effect on its larger cannibal.
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- In the wake of each Sagittarius’ crash through the Milky Way’s disc, stars formation in the galaxy accelerated. In fact, one of those periods roughly coincided with the formation of our Sun and the Solar System some 4.7 billion years ago.
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- Each collision caused ripples in the interstellar medium, like a rock thrown into water. As a result, the concentration of gas and dust in some areas of the Milky Way increased to the level that triggered more star formation.
- After this initial violent epoch of star formation, partly triggered by an earlier merger, the Milky Way had reached a balanced state in which stars were forming steadily. The galaxy was relatively quiet. Suddenly, Sagittarius fell in and disrupted the equilibrium, causing all the previously still gas and dust inside the larger galaxy to slosh around like ripples in water.
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- Further research of this data calculated the true size and brightness of millions of stars observed by Gaia. From the information about the size and brightness, astronomers could deduce the age of individual stars. The study concluded that star formation in the Milky Was had been declining since its formation until about five billion years ago when it suddenly ramped up.
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- Up to half of the total mass of all the stars ever created in the Milky Way’s thin disk, which contains most of the Galaxy’s stars, was produced during this period. It now appears that without the “in fall” of the relatively small Sagittarius dwarf galaxy, the Milky Way would have looked very different today and would have had much fewer stars.
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- Before Gaia, astronomers were limited in their attempts to study the structure of the Milky Way. They knew that the Milky Way was a spiral galaxy, a pancake-like disc of stars with a pattern of spiral arms rotating around a much denser core.
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- The spiral arms are areas of densely packed gas and stellar matter that appear, when observed in other galaxies, in bluer shades than the rest of the disc, which indicates the hotter temperature of the stars within them. Since hot stars are massive stars and since massive stars are young stars, astronomers can tell that spiral arms are areas where stars are being formed.
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- The Sun is located in one of the two smaller arms, named the Orion arm, some 26,000 light years away from the Milky Way’s center, completing one rotation around the center in about 230,000,000 years.
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- In January 2020 patterns in the density of stars in the Milky Way were compared to those with previously identified. There are several theories for spiral-arm structure formation. Many astronomers believe that spiral arms are short-lived structures caused by some sort of gravitational instability and that they disappear within a couple of rotations and then re-emerge with some different pattern.
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- Spiral arms do not contain the same stars throughout their billions years of existence. Spiral arms are like traffic jams, areas where stars concentrate while waiting to squeeze through some sort of a bottleneck. Stars are moving out at the front but the traffic jam stays because stars are piling up at the back.
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- In addition to these main structures, research based on Gaia data has revealed that the entire disc of the galaxy is alive and moving in many different ways and directions as a result of the various forces that act within the galaxy.
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- The Milky Way is constantly being disturbed by other bodies, dwarf galaxies and stellar clusters, that orbit around it. These interactions leave lasting imprints on the Milky Way, which can be observed hundreds of millions of years later.
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- In 2018, astronomers found millions of stars following a snail-shape position and motion pattern in the Milky Way’s disc. Their calculations revealed that this ripple was most probably a result of one of the past collisions with the Sagittarius dwarf galaxy.
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- The periodic collisions with Sagittarius are believed to have had a profound effect on how stars move in the Milky Way. Some even claim that the 10,000 times more massive Milky Way’s trademark spiral structure might be a result of the crashes with Sagittarius.
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- The Milky Way is also constantly stripping stars from dwarf galaxies and stellar clusters with which it interacts. Astronomers have identified streams of stars ripped from these bodies that frequently stretch over distances of thousands of light-years, covering a large portion of the sky.
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- Data about such streams can help astronomers assess the gravitational force and therefore the mass distribution of the Milky Way and hence reveal how galaxies acquire stars.
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- Gaia has also found stars in the Milky Way’s disc that are traveling at such high speeds that they might be able to escape the galaxy’s gravitational pull or might have been expelled from other galaxies and subsequently captured by the Milky Way.
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- Of the seven million Gaia stars with full 3D velocity measurements, some twenty stars could be traveling fast enough to eventually escape from the Milky Way. But rather than flying away from the galactic center, most of the high-velocity stars seem to be racing towards it.
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- It is possible that these intergalactic interlopers come from the Large Magellanic Cloud, a relatively small galaxy orbiting the Milky Way, or they may originate from a galaxy even further away.
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- Stars can be accelerated to high velocities when they interact with a supermassive black hole. The presence of these stars might be a sign of such black holes in nearby galaxies. But the stars may also have once been part of a double-star system in another galaxy, flung towards the Milky Way when their companion star exploded as a supernova.
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- If these stars do come from other galaxies, they would carry the imprint of their place of origin. They could provide unique insight into distant universes that otherwise would be much more difficult to study.
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- The astronomers, however, admit that these sprinting stars could be native to our Galaxy’s halo, accelerated and pushed inwards through interactions with one of the dwarf galaxies that fell towards the Milky Way during its build-up history.
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- A star from the Milky Way halo is likely to be fairly old and mostly made of hydrogen, whereas stars from other galaxies could contain lots of heavier elements. Looking at the colors of stars tells us more about what they are made of.
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- In 2019, scientists discovered that interstellar gas clouds in the Sun’s galactic neighborhood form a 9,000 light-years long wave that undulates about 500 light-years above and below the galactic disc. The 400 light-years wide wave is part of what astronomers describe as the “Local Arm“, a small spiral arm of the Milky Way close to the Sun.
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- Interstellar gas clouds interest researchers because they give birth to stars when they collapse. Prior to the 2019 discovery, it was believed that such clouds in the solar neighborhood are concentrated in the so-called Gould Belt, a ring of young stars, gas, and dust which arches above and below the galactic plane.
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- The Sun lies only 500 light-years from the wave at its closest and almost appears as if it is surfing it. In fact, the Sun crossed the wave only some 13 million years ago and will cross it again and again in the future.
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- The scientists do not know what caused this unexpected, undulating shape. A past collision with a massive body, a dwarf galaxy, could be a possible explanation but further studies and more Gaia data are needed to really get a good understanding.
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- In any case, the findings challenge astronomers to revise theories about the distribution of gas in the Sun’s neighborhood.
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- The Milky Way is also our home and you should learn more about it.
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------------------------------ Other Reviews available;
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- 2655 - MILKY WAY - our home in the cosmos? - This center of the Milky Way is located in the direction of the constellation Sagittarius, the “Teapot” group of stars low in the southern sky. The exact center is located just above the spout of the teapot. The center bulge would be much, much brighter if we could see through the enormous dust cloud of intergalactic dust that lies between us and the center.
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- 2411 - Our Milky Way, the Galaxy that is our home. It is an awe-inspiring place full of stars, supernovas, nebulas, energy and dark matter. But, many aspects of it remain mysterious, even to scientists.
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- 2325 - Calculating Milky Way’s Black Hole. Black Holes are both simple and complex. We can calculate their mass, radius, lifetime, energy consumption using simple algebra. At the same time, their immense gravity causes space to bend, lengths to shorten, time to slow and mass to increase.
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- 2324 - MILKY WAY - how to explain its rotation? This Review is about new things we are learning about our own Milky Way Galaxy. What is mysterious is that what we learn seems to defy the physics that we think we know. Certainly our solar system’s planets are rotating with different physics than stars in our galaxy or other galaxies. What’s going on? What are we missing?
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- 2057 - Our closest galaxy, not the Andromeda Galaxy. - The Andromeda Galaxy is our closest large spiral galaxy. The closest small galaxies are a formation that is actually within the Milky Way itself. These are dwarf galaxies that we’ve only known about the last ten years. The closet known galaxy in the Milky Way is the Canis Major Dwarf Galaxy.
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- 1919 - How did planets form? Theory is that gas and dust orbiting the star (our Sun) coalesced and multiple collisions created little planets ( planetesimals) that with further collisions created the large planets. Stars form into galaxies much the same way.
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- 1889 - Evolution of the galaxies Computer modeling and new extensive observations of our Milky Way Galaxy are telling us a story of galactic creation. The greatest mystery still is the you are here learning about it.
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- 1804 - Placing 1,000 Milky Ways end for end spans 100 million lightyears which is the size of the Virgo Cluster of galaxies.
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- 1703 - Many of the dwarf galaxies that were swallowed up by the Milky Way have left streams of stars stretching across the sky.
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- 1526 - The galaxy center is like a pin ball machine.
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- 1492 - The diameter of the bulge at the center of the galaxy is 8,000 lightyears.
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- 1348 - During the time it took light from 13.7 billion lightyears to reach us the Universe has grown to 45 billion lightyears.
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- 1337 - The gas falling into the Blackhole at the center emits 50 times more energy than hydrogen-helium fusion would. This review lists 8 more earlier reviews about the Milky Way Galaxy
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- October 6, 2020 2854
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--------------------- --- Tuesday, October 6, 2020 ---------------------------
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