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EI2GYB > ASTRO 27.08.21 10:21l 115 Lines 5943 Bytes #999 (0) @ WW
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Subj: Astronomers find a 'break' in one of the Milky Way's spiral
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Astronomers find a 'break' in one of the Milky Way's spiral arms
Scientists have spotted a previously unrecognized feature of our Milky Way
galaxy: A contingent of young stars and star-forming gas clouds is sticking
out of one of the Milky Way's spiral arms like a splinter poking out
from a plank of wood.
Stretching some 3,000 light-years, this is the first major structure
identified with an orientation so dramatically different than the arm's.
Astronomers have a rough idea of the size and shape of the Milky Way's arms,
but much remains unknown: They can't see the full structure of our home
galaxy because Earth is inside it.
It's akin to standing in the middle of Times Square and trying to draw a
map of the island of Manhattan.
Could you measure distances precisely enough to know if two buildings
were on the same block or a few streets apart? And how could you hope to
see all the way to the tip of the island with so many things in your way?
To learn more, the authors of the new study focused on a nearby portion of
one of the galaxy's arms, called the Sagittarius Arm.
Using NASA's Spitzer Space Telescope prior to its retirement in January
2020, they sought out newborn stars, nestled in the gas and dust clouds
(called nebulae) where they form.
Spitzer detects infrared light that can penetrate those clouds, while
visible light (the kind human eyes can see) is blocked.
Young stars and nebulae are thought to align closely with the shape of the
arms they reside in.
To get a 3D view of the arm segment, the scientists used the latest data
release from the ESA (European Space Agency) Gaia mission to measure the
precise distances to the stars.
The combined data revealed that the long, thin structure associated with
the Sagittarius Arm is made of young stars moving at nearly the same
velocity and in the same direction through space.
"A key property of spiral arms is how tightly they wind around a galaxy,"
said Michael Kuhn, an astrophysicist at Caltech and lead author of the
new paper. This characteristic is measured by the arm's pitch angle.
A circle has a pitch angle of 0 degrees, and as the spiral becomes
more open, the pitch angle increases.
"Most models of the Milky Way suggest that the Sagittarius Arm forms
a spiral that has a pitch angle of about 12 degrees, but the structure
we examined really stands out at an angle of nearly 60 degrees."
Similar structures - sometimes called spurs or feathers - are commonly
found jutting off the arms of other spiral galaxies.
For decades scientists have wondered whether our Milky Way's spiral
arms are also dotted with these structures or if they are relatively smooth.
Measuring the Milky Way
=======================
The newly discovered feature contains four nebulae known for their
breathtaking beauty: the Eagle Nebula (which contains the Pillars of Creation),
the Omega Nebula, the Trifid Nebula, and the Lagoon Nebula.
In the 1950s, a team of astronomers made rough distance measurements to
some of the stars in these nebulae and were able to infer the existence
of the Sagittarius Arm.
Their work provided some of the first evidence of our galaxy's spiral
structure.
"Distances are among the most difficult things to measure in astronomy,"
said co-author Alberto Krone-Martins, an astrophysicist and lecturer
in informatics at the University of California, Irvine and a member of
the Gaia Data Processing and Analysis Consortium (DPAC).
"It is only the recent, direct distance measurements from Gaia that make
the geometry of this new structure so apparent."
In the new study, researchers also relied on a catalog of more than a
hundred thousand newborn stars discovered by Spitzer in a survey of the
galaxy called the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire
(GLIMPSE).
"When we put the Gaia and Spitzer data together and finally see this
detailed, three-dimensional map, we can see that there's quite a bit of
complexity in this region that just hasn't been apparent before,"
said Kuhn.
Astronomers don't yet fully understand what causes spiral arms to
form in galaxies like ours.
Even though we can't see the Milky Way's full structure, the ability
to measure the motion of individual stars is useful for understanding
this phenomenon: The stars in the newly discovered structure likely
formed around the same time, in the same general area, and were
uniquely influenced by the forces acting within the galaxy, including
gravity and shear due to the galaxy's rotation.
"Ultimately, this is a reminder that there are many uncertainties about
the large-scale structure of the Milky Way, and we need to look at the
details if we want to understand that bigger picture," said one the
paper's co-authors, Robert Benjamin, an astrophysicist at the University
of Wisconsin-Whitewater and a principal investigator on the GLIMPSE survey.
"This structure is a small piece of the Milky Way, but it could tell
us something significant about the Galaxy as a whole."
The study was published in Astronomy & Astrophysics.
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