- 3794 - MILKY WAY GALAXY - 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.
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- Data from the star-mapping “Gaia satellite” are helping scientists unlock the mystery of our Milky Way galaxy's spiral arms.
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- 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. It's like being in a forest and looking around. At some point, the trees are in front of each other. Plus the forest is a bit foggy, so you really can’t see what the whole forest looks like.
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- 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.
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- In addition to the fixed positions, the Gaia 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.
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- Astronomers derive the distance of the stars from measuring the parallax. This parallax measurement is 20% better with the latest release. That means that stars that previously we may have seen as part of the same structure now clearly belong to different structures.
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- 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.
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- Astronomers looked at concentrations of hot bright blue stars, called the OBA-type stars, in the Milky Way's disc. In areas where they could see a higher-than-average concentration of these stars, they could assume the existence of a spiral arm. They then compared their analysis with previously developed models of the galaxy.
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- The Milky Way is known to have two main spiral arms, the Perseus arm and the Scutum-Centaurus arm. It possesses two less pronounced arms, or spurs, called the Sagittarius and the Local Arm ,which passes close to the sun.
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They looked at concentrations of 600,000 young stars to determine the precise position of the spiral arms. Young stars are especially valuable when studying the spiral arms because spiral arms, with their dense concentration of dust and gas, are believed to be where the majority of stars form.
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- They calculated for each position in the disc, whether that region was more or less populated with respect to the average. Using that approach, 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.
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- 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.
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- 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.
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- This “density wave theory“, however, 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.
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- To find which theory is correct they looked at open clusters, 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.
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- The spiral structure appears to contain the younger population of stars but disappears if you look at the older stars. We see that 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.
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- The spiral arms may exist for only about 80 to 100 million years, a small fraction of time in the 13-billion-year life of our galaxy.
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- In the future we might be able to find out why those spiral arms in the Milky Way exist in the first place. 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.
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- 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, 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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- Every 200 million years, high-energy comets may pelt our planet as it passes through our galaxy's spiral arms. Earth's journey through the Milky Way may have had a profound impact on our planet's geology. New research indicates that 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 thicken Earth's continental crust.
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- The dense clouds of gas in the spiral arms interact with comets at the edge of the solar system, sending them hurtling toward Earth. The team reached their conclusion by examining zircon crystals from two of Earth's oldest continents and regions, where the planet's earliest continental history is preserved: the North American Craton, in Greenland, and the Pilbara Craton, in Western Australia.
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- 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.
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- 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, a sign of crust production.
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- 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.) Therefore, Earth's journey around the Galactic Center helps shape the planet's geology.
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- 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, much like fans doing "the wave" at a stadium.
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- When the solar system moves into the spiral arms, icy planetesimals in the Oort cloud at its outer edge (around 4.6 trillion milesfrom the sun) interact with dense gas clouds of the whip-like arms, sending icy material hurtling toward the inner solar system and our planet.
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- 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 is.
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- The influence of impacts on rock formation and increased crustal generation was also apparent in the team's examination of 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.
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- Determining the ages of more deposits in spherule beds could further support the team's findings and, in turn, encourage geologists and astrophysicists to start thinking more about the influence of Earth's wider cosmic environment on the planet's geology.
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December 20, 2022 MILKY WAY GALAXY 3794
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