Saturday, December 31, 2022

3800 - MILKY WAY GALAXY

 

 -  3800  -  MILKY  WAY  GALAXY  -   The "poor old heart of the Milky Way"is a population of stars left over from the earliest history of our home galaxy, which resides in our galaxy's core regions.


---------------------  3800  -   MILKY  WAY  GALAXY

- "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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-  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

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-  For almost all stars, there is a "building style" that allows a general verdict on age: a star's so-called metallicity, defined as the amount of chemical elements heavier than helium that the star's atmosphere contains.

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-   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. 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 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.

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-  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.

 

-  The Miky Way Galaxy teenage years coincided with the last significant merger of another galaxy, called Gaia Enceladus/Sausage, whose remnants were found in 2018.   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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-   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 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 search of ancient core stars stars from our galaxy's ancient core the LAMOST telescope was used in due to its location on Earth and its inability to observe during the monsoon months in summer, cannot observe the Milky Way's core regions at all.

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-  Spectra are where astronomers find information about the chemical composition of a star's atmosphere, including metallicity.  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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-   It proved comparatively easy to identify the ancient heart of the Milky Way galaxy, the "poor old heart," given their low metallicity, inferred old age, and central location. On a sky map, these stars appear to be concentrated around the galactic center.

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-   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 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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-  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.

 

-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 24, 2022     MILKY  WAY  GALAXY      3800                                                                                                                                

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--------------------- ---  Saturday, December 31, 2022  ---------------------------

 

 

 

 

         

 

 

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