- 4112 - MILKYWAY - new chemicals and ghost stars? “Chemical cartography”, has unveiled new regions of our galaxy's stunning radial features populated by dense patches of young stars. This could be crucial for astronomers seeking to understand our galaxy's evolution, shape and structure.
-------------- 4112 - MILKYWAY - new chemicals and ghost stars?
- A mysterious alignment of stellar dead
stars haunts the heart of the Milky Way.
These exist in the form of “planetary nebulas”, clouds of gas that are
expelled by dying stars at the end of their lives.
-
- These “nebulae” can resemble butterflies or
hourglasses with the smoldering remains of the star at their heart. The sun,
when it runs out of fuel for nuclear fusion at its core and after it has
swelled out as a red giant and swallowed the inner planets in around 5 billion
years, will leave similar gaseous remains around a white dwarf star.
-
- Astronomers know a great deal about
planetary nebulas, but an arrangement of such clouds in the galactic bulge at
the center of our Milky Way galaxy has still been a puzzle since its discovery
10 years ago.
-
- Studying 136 planetary nebulas in the
thickest part of the Milky Way, the galactic bulge, with the “Very Large
Telescope” (VLT), the team discovered that each is unrelated and comes from
different stars, which died at different times and spent their lives in
different locations.
-
- The researchers also found that the shapes
of these planetary nebulas line up in the sky in the same way. Not only this,
but they are also aligned almost parallel to the plane of the Milky Way.
-
- This alignment is only present in the
planetary nebulas that have a close stellar companion. In these cases, the companion
stars orbit the stellar remnant at the heart of the planetary nebulas at a
distance closer than our solar system's innermost planet Mercury is to the sun.
-
- The alignment is absent in planetary
nebulas that lack such a companion star, and this implies that the alignment
could be created as a result of the rapid orbital motion of the companion star,
which may even end up orbiting inside the remains of the main star.
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- The formation of stars in the bulge of our
galaxy is a complex process that involves various factors such as gravity,
turbulence, and magnetic fields. Until now, we have had a lack of evidence for
which of these mechanisms could be causing this process to happen and
generating this alignment.
-
- Chemical cartography shows the distribution
of elements throughout the Milky Way
from lighter elements, such as hydrogen and helium, to heavier ones,
such as carbon, nitrogen and oxygen. Astronomers refer to any elements heavier
than helium as metals. This allows astronomers to locate stars according to
their chemical compositions rather than merely the light they emit.
-
- Over the course of their lives, stars fuse
hydrogen to create helium, then fuse that helium to create other metals. This
means metal levels associated with individual stars can give astronomers
information about their ages.
-
- We are now creating better and better maps
of the Milky Way. Chemical cartography
isn't a new process, but only recently
did scientists manage to develop telescopes with enough observing power to get
significant results using the technique.
-
- For at least seven decades, astronomers have
understood that our galaxy has spiral arms that extend out from the dense
concentration of stars, gas, and dust which lie at its heart, known as the
"central bulge." However, the
exact shape of this striking structure ,down to the number of arms our galaxy
has, remains uncertain.
-
- The difficulty in assessing the Milky Way's
morphology comes from the fact that we live in it. We're basically analyzing it
from the inside, with Earth sitting in the Orion Arm around two-thirds of the
way from the central bulge.
-
- We simply can't get far enough to observe
our galaxy from an outsider's perspective.
One
traditional way of mapping the Milky Way involves monitoring the concentrations
of young stars that are created as the galaxy's very dense spiral arms rotate.
As this rotation occurs, it compresses gas and dust to ultimately trigger star
births.
-
- Identifying an overabundance of young stars
implies the location of a spiral arm.
Though young stars can be detected by tracking the bright blue light
they emit, observations of this kind can be obscured by thick clouds of dust
which present a challenge to even the most advanced telescopes. That means some
regions of the Milky Way’s spiral arms go unobserved.
-
- One way of working around this dust veil is
by observing exactly how metal-rich the stars that lurk in hidden regions
are. This “metalicity” serves as an age measurement because
the early cosmos was filled with hydrogen and helium, but little in the way of
metals. That means the oldest stars are also composed of mostly hydrogen and
helium and are thus "low metalicity" or "metal-poor."
-
- During their lives, these older stars forge
heavier elements via nuclear fusion, but,
when they run out of such fuel, are ripped apart by supernova explosions
that spread the metals throughout their cosmic environment. Therefore, when
metal-enriched clouds of dust and gas collapse to birth stars, this next
generation of stars is richer in metals than the last.
-
- This stellar cycle of life and death has
continued throughout the 13.8 billion-year history of the universe, with every
subsequent generation of stars being more metal-rich than the last. Thus, young
stars are expected to be "metal-rich" or hold a "high
metalicity."
-
- If the Milky Way's spiral arms trigger star
births as predicted, then they should be marked by young stars, metal-rich
stars. Conversely, spaces between the arms should be marked by metal-poor
stars. The spiral arms are indeed richer
in metals.
-
- Since Gaia launched in 2013, the spacecraft
has observed around 2 billion cosmic objects allowing astronomers to
considerably widen their view of the universe. It dropped its latest and third
data release in June 2022, which was especially important for chemical
cartography because it offers the most precise and comprehensive survey of the
Milky Way ever conducted.
-
- The sheer volume of data available from
Gaia allows us to do chemical cartography at a galactic scale now. Data on both the positions for billions of
stars and their chemical makeup wasn’t available until recently.
-
- Gaia’s chemical data is still represent just
around 1% of the Milky Way. Going forward, not only will Gaia continue to scour
our galaxy collecting this data, but new telescopes are also coming online to
collect data ripe for chemical cartography endeavors.
-
- As telescope technology becomes more
advanced, the power of chemical cartography will also increase, meaning
astronomers stand to learn more about the structure of our galaxy and its
previously obscured regions.
-
-
August 6, 2023 MILKYWAY
- new chemicals and ghost
stars? 4112
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