Transcript The Milky Way Galaxy - d_smith.lhseducators.com

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The Milky Way Galaxy
Structure & Evolution
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Milky Way’s Structure
Nuclear Bulge in center
– 12,000 LY wide x 10,000 LY thick
– Made of old (mostly yellow & red) stars
– Contains much of the visible mass in the
galaxy.
The Galactic Center
The center of the galaxy is a very
busy place.
It’s obscured by dust and hard to see,
but we do know there’s a 4 million
solar mass black hole inside!
This intense X-ray and radio source is
called Sagittarius A.
Center of the galaxy
There are so many old, red stars
there that “night” on a planet near the
galactic center would be twice as
bright as twilight is to us.
A fun place to live, except for all that
pesky deadly X-ray radiation.
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Thin Disk surrounds nuclear bulge
– 120,000 LY wide, but <10,000 LY thick
– Most new star formation goes on here,
so stars are much younger (Population I
type stars – all colors)
– Much free-floating gas & dust.
– BUT…contains < 50% of the visible
mass of the galaxy.
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A halo of stars and globular clusters
surrounds the entire galaxy.
– Contains almost exclusively VERY old
stars (population II stars).
– These stars are low in heavy elements,
meaning that they’re as old as the
universe (before supernovas formed
heavier elements.)
– Contains little visible mass, but about ½
total mass of galaxy.
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Total Mass of the galaxy is equivalent
to 1 trillion stars, but the galaxy
actually contains only about 200
billion stars.
Loose gas & dust only account for
about another 200 billion stars’ worth.
Where and what is the missing 600
billion stars’ worth of mass?
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Where is our sun in the MW?
We have a difficult problem in trying
to understand where we live in the
galaxy.
Imagine yourself being plopped down
on a street corner in an unfamiliar
city, and being told to map the entire
city.
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Here’s our galactic street corner.
Where do we live in the galaxy?
You’d only be able to clearly map the
area right around you.
You might be able to guess at some
other streets if you could see light
poles, trees, or rooftops.
The other side of town would remain
unknown.
But if you could go 1000’ up in a
helicopter, it would be much easier.
From up above, we can see the layout.
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It’s hard for us to know the layout of
our own galaxy, because we live in
the plane of the galaxy, not above it.
We have to find out indirectly…
– by looking at other similar galaxies…
– by looking at globular clusters…
– by mapping the positions of bright
sources (nebulae, bright stars.)
The Andromeda Galaxy is similar to ours.
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We believe that our solar system is
about 2/3 out from the center toward
the edge of the galaxy. Here’s why:
Our galaxy is surrounded by a halo of
evenly-distributed globular star
clusters.
A globular cluster from a nearby
planet.
If we lived near the center of our galaxy,
we’d see globular star clusters evenly
distributed everywhere in the sky…
If we live nearer the edge of the galaxy,
globular star clusters won’t be evenly
distributed – this is what we actually see.
How fast are we moving?
By comparing how our position changes
relative to the average speed of the
galaxy’s globular clusters, we know that
the sun moves at a speed of 220
km/second around the galaxy’s center.
The sun takes 1000 years to move 1 light
year, or it takes 280 million years to orbit
the nucleus of the galaxy once.
The galaxy has only made about 50
complete rotations since the universe
began!
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Here’s a problem: by looking at the
visible mass in the galaxy, and the
way that the galaxy rotates, we can
predict how fast the sun should move
through space, revolving around the
center of the galaxy.
It moves 220 km/sec, but it should
only move at 160 km/sec.
So what does this mean?
The galaxy rotates so fast, that it
should fly apart. (There’s not enough
visible mass to hold it together.)
The way that the galaxy rotates tells
us where the galaxy’s mass is
located.
If it rotated like a
rigid object, (a
spinning DVD),
where most of the
mass is located in
the outer half of
the disk, then the
outer edges would
move faster than
the center.
If the galaxy was
like the solar
system, where
most of the mass
is in the center,
the edges would
rotate much slower
than the center.
Eventually, the galaxy’s arms would wind
up, like a watch spring twisted too tight.
However, the
galaxy rotates at
just about the
same speed
everywhere.
This means that
the mass is evenly
distributed inside
and outside the
sun’s orbit.
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Dark Matter
Since more than half of the visible
(bright) mass is inside the sun’s orbit,
this means is that there is a vast
amount of dark matter in our galaxy,
about 600 billion sun’s worth.
We can’t see this dark matter. We
don’t even know what it is, but most
of it is found in the galaxy’s halo.
There are >30 candidates for
the identity of the dark matter.
black holes
dust
neutrinos
tiny red dwarf stars
types of elementary particles that we
don’t yet know anything about
dark energy – a strange repulsive
“anti-gravity”