- 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.
-
- 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.
-
- 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.
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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
-
- 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.
-
- 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.
-
- 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.
-
December 24, 2022 MILKY
WAY GALAXY 3800
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