Wednesday, January 11, 2023

3822 - MILKY WAY - how did it get its shape?

 

     -  3822 -    MILKY  WAY  -  how did it get its shape?   Why does the Milky Way have spiral arms? New Gaia data are helping solve the puzzle.  Recently published studies exploring the Early Data Release 3 (EDR3), a batch of observations made available to the scientific community last December, 2022, reveal the spiral structure of our galaxy with a greater precision and detail than was possible before.

            


            ---------  3822  -  MILKY  WAY  -  how did it get its shape?

            -    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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            -    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.  The Gaia mission has been mapping the Milky Way since 2014, measuring the precise positions and distances from Earth of nearly two billion stars.

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            -   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. In addition to the fixed positions, the spacecraft 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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            -    The latest data release, EDR3, improves the accuracy of the previous data sets. And it's this precision that is enabling astronomers to disentangle the spiral arms from the rest of the stars in the galactic disc with better precision.

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            -   We derive the distance of the stars from a measure called 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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            -    The Milky Way is known to have two main spiral arms, the Perseus arm and the Scutum-Centaurus arm. Our galaxy also possesses two less pronounced arms, or spurs, called the Sagittarius and the Local Arm,which passes close to the sun.

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            -    Astronomers 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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            -    For each position in the disc, whether that region was more or less populated with respect to the average, astronomers 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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            -    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 is the “density wave theory”, which 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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            -    The spiral arms may exist for about 80 to 100 million years, a small fraction of time in the 13-billion-year life of our galaxy.   Scientists 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 (measured by the number of scientific papers it produces), 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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            -    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.) These findings were further supported by examinations of oxygen isotopes, which revealed a similar pattern.

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            -   Therefore, Earth's journey around the Galactic Center helps shape the planet's geology, the team concluded.  Not only does the solar system travel around the Galactic Center, but the spiral arms that radiate from it also turn, albeit at a different rate.

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            -   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 miles from 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.  That's important, because more energy will result in more melting. When it hits, it causes larger amounts of decompression melting, creating a larger uplift of material.

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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 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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            -  Astronomers have managed to identify the "poor old heart of the Milky Way",  a population of stars left over from the earliest history of our home galaxy, which resides in our galaxy's core regions.

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            -    For this feat of "galactic archaeology," the researchers analyzed data from the most recent release of ESA's Gaia Mission, using a neural network to extract metallicities for two million bright giant stars in the inner region of our galaxy. The detection of these stars, but also their observed properties, provides corroboration for cosmological simulations of the earliest history of our home galaxy.

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            -    Our home galaxy, the Milky Way, gradually formed over nearly the entire history of the universe, which spans 13 billion years. Over the past decades, astronomers have managed to reconstruct different epochs of galactic history in the same way that archaeologists would reconstruct the history of a city with explicit dates of construction.

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            -    The basic building blocks of a galaxy are its stars. For a small subset of stars, astronomers can deduce precisely how old they are.  This is true for so-called sub-giants, a brief phase of stellar evolution where a star's brightness and temperature can be used to deduce its age.

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            -    A star's metallicity, defined as the amount of chemical elements heavier than helium that the star's atmosphere contains, which astronomers call "metals," are produced inside stars through nuclear fusion and released near or at the end of a star's life, some when a low-mass star's atmosphere disperses, the heavier elements more violently when a high-mass star explodes as a supernova. In this way, each generation of stars "seeds" the interstellar gas from which the next generation of stars is formed, and generally, each generation will have a higher metallicity than the rest.

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            -    Milky Way stars may be confined to the central regions, or they may be part of an orderly rotating motion in the Milky Way's thin disk or thick disk. Or else, they may form part of the chaotic jumble of orbits of our galaxy's extended halo of stars, including very eccentric ones, which repeatedly plunge through the inner and outermost regions.

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            -     Galaxy history is shaped by mergers and collisions, as well as by the vast amounts of fresh hydrogen gas that flow into galaxies over billions of years, the raw material for a galaxy to make new stars. A galaxy's history starts with smaller proto-galaxies: over-dense regions shortly after the Big Bang, where gas clouds collapse to form stars.

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            -    Proto-galaxies collide and merge, they form larger galaxies. Add another proto-galaxy to these somewhat larger objects, namely a proto-galaxy that flies in sufficiently off-center ("large orbital angular momentum"), and you may end up with a disk of stars. Merge two sufficiently large galaxies ("major merger"), and their gas reservoirs will heat up, forming a complicated elliptical galaxy combining a dearth of new star formation with a complex pattern of orbits for the existing older stars.

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            -    Data from ESA's Gaia satellite and from the LAMOST spectral survey to determine the ages of stars in an unprecedented sample of 250,000 so-called sub-giants. From this analysis, the astronomers had been able to reconstruct the consequences of the Milky Way's exciting teenage years 11 billion years ago and its subsequent more settled (or boring) adulthood.

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            -    The teenage years coincided with the last significant merger of another galaxy, called Gaia Enceladus/Sausage, whose remnants were found in 2018, with the Milky Way.  It sparked a phase of intensive star formation and led to a comparatively thick disk of stars we can see today. Adulthood consisted of a moderate inflow of hydrogen gas, which settled into our galaxy's extended thin disk, with the slow, but the continual formation of new stars over billions of years.

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            -    What the astronomers noticed back then was that the oldest stars in their teenage sample already had considerable metallicity, about 10% as much as the metallicity of our sun. Clearly, before those stars formed, there must have been even earlier generations of stars that had polluted the interstellar medium with metals.

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            -    These simulations, the initial formation of what later became our Milky Way involved three or four proto-galaxies that had formed in close proximity and then merged with each other, their stars settling down as a comparatively compact core, no more than a few thousand light-years in diameter.

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            -    Later additions of smaller galaxies would lead to the creation of the various disk structures and the halo. But according to the simulations, part of that initial core could be expected to survive these later developments relatively unscathed. It should be possible to find stars from the initial compact core, the ancient heart of the Milky Way, in and near the central regions of our galaxy even today, billions of years later.

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            -     In June , 2022,  came the Data Release 3 (DR3) of ESA's Gaia mission. Since 2014, Gaia has been measuring highly accurate position and motion parameters, including distances, for more than a billion stars, revolutionizing (among other sub-fields) galactic astronomy. DR3 was the first data release to include some of the actual spectra Gaia had observed: spectra for 220 million astronomical objects.

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            -    Spectra are where astronomers find information about the chemical composition of a star's atmosphere, including metallicity. But while Gaia's spectra are of high quality, and there is an unrivaled number of them, the spectral resolution—how finely the light of an object is split by wavelength into the elementary rainbow colors—is comparatively low by design.

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            -    Extracting reliable metallicity values from the Gaia data would require extra analysis.  Astronomers specifically looked at red giant stars in the Gaia sample. Typical red giants are about a hundred times brighter than sub-giants and readily observable even in the distant core regions of our galaxy. These stars also have the added advantage that the spectral features that encode their metallicity are comparatively conspicuous, making them particularly suitable for the kind of analysis the astronomers were planning.

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            -    The researchers had access to a sample of accurate metallicities of unprecedented size, consisting of 2 million bright giants in the inner galaxy. With that sample, it proved comparatively easy to identify the ancient heart of the Milky Way galaxy, a population of stars that Rix has dubbed the "poor old heart," given their low metallicity, inferred old age, and central location.

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            -    On a sky map, these stars appear to be concentrated around the galactic center. The distances conveniently supplied by Gaia (via the parallax method) allow for a 3D reconstruction that shows those stars confined within a comparatively small region around the center, approximately 30,000 light-years across.

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            -    The stars in question have just the right metallicity to have brought forth the metal-poorest of those stars that, later on, formed the Milky Way's thick disk. Since that earlier study provided a chronology for thick-disk formation, this makes the ancient heart of the Milky Way older than about 12.5 billion years.

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            -     The abundance of elements like oxygen, silicon, and neon can be obtained by successively adding alpha particles (helium-4 nuclei) to existing nuclei in a process called "alpha enhancement." Their presence in such quantities indicates that the early stars obtained their metals from an environment in which heavier elements were produced on comparatively short time scales via the supernova explosions of massive stars.

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            -    Can astronomers obtain more detailed spectra for many more or even all of those stars, which allow for a more detailed analysis of their chemical composition? Will they all show alpha enhancement, consistent with their formation in the Milky Way's initial core? Follow-up spectra taken as part of the recently launched SDSS-V survey or the upcoming 4MOST survey, in both of which MPIA is a partner, promise to allow the group to obtain the information necessary for answering these key questions.

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            -    The combination of a star's position in the sky and its distance gives us the star's three-dimensional location within the Milky Way. The information about the stars' motion towards or away from us, measured by the Doppler shift of their spectral lines, combined with their apparent motions on the sky permits the reconstruction of the stars' orbits within our home galaxy.

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            -    If such an analysis shows that the stars of the poor old heart belong to two or three different groups, each with its own pattern of motion, those groups are likely to correspond to the different two or three progenitor galaxies whose initial merger created the archaic Milky Way.

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            January 11, 2022         MILKY  WAY  -  how did it get its shape?               3822                                                                                                                             

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