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Journal 3/28/17
What do you think is beyond the Orion Spur on our map?
Take a guess.
Objective
Tonight’s Homework
To learn about the shape and
structure of the Milky Way
Read section 23.3
Do p 600: 9, 10
Milky Way
There’s a more complete view of our local Orion Spur.
Milky Way
Since we live in the Milky Way, we only see part of it
edge on and it becomes hard to tell its structure. From
Earth, we see it only as a bright band. Even so, we can
see a few features:
Milky Way
Since we live in the Milky Way, we only see part of it
edge on and it becomes hard to tell its structure. From
Earth, we see it only as a bright band. Even so, we can
see a few features:
spiral arm
dust lanes
nuclear bulge
Milky Way
If we could fly above the galaxy, we’d get a view more like
this (make a rough sketch):
Milky Way
If we could fly above the galaxy, we’d get a view more like
this (make a rough sketch):
nuclear bulge: the bulge is
composed of old population II
stars. Recent evidence has led
astronomers to believe that
our bulge is shaped like a bar.
Milky Way
If we could fly above the galaxy, we’d get a view more like
this (make a rough sketch):
nuclear bulge: the bulge is
composed of old population II
stars. Recent evidence has led
astronomers to believe that
our bulge is shaped like a bar.
spiral arms: the arms are full
of young population I stars
and star forming dust clouds.
Milky Way
If we could fly above the galaxy, we’d get a view more like
this (make a rough sketch):
nuclear bulge: the bulge is
composed of old population II
stars. Recent evidence has led
astronomers to believe that
our bulge is shaped like a bar.
spiral arms: the arms are full
of young population I stars
and star forming dust clouds.
The Orion Spur: this is our
local section of the galaxy. A
small fraction of an arm sitting
between two larger arms.
99% of the stars we see in the
sky are in this spur.
Milky Way
The Milky Way also has a “halo”. A rough sphere
around it of globular clusters and occasional
stars. While these clusters don’t make up even a
small percent of the galaxy, we have to account
for them.
Milky Way
One of the weird things about the Milky Way is
how it spins with its spiral arms. Most things that
spin in this kind of way eventually get all “wound
up” because the middle spins faster than the
outside edge. But our galaxy hasn’t become
wound up
for some
reason.
In fact,
pretty
much no
galaxies
seem to
have
wound up.
Milky Way
But the solution to this is even more confusing.
If we make a graph of how fast stars in the arms
move compared to distance, we get line “B”. The
fact that all the stars go the same speed
explains why the spiral arms don’t twist up, but
it doesn’t make sense. According to physics, the
speeds should be following line “A”.
Something is
making the
stars move too
fast and we
don’t know
what it is!
Milky Way
The only thing we can think of is that there’s a
ton of mass beyond the edge of the galaxy that’s
pulling on everything and keeping it moving
faster than normal.
Since we don’t see anything like this, we call this
stuff “dark matter”.
At present, we think dark matter is mostly tons
of subatomic particles. Maybe planets that don’t
have stars, so they look dark. Stuff like that.
Milky Way
It’s hard to estimate the age of the galaxy, but
our best estimates are around 9 billion years. To
compare, our estimates for the Universe are that
it is around 13 billion years old. So the Milky
Way has been around for most of the life of the
Universe.
Milky Way
The Milky Way is the largest galaxy nearby. It
has a number of smaller satellite galaxies that
orbit around it, like moons around a planet.
ursa minor
sagittarius dwarf
magellanic clouds
sculptor
Milky Way
But this isn’t peaceful. Nearly all our satellite
galaxies were almost certainly once galaxies of
their own that got torn apart by the Milky Way.
The Large and Small Magellanic clouds in
particular likely used to be decent galaxies. All
that’s left, however, are wispy collections of
stars.
Worktime
You have the rest of class to finish your map of the local
galaxy.
Exit Question
What kinds of stars do we find in the middle area of
the galaxy?
Old ones
New ones
Red ones
Blue ones
All of the above
None of the above