- 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?
-
- 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?
-
- 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.
-
-
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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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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