Harry Kroto 2004

ifa.hawaii.edu

Harry Kroto

Harry Kroto 2004

Interstellar gas and dust moves towards the gravitational wave region

Harry Kroto 2004 Interstellar gas and dust moves towards the gravitational wave region The gas and dust clouds compressed and localised very high density regions develop

Harry Kroto 2004 Interstellar gas and dust moves towards the gravitational wave region The gas and dust clouds compressed and localised very high density regions develop Stars are born in the very high density regions and the hot blue stars burn up before leaving the arm

Harry Kroto 2004

image at:

Harry Kroto 2004 image at: Interstellar gas and dust moves towrds the gravitationl wave region gas and dust cloud compreesed and localised v high denstry regions develop Stars are born in the v high density regions

Harry Kroto 2004 image at:

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 locations of new star formation.

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 10 5 L. 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. \

Harry Kroto 2004 image at:

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. 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 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.

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 locations of new star formation.

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 = (GM int /r)1/2 M int = 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 M int = M. 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