- 4320 - MILKY WAY GALAXY - more than we thought? Milky Way galaxy may be a different shape than we thought. The shape may reveal a history of collisions with other galaxies or even galactic clusters.
4320 - MILKY WAY GALAXY - more than we thought?
- Astronomers have
increasingly discovered that galaxies seem to come in three main shapes:
Elliptical, irregular and spiral. The majority of known galaxies that fit in
this last category seem to have two prominent "arms" that branch out
and split into lesser arms.
-
- The traditional
portrayal of the Milky Way is that of a galaxy with four major spiral arms
extending out from a thick centralized bulge of stars. This makes our spiral
galaxy stand out as an extremely rare outlier with an odd shape that must have
some very unique properties to grant it four major arms.
-
- That portrayal
could be wrong. New research suggests we
have been wrong about the shape of the Milky Way for decades, with our galaxy
instead having two main arms just like its contemporary spiral galaxies.
-
- In the last two
decades, accurate distance measurements have provided us with an opportunity to
solve this issue. Data from a new
generation of space instruments that can better measure the distance to
individual stars has allowed measurements of the distances to around 200 stars.
Astronomers added data from the European Space Agency (ESA)'s Gaia space
telescope which precisely observes the movement of stars and their location in
relation to Earth.
-
- The astronomers
honed in on hot and massive stars called “OB stars” in the Gaia data. Because
these stars are short-lived they move very little during their main-sequence
hydrogen-burning lifetime which makes them useful for mapping purposes. Data
collected from 24,000 OB stars was added to the map as were Gaia observations
of over 1,000 open galactic clusters.
-
- Using the precise
locations of very young objects, for the first time, they propose that our
galaxy has a multiple-arm morphology that consists of two-arm symmetry. The Norma and Perseus Arms are likely the two
symmetric arms in the inner Milky Way. As they extend from the inner galaxy to
the outer parts, they bifurcate, and connect to the Centaurus and Sagittarius
Arms.
-
- At the outskirts of
the Milky Way are distant and fragmented irregular arms that are not connected
to the central bulge of the galaxy where the majority of its stars are located.
The fragmentation of spiral arms may have been caused by our galaxy colliding
with other galaxies or even galactic clusters in its ancient history.
-
- This new model of
the Milky Way's shape could provide an alternative basis for future studies of
galactic structure. More details
should be revealed by further observations of nearby radio sources taken by
multiple telescopes that would allow their distances from Earth to be
calculated, and by improved data from the Gaia spacecraft. Gaia launched in
2013 and is expected to observe the universe for at least another two years
until 2025.
-
- Although our
telescopes have captured some truly stunning images of the Milky Way,
astronomers have only a vague understanding of our home galaxy. It took a lot
of work even to get that sketch, and it's amazing what we've been able to learn
from our limited vantage point.
-
- Here on Earth's
surface, the Milky Way galaxy appears to the naked eye as a nebulous band
across the sky. While astronomers and philosophers have debated the true nature
and location of the Milky Way for ages, the great astronomer, physicist and
all-around genius Galileo Galilei was the first to discover the galaxy's true
nature: countless stars so small that their light blends together.
-
- In the mid-1700s,
philosopher Immanuel Kant correctly guessed that the Milky Way was a rotating
disk of stars, and because we were embedded in that disk, it appeared as a band
to us. A few decades later, astronomer William Herschel attempted to create a
map of the universe, with little success.
-
- It wasn't until the
early 1900s that we really started to piece together the true nature of our
galaxy. That's when astronomer Edwin Hubble discovered that the Andromeda
nebula was, in fact, the Andromeda galaxy, an "island universe"
sitting millions of light-years from us. The Milky Way wasn't just a disk of
nearby stars. It formed the bulk of our own galaxy, so our galaxy took on the
name of that familiar sky feature.
-
- Observations of
other galaxies helped us piece together what our home galaxy looked like. Most
disk galaxies host spiral arms and a dense central bulge, so it's natural to
assume that the Milky Way has those features as well.
-
- Directly mapping
the Milky Way is an extremely challenging task. For one, it's big , roughly
100,000 light-years at its widest point. And there's a lot of stuff inside
it, between 100 billion and 400 billion
stars, hundreds of thousands of star-forming regions, and countless planets,
black holes, neutron stars and lots more. So surveys of even small portions of
the Milky Way require enormous amounts of resources.
-
- And then there's
dust. Hanging out in interstellar space, dust has the annoying property of
dimming and scattering light. Because we are embedded in the Milky Way, the
farther we try to look, the more our view is obscured by dust. Even the most
powerful telescopes in the world can't directly examine regions on the opposite
side of the galaxy.
-
- To map the Milky
Way, researchers use many kinds of observations and combine those with
comparisons with other galaxies and clever theoretical modeling to put together
a complete picture.
-
- Globular clusters
orbit the center of the Milky Way in a roughly spherical arrangement. By
plotting their positions in three-dimensional space, we can find where the
center is roughly 25,000 light-years away from us.
-
- We can also examine
the motions of stars as they orbit the center and use our understanding of
gravity to model what the core must look like. It's through this technique that
we think our galaxy is a "barred" spiral, the core is elongated, and
maybe even peanut-shaped. This is confirmed by observations showing that a
certain kind of red giant star that lies near the core is split into two
populations and that infrared light coming from the core isn't symmetrical.
-
- The Gaia
spacecraft, launched in 2013, has one main mission: to create a census of as
many stars as possible, recording their distances, motion, brightness and
colors. To date, that mission has cataloged nearly 2 billion stars, which,
while impressive, still represents only roughly 1% of all the stars in the
Milky Way.
-
- Despite their
dramatic appearance, the spiral arms are only about 10% denser than their
surroundings. Instead, they appear visually striking because they are regions
of active star formation, hosting lots of newly formed, big, bright stars.
-
- We know that the
Milky Way has at least two prominent spiral arms, and those arms are anchored
on a central bar, or peanut-like core in a giant S shape. Beyond that, however,
things are a little fuzzier.
-
- The galaxy might
have two additional medium-intensity arms, or just a tangled mess of spurs and
branches. Any "map" you might encounter of the Milky Way is mostly
conjecture and will likely change every few years as we improve our techniques
and gain a better understanding.
-
- From a distance,
our galaxy would be beautiful, if a bit messy.
Astronomers have used the youngest objects in the Milky Way to build a
new map of the galaxy's spiral arms, and the results are far messier than
expected.
-
- Even though the
spiral arms of a galaxy look stunning and impressive, they are not much denser
with stars than the gaps between them. A typical spiral arm has only about 10%
more stars than average, so spiral arms aren't regions of greater stellar
concentration.
-
- Instead, the arms
represent places of greater-than-average star formation. The arms are density
waves, their formation triggered by gravitational interactions with satellite
galaxies, that travel through a galaxy at their own speed, like ripples in a
pond. As the waves travel, they slightly increase the density in that region of
space as they pass by. When that happens, clouds of gas destabilize and
collapse, leading to a new round of star formation.
-
- Those clouds go on
to form all sorts of stars, from small red dwarfs to blue giants. Those giant
stars don't last long; a typical star of that size lasts only a few million
years. By the time the spiral density wave finishes passing through, all of
those stars will be gone, leaving behind only their smaller, dimmer siblings.
-
- We look at a
galaxy from afar, the bright giant stars outshine all the others, highlighting
the appearance of the spiral arms.
Because we are embedded within the same galactic disk as the arms, we
have to contend with tens of thousands of light-years of gas and dust, which
are very effective at blocking many wavelengths of light, to make our
observations. And so our maps of the Milky Way are surprisingly vague and
filled with guesswork.
-
- To build their map,
the astronomers focused on observations of three kinds of objects: high-mass
star-forming regions, bright young stars, and young open clusters. All three of
these object types represent fresh rounds of star formation in their own way,
so they all should prove to be relatively reliable tracers of spiral arms.
-
- The high-mass
star-forming regions are dense clouds of gas and dust that are actively forming
giant stars. The interplay of radiation from the stars with the gas in the
clouds generates "masers," naturally occurring microwave lasers. Very
long baseline interferometers, which are networks of telescopes scattered
across the globe, have allowed researchers to measure the distances to dozens
of these masers throughout the galaxy.
-
- Because giant
stars do not live long, they do not have a lot of time to move away from their
birth locations. Thus, where we see them today should be near the location of a
spiral arm. The team used the positions of over 23,000 young stars from the
Gaia catalog. By using only young stars with high-precision positions, the
astronomers could ensure that they were closely mapping the locations of spiral
arms.
-
- The team used the
locations of known young open clusters. Open clusters are associations of stars
that formed from the same gas cloud but have not yet had enough time to drift
away from each other. So these clusters, of which the team used nearly 1,000
samples, should be able to tell us where the spiral arms are.
-
- Despite the
volume of data, the locations of all of these young objects doesn't even come
close to covering the entire Milky Way. Instead, the researchers had to take
the best-fit locations of the spiral arms in our local vicinity and extend
them, reconstructing the entire lengths of the arms all the way from the edge
of the galaxy back to the core.
-
- They found a total
of seven spiral arms. Two of these, the Perseus and Norma arms, dominated the
others; a distant observer would see these two arms stand out much more
clearly. These arms begin at opposite ends of the Milky Way's elongated core
and wrap around each other, forming a symmetrical S-shaped pattern.
-
- The other five
arms, the Carina, Sagittarius, Centaurus, Outer and Local arms, don't extend
nearly as far as the major two. Instead, they begin either as forks of the
major arms or as loose segments of their own, neither of which wraps entirely
around the galaxy.
-
-
January 18, 2023
MILKY WAY
GALAXY - more than we thought? 4312
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