- 2941 - MILKY WAY - it ain’t what it used to be? . Even after 14 billion years, mergers continue to sculpt the overall shape of our galaxy. This realization is just the latest change in how we understand the great stream of milk across the sky. Everything we thought we knew about the future and the history of the Milky Way, we need a new model to describe this.
------------------ 2941 - MILKY WAY - it ain’t what it used to be?
- Gazing at the night sky, what do you see in the stars? The Khoisan hunter-gatherers of sub-Saharan Africa gazed upon the meandering trail of stars and dust that split the night sky, they saw the embers of a campfire. Polynesian sailors perceived a cloud-eating shark. The ancient Greeks saw a stream of milk, “gala‘ in Greek, which would eventually give rise to the modern term “galaxy.”
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- In the 20th century, astronomers discovered that our silver river is just one piece of a vast island of stars, and they penned their own galactic origin story. It held that our Milky Way galaxy came together nearly 14 billion years ago when enormous clouds of gas and dust coalesced under the force of gravity.
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- Over time, two structures emerged: first, a vast spherical “halo,” and later, a dense, bright disk. Billions of years after that, our own solar system spun into being inside this disk, so that when we look out at night, we see spilt milk, an edge-on view of the disk splashed across the sky.
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- Over the past two years, researchers have rewritten nearly every major chapter of the galaxy’s history. What happened? They got better data. Here is what they learned:
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- On April 25, 2018, a European spacecraft by the name of ‘Gaia’ released a staggering quantity of information about the sky. Gaia’s years-long data set described the detailed motions of roughly 1 billion stars, 1,000,000,000 stars.
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- Previous surveys had mapped the movement of just thousands. The data brought a previously static swath of the galaxy to life. “Gaia started a new revolution in astronomy.
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- Astronomers raced to download the dynamic star map, and a flurry of discoveries followed. They found that parts of the disk appeared impossibly ancient. They also found evidence of epic collisions that shaped the Milky Way’s violent youth, as well as new signs that the galaxy continues to churn in an unexpected way.
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- Taken together, these results have spun a new story about our galaxy’s turbulent past and its ever-evolving future. The Milky Way is not a static object. Things are changing rapidly everywhere, even today.
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- To see back to the galaxy’s earliest days, astronomers seek stars that were around back then. These stars were fashioned only from hydrogen and helium, the cosmos’s rawest materials with no heavier elements like calcium and oxygen. Fortunately, the smaller stars from this early stock are also slow to burn, so many are still shining after billions of years. Larger stars are the first to go supernova after several million years.
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- Researchers have assembled a catalog of 42 such ancients, known as ultra metal-poor stars (to astronomers, any atom bulkier than helium qualifies as metallic). According to the standard story of the Milky Way, these stars should be swarming throughout the halo, the first part of the galaxy to form.
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- By contrast, stars in the disk, which was thought to have taken perhaps an additional billion years to spin itself flat, should be contaminated with heaver elements such as carbon and oxygen.
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- In late 2017, Astronomers found that most stars were streaming through the halo, as predicted. But some 23% weren’t. Rather, they appeared stuck in the disk, with the Milky Way’s youngest stars.
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- Further research confirmed that the stars really are long-term residents of the disk, and not just tourists passing through. From two recent surveys of roughly 5,000 metal-poor stars, a few hundred of them appear to be permanent residents of the disk.
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- Surveys of about 500 stars identified finding that about 1 in 10 of these stars lie flat in circular, sunlike orbits. And a third research survey found stars of various metallic ties ,and therefore various ages, moving in flat disk orbits.
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- The astronomers speculated that perhaps pockets of pristine gas managed to dodge all the metals expelled from supernovas for eons, then collapsed to form stars that looked deceptively old. Or, the disk may have started taking shape when the halo did, nearly 1 billion years ahead of schedule.
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- In digital simulations, a Milky Way like galaxy forms and evolves over 13.8 billion years from the early universe to the present day. A Supercomputing simulation required three months of computing time. Astronomers repeated the exercise six times. How did those ancient stars get into the disk?
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- Simply put, they were stellar immigrants. Some of them were born in clouds that predated the Milky Way. Then the clouds just happened to deposit some of their stars into orbits that would eventually form part of the galactic disk. Other stars came from small “dwarf” galaxies that slammed into the Milky Way and aligned with an emerging disk.
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- The results suggest that the classic galaxy formation models were incomplete. Gas clouds do collapse into spherical halos, as expected. But stars arriving at just the right angles can kick-start a disk at the same time.
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- With Gaia, astronomers have found direct evidence of cataclysmic collisions. Astronomers assumed that the Milky Way had a hectic youth, they used the Gaia data to help pinpoint specific debris from one of the largest mergers.
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- A huge number of halo stars were ping-ponging back and forth in the center of the Milky Way in the same peculiar way, a clue that they had come from a single dwarf galaxy.
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- The galactic wreckage was everywhere. Perhaps half of all the stars in the inner 60,000 light-years of the halo that extends hundreds of thousands of light-years in every direction came from this lone collision, which may have boosted the young Milky Way’s mass by as much as 10%.
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- The incoming galaxy was named Gaia-Enceladus, after the Greek goddess Gaia and her Titan son Enceladus. When the Milky Way and Gaia-Enceladus collided, perhaps 10 billion years ago, the Milky Way’s delicate disk may have suffered widespread damage.
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- Astronomers debate why our galactic disk seems to have two parts: a thin disk, and a thicker one where stars bungee up and down while orbiting the galactic center. Hints of additional mergers have been spotted in bundles of stars known as globular clusters.
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- All these mergers have led some astronomers to suggest that the halo may be made almost exclusively of immigrant stars. Models from the 1960s and ’70s predicted that most Milky Way halo stars should have formed in place. But as more and more stars have been identified as galactic interlopers, astronomers may not need to assume that many, if any, stars are natives.
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- The Milky Way has enjoyed a relatively quiet history in recent eons, but newcomers continue to stream in. Stargazers in the Southern Hemisphere can spot with the naked eye a pair of dwarf galaxies called the Large and Small Magellanic Clouds. Astronomers long believed the pair to be our steadfast orbiting companions, like moons of the Milky Way.
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- Then a series of Hubble Space Telescope observations between 2006 and 2013 found that they were more like incoming meteorites. Astronomer clocked the clouds as coming in hot at about 330 kilometers per second, nearly twice as fast as had been predicted.
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- They concluded that the speedy clouds must be extremely big, perhaps 10 times bulkier than previously thought. Various groups have predicted that the unexpectedly beefy dwarfs might be dragging parts of the Milky Way around.
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- Astronomers had to figure out how to neutralize the motions of the Earth and the sun, and how to average out the motion of halo stars so that the halo’s outer fringe could serve as a stationary backdrop.
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- When they calibrated the data in this way, they found that the Earth, the sun, and the rest of the disk in which they sit are lurching in one direction, not toward the Large Magellanic Cloud’s current position, but toward its position around a billion years ago
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- The sliding of the disk against the halo undermines a fundamental assumption that the Milky Way is an object in balance. It may spin and slip through space, but most astronomers assumed that after billions of years, the mature disk and the halo had settled into a stable configuration.
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- This analysis proves that assumption were wrong. With new data new theories of how we came to be materialize must change. Stay tuned we are not done learning.
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----------------------------------- Other reviews on the Milky Way galaxy:
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- 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.
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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 - MILKY WAY - 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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- December 18, 2020 MILKY WAY - it ain’t what it used to be? 2941
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--------------------- --- Saturday, December 19, 2020 ---------------------------
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