- 3798 - GALAXY - Milky Way by computer simulation? Our home galaxy, the Milky Way, gradually formed over nearly the entire history of the universe, which spans 13,000,000,000 years. Over the past decades, astronomers have managed to reconstruct different epochs of this galactic history.
---------------- 3798 - GALAXY - Milky Way by computer simulation?
- Astronomers identified the "poor old heart of the Milky Way", a population of stars left over from the earliest history of our home galaxy that resides in our galaxy's core regions.
"Galactic archaeology," 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.
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- Cosmic archaeology used the basic building blocks of a galaxy, its stars. For a small subset of stars, astronomers can deduce precisely how old they are. For example, this is true for “sub-giants stars“, a brief phase of stellar evolution where a star's brightness and temperature can be used to deduce its age.
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- For almost all stars, there is a "building style" that allows a general verdict on age: a star's metallicity, defined as the amount of chemical elements heavier than helium that the star's atmosphere contains. Such elements, 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. Hydrogen is 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, then 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.
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- Merge two sufficiently large galaxies, 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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- What came before the Milky Way's exciting teenage years? Astronmers use the data from ESA's Gaia satellite and from the “LAMOST spectral survey” to determine the ages of stars in a 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 adulthood.
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- The teenage years 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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- 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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- The existence of those earlier generations was in line with the predictions from simulations of cosmic history. Those simulations predicted where surviving representatives of those earlier generations might reasonably be found. 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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- Since 2014, Gaia has been measuring highly accurate position and motion parameters, including distances, for more than a billion stars, revolutionizing galactic astronomy. Spectra are where astronomers find information about the chemical composition of a star's atmosphere, including metallicity.
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- But while Gaia's spectra are of high quality, and there is an unrivaled number of them, the spectral resolution which is 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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- The astronomers turned to machine learning methods. The key property of machine learning is that the solution strategies are not programmed explicitly. Instead, at the core of the algorithm is a so-called neural network, with superficial similarities to the way that neurons are arranged in human brains. That neural network is then trained: given combinations of tasks and their solutions, and the connections between input and output adjusted so that, for the training set at least, the network produces the correct output given a specific input.
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- The neural network was able to deduce precise and accurate metallicities even for stars it had never encountered before. With that sample, it proved comparatively easy to identify the ancient heart of the Milky Way galaxy, a population of stars, 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, approximately 30,000 light-years across. This 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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- For an older star, like those in the poor old heart, the additional data about chemical composition and temperature allows for a reliable estimate of the star's luminosity. By comparison with how bright that star is in the sky, one can deduce the star's distance. For the comparatively distant stars in question, distance values obtained in this way are considerably more precise than the results of Gaia's parallax measurements.
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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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December 21, 2022 GALAXY - Milky Way by computer simulation? 3798
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