Transcript Spiral Arms - Harry Kroto

Harry Kroto 2004
ifa.hawaii.edu
Harry Kroto 2004
http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/pics/wind3.gif
Harry Kroto 2004
Harry Kroto 2004
Interstellar gas and dust
moves towards the
gravitational wave region
Harry Kroto 2004
The gas and dust clouds
compressed and localised very
high density regions develop
Interstellar gas and dust
moves towards the
gravitational wave region
Harry Kroto 2004
Stars are born in the very high
density regions and the hot blue
stars burn up before leaving the
arm
The gas and dust clouds
compressed and localised very
high density regions develop
Interstellar gas and dust
moves towards the
gravitational wave region
Harry Kroto 2004
Harry Kroto 2004
Harry Kroto 2004
Harry Kroto 2004
image at: www.nicolascretton.ch/.../Disks/Disks.html
Harry Kroto 2004
Stars are born in the v high density regions
gas and dust cloud compreesed
and localised v high denstry
regions develop
Interstellar gas and dust
moves towrds the
gravitationl wave region
image at: www.nicolascretton.ch/.../Disks/Disks.html
Harry Kroto 2004
image at: www.nicolascretton.ch/.../Disks/Disks.html
Harry Kroto 2004
Harry Kroto 2004
Harry Kroto 2004
Harry Kroto 2004
Spiral Arms
In the disk the density of stars in the spiral arms in only about 5% greater than the density of stars in the rest of the disk. So why are they so
much brighter? Newborn clusters are all in the spiral arms. These clusters contain the most luminous and blue stars. So even though the density
contrast is not great the luminosity contrast is. O and B stars are as much as 105L .
What are the spiral arms? Are they physical arms?
Winding Problem: Imagine that the arms are physical and we start them out in a line. Now we watch them wrap into a spiral due to the fact that
the inner parts will revolve around the Galactic center faster than the outer parts. We can see that a nice spiral forms very quickly. The problem is
that the spiral continues wind up tighter and tigher. In only a few revolutions the spiral gets extremely wound up. At its distance from the Galactic
Center the Sun has gone around the Galaxy around 18 times. In so many turns the spiral pattern would be extremely wound up. This is not the
case and such tight spirals arms are not observed in other galaxies.
Density Waves: We now think that the spiral arms are not physical at all but are rather a pattern of overdensity rotating around the Galaxy. Such
a pattern of overdensity is called a Density wave.
The concept of a density wave is very simple. Here's an analogy. Imagine that you are in a traffic helicopter hovering over a multilane freeway at
night. Ahead you see that there is a truck moving slow in the far right lane. The cars behind it are slowing down ad bunching up behind it. The cars
try to switch lanes to get around the truck. As they do they interupt the flow of traffic in the adjacent lanes as well. The result is a region around
the truck of more cars bunched up together going slower than average. You can see this in the helicopter as the region where the tail lights all
brighten as the drivers apply their brakes and they are squished closer together. The individual cars move through this bunched up region and new
cars move in and take their place. The bunched up region persists. It moves along at about the speed of the slow truck. The bunched up region is
an overdensity in cars and it is a density wave.
The spiral arms are regions where stars' and gas clouds' orbits bunch up closer to one another and the region becomes overdense. Stars go in and
move out of the pattern, but the pattern persists and moves at its own rate.
Since the region is overdense when gas clouds enter it they are compressed and begin to collapse gravitationally. This causes star formation to
occur. New stars are born in the Spiral Arms. The new star clusters contain very luminous O and B stars.
The O and B stars don't live for very long. The cluster might form at one edge of the spiral arm and then exit the other edge a few million years
later. But that's all the longer the O and B stars live. They die before ever leaving the region of their birth. They die in supernova explosions. This is
why massive star supernovae are only seen in spiral arms of galaxies.
The other stars move out into the rest of the disk and over their lifetimes move in and out of other spiral arms.
sl.berkeley.edu/bmendez/ay10/2002/notes/pics/wind3.gif
Harry Kroto 2004
Harry Kroto 2004
Spiral Arms
In the disk the density of stars in the spiral arms in only about
5% greater than the density of stars in the rest of the disk. So
why are they so much brighter? Newborn clusters are all in the
spiral arms. These clusters contain the most luminous and blue
stars. So even though the density contrast is not great the
luminosity contrast is. O and B stars are as much as 105L .
What are the spiral arms? Are they physical arms?
Winding Problem: Imagine that the arms are physical and
we start them out in a line. Now we watch them wrap into a
spiral due to the fact that the inner parts will revolve around
the Galactic center faster than the outer parts. We can see that
a nice spiral forms very quickly. The problem is that the spiral
continues wind up tighter and tigher. In only a few revolutions
the spiral gets extremely wound up. At its distance from the
Galactic Center the Sun has gone around the Galaxy around 18
times. In so many turns the spiral pattern would be extremely
wound up. This is not the case and such tight spirals arms are
not observed in other galaxies. \
http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/pics/wind3.gif
Harry Kroto 2004
image at: www.nicolascretton.ch/.../Disks/Disks.html
Harry Kroto 2004
Harry Kroto 2004
Spiral Arms
Density Waves: We now think that the spiral arms are not physical at
all but are rather a pattern of overdensity rotating around the Galaxy.
Such a pattern of overdensity is called a Density wave. The concept of
a density wave is very simple. Here's an analogy. Imagine that you are
in a traffic helicopter hovering over a multilane freeway at night. Ahead
you see that there is a truck moving slow in the far right lane. The cars
behind it are slowing down ad bunching up behind it. The cars try to
switch lanes to get around the truck. As they do they interupt the flow
of traffic in the adjacent lanes as well. The result is a region around the
truck of more cars bunched up together going slower than average.
You can see this in the helicopter as the region where the tail lights all
brighten as the drivers apply their brakes and they are squished closer
together. The individual cars move through this bunched up region and
new cars move in and take their place. The bunched up region persists.
sl.berkeley.edu/bmendez/ay10/2002/notes/pics/wind3.gif
It moves along at about the speed of the slow truck. The bunched up
region is an overdensity in cars and it is a density wave. The spiral
arms are regions where stars' and gas clouds' orbits bunch up closer to
one another and the region becomes overdense. Stars go in and move
out of the pattern, but the pattern persists and moves at its own rate.
Harry Kroto 2004
Spiral Arms
ISince the region is overdense when gas clouds enter it they
are compressed and begin to collapse gravitationally. This
causes star formation to occur. New stars are born in the Spiral
Arms. The new star clusters contain very luminous O and B
stars.
The O and B stars don't live for very long. The cluster might
form at one edge of the spiral arm and then exit the other
edge a few million years later. But that's all the longer the O
and B stars live. They die before ever leaving the region of
their birth. They die in supernova explosions. This is why
massive star supernovae are only seen in spiral arms of
galaxies.
The other stars move out into the rest of the disk and over
their lifetimes move in and out of other spiral arms.
Spiral arms are only a pattern. Stars and gas clouds move in
and out of them, and they are the locations of new star
formation.
http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/pics/wind3.gif
Harry Kroto 2004
Harry Kroto 2004
Harry Kroto 2004
Harry Kroto 2004
Harry Kroto 2004
Spiral Arms
In the disk the density of stars in the spiral arms in only about 5% greater than the density of stars in
the rest of the disk. So why are they so much brighter? Newborn clusters are all in the spiral arms. These
clusters contain the most luminous and blue stars. So even though the density contrast is not great the
luminosity contrast is. O and B stars are as much as 105L .
What are the spiral arms? Are they physical arms?
Winding Problem: Imagine that the arms are physical and we start them out in a line. Now we watch
them wrap into a spiral due to the fact that the inner parts will revolve around the Galactic center faster
than the outer parts. We can see that a nice spiral forms very quickly. The problem is that the spiral
continues wind up tighter and tigher. In only a few revolutions the spiral gets extremely wound up. At its
distance from the Galactic Center the Sun has gone around the Galaxy around 18 times. In so many
turns the spiral pattern would be extremely wound up. This is not the case and such tight spirals arms
are not observed in other galaxies.
Density Waves: We now think that the spiral arms are not physical at all but are rather a pattern of
overdensity rotating around the Galaxy. Such a pattern of overdensity is called a Density wave.
The concept of a density wave is very simple. Here's an analogy. Imagine that you are in a traffic
helicopter hovering over a multilane freeway at night. Ahead you see that there is a truck moving slow in
the far right lane. The cars behind it are slowing down ad bunching up behind it. The cars try to switch
lanes to get around the truck. As they do they interupt the flow of traffic in the adjacent lanes as well.
The result is a region around the truck of more cars bunched up together going slower than average. You
can see this in the helicopter as the region where the tail lights all brighten as the drivers apply their
brakes and they are squished closer together. The individual cars move through this bunched up region
and new cars move in and take their place. The bunched up region persists. It moves along at about the
speed of the slow truck. The bunched up region is an overdensity in cars and it is a density wave.
The spiral arms are regions where stars' and gas clouds' orbits bunch up closer to one another and the
region becomes overdense. Stars go in and move out of the pattern, but the pattern persists and moves
at its own rate.
Since the region is overdense when gas clouds enter it they are compressed and begin to collapse
gravitationally. This causes star formation to occur. New stars are born in the Spiral Arms. The new star
clusters contain very luminous O and B stars.
The O and B stars don't live for very long. The cluster might form at one edge of the spiral arm and then
exit the other edge a few million years later. But that's all the longer the O and B stars live. They die
before ever leaving the region of their birth. They die in supernova explosions. This is why massive star
supernovae are only seen in spiral arms of galaxies.
The other stars move out into the rest of the disk and over their lifetimes move in and out of other spiral
arms.
Spiral arms are only a pattern. Stars and gas clouds move in and out of them, and they are the
sl.berkeley.edu/bmendez/ay10/2002/notes/pics/wind3.gif
Harry Kroto 2004
Galactic Rotation and Dark Matter
The Galaxy is expected to rotate differentially. Different speed
in orbit for different distance from the center of the Galaxy.
Inner parts orbit faster than outer parts. If orbits of stars are
Keplerian then we can measure the mass interior to the orbit
of a star from its speed.
P2 = 4 2/G(m1 + m2) * a3
Now, we replace a with the radius of circular orbits, r, and
since the stars are far less massive than the Galaxy we need
only concern ourselves with the mass interior to the orbit,
Mint. Thus Kepler's 3rd law now reads
P2 = (4 2/GMint)*r3
Now, we can write the orbital velocity, v, in terms of the
period, P, by recognizing that the distance traveled by the star
during one period at speed v is given by: d = vt. t is the
period, so t = P. d is the circumference of the circular orbit, so
d = 2 r. Thus we have 2 r = vP and putting it in terms of the
period: P = 2 r/v. \
Harry Kroto 2004
Galactic Rotation and Dark Matter
Now we substitute the period into the expression for Kepler's
3rd law and get
4 2r2/v2 = (4 2/GMint)*r3
Which simplifies to: v2 = GMint/r
We can write the result in two different ways to examine the
mass of the Galaxy.
v = (GMint/r)1/2
Mint = v2r/G
We can use the Doppler shift of radio emission from hydrogen
gas in the disk of the galaxy to measure the rotation curve:
the orbital velocity of the gas in the disk at a given radius. At
the distance of the Sun we find that there should be Mint =
1011M . If the average star has 0.5M then there are about
200 billion stars within the Sun's orbit.
Rotation Curves: Measuring the rotation curve tells you how
the matter is distributed in the Galaxy.
Harry Kroto 2004
Galactic Rotation and Dark Matter
Rotation Curves: Measuring the rotation curve tells you how the matter is
distributed in the Galaxy.
A solid body rotates with a straight line. Velocity increases with distance.
The Solar System follows a fall off of velocity with distance following
Kepler's 3rd Law because it is centrally condensed. The rotation of curve
of the Galaxy is initially straight (Bulge rotates like a solid body), then
Keplerian for a short distance, then it flattens off at large radius.
The fact that the rotation curve is approximately flat out to very large
radii implies that the mass of the Galaxy grows linearly with distance
beyond the Sun's orbit... Yet we do not see enough stars out there to
account for all of this mass. The mass must therefore be mostly invisible -> Dark Matter
What is it? Black Holes? Neutrinos? Brown Dwarfs? Exotic Particles? Jello
Pudding Pops?
MACHOs: MAssive Compact Halo Objects
"Jupiters", Brown Dwarfs, Red Dwarfs (M and L stars), White Dwarfs,
Neutron Stars, Black Holes. Look for these using gravitational lensing of
light of stars in LMC.
WIMPs: Weakly Interacting Massive Particles
Neutrinos, Axions, ?
All other spiral galaxies also have flat rotati
Harry Kroto 2004