Sunday, March 19, 2023

3920 - MILKY WAY GALAXY - are we unique?

 

-   3920  -  MILKY  WAY  GALAXY  -  are we unique?    Is the Milky Way’s bulge-formation history unique or common in galaxy evolution? To answer that question, astronomers will have to look at galaxy assembly in the distant, young universe, a task for which NASA’s James Webb Space Telescope was specifically designed.


------------  3920  -    MILKY  WAY  GALAXY  -  are we unique?

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-    Our Milky Way galaxy has a central bulge of stars sits in the middle of a sprawling disk of stars. This is a common feature among spiral galaxies and astronomers have spent decades puzzling out how and when the Milky Way’s central bulge might have formed.

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-    Were the stars within the bulge born early in our galaxy’s history, 10 to 12 billion years ago? Or did the bulge build up over time through multiple episodes of star formation?

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-    Some studies have found evidence for at least two star-forming bursts, leading to stellar populations as old as 10 billion years or as young as 3 billion.   A new survey of millions of stars instead finds that most stars in the central 1,000 light-years of the Milky Way’s hub formed when it was engorged with infalling gas more than 10 billion years ago. This process might have been triggered by simple accretion of primordial material, or something more dramatic like merging with another young galaxy.

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-    Many other spiral galaxies look like the Milky Way and have similar bulges, so if we can understand how the Milky Way formed its bulge then we’ll have a good idea for how the other galaxies did too.

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Astronomers studied the stars’ chemical compositions.   Stars in the galactic bulge look like they’ve undergone a cosmic Botox treatment.  They appear younger than they are. That’s because they contain about the same amount of heavy elements (heavier than hydrogen and helium) as the Sun.

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-   That’s surprising because metals take time to accumulate. They must be created by earlier generations of stars, ejected through stellar winds or supernovas, and then incorporated into later generations.

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-    Our Sun, at 4.5 billion years old, is a relative newcomer, so it makes sense that it would be replete in metals. In contrast, most old stars within our galaxy are lacking in heavy elements. And yet these bulge stars are metal enriched despite their advanced age.

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-    Something different happened in the bulge. The metals there built up very, very quickly, possibly in the first 500 million years of its existence. Astronomers used the measured brightness of stars at different wavelengths of light, particularly in the ultraviolet, to determine their metal content.

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-   Stars forming at different times would be expected to have different metallicities on average. Instead, they found that stars within 1,000 light-years of the galactic center showed a distribution of metals clustered around a single average.  This suggests that those stars formed in a brief firestorm of star birth.

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-   They surveyed a portion of the sky covering more than 200 square degrees, an area approximately equivalent to 1,000 full Moons. They used the Dark Energy Camera (DECam) on the Victor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile, a program of NSF’s NOIRLab. This wide-field camera is capable of capturing 3 square degrees of sky in a single exposure.

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-   They collected more than 450,000 individual photographs that allowed them to accurately determine chemical compositions for millions of stars. A subsample of 70,000 stars were analyzed for this study.

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-    This survey is unique because we were able to scan a continuous section of the bulge at wavelengths of light from ultraviolet to visible to near-infrared. That allows us to get a clear understanding of what the various components of the bulge are and how they fit together.

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-    The researchers are looking into the possibility of measuring stellar distances to make a more accurate 3D map of the bulge. They plan to seek correlations between their metallicity measurements and stellar orbits. That investigation could locate “flocks” of stars with similar orbits, which could be the remains of disrupted dwarf galaxies, or identify signs of accretion like stars orbiting opposite the galaxy’s rotation.

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                   March 11, 2023     MILKY  WAY  GALAXY  -  are we unique?             3913                                                                                                                         

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--------------------- ---  Sunday, March 19, 2023  ---------------------------

 

 

 

 

         

 

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