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The nearest stars to the Sun.
Barnard s Star
Brightest stars in the sky
From the two H-R diagrams what can you conclude about the types of stars in the Galaxy? 1. Most stars must be low mass because that is what we find near us 2. Most stars must be high mass because throughout the galaxy we mostly see luminous stars 3. If we combine the two H-R diagrams there are about the same number of high and low mass stars in the Galaxy
We see a lot of high mass or luminous evolved stars in the sky. This is because they have extremely large luminosity. They can be readily seen at great distances. The stars in our little volume of the Galaxy are almost completely, low mass stars. This means if we increased the volume that we are using to search for stars, we would start to bring in a few high mass stars, but the number of low mass stars would skyrocket. Most stars in the Galaxy are low-mass, red dwarfs. ~90% of all stars.
Why are there so many little red stars in the Galaxy? 1. Little low mass stars live much longer than high mass stars 2. Star forming regions make more low mass stars than high mass 3. It is the combination of 1 & 2
In young stars clusters, there are typically 5 10 really massive, luminous stars. But there might be several hundred or more low mass stars. The massive stars die rapidly. The really small stars (less massive than the Sun) have never died since the start of the universe. Both are important.
The stellar populations throughout the Galaxy We have discussed three components of a disk galaxy. The disk, the bulge, and the halo. We have also discussed the types of stars we find in the disk of the Milky Way. There are very luminous main-sequence stars (O & B) and there are giants and super giants (I & III) these are in the minority. The greatest number of stars are the cool, red dwarf stars.
Let s look at a few color images of spiral-disk galaxies. For the images that follow, I want you to write down the color you see for the disk of the galaxy, and the color for the bulge of the galaxy.
In the survey of stars in the disk of the Milky Way we find that 90% of the stars are low luminosity red stars. Fewer than 1% are high mass, high luminosity, O & B type stars. How can this be explained from the images of disks of galaxies that you have just viewed?
How can this be explained from the images of disks of galaxies that you have just viewed? 1. Our Galaxy is not typical. Most galaxies have many more blue stars than red stars. 2. When even a small percentage of blue stars are present, their light dominates all other stars. 3. The galaxies I showed you all had spiral arms. The Milky Way must not have spiral arms.
Only about 1 in 100 stars in the disk are high mass, blue stars. But those stars have luminosities that are 10,000 to several hundred thousand times the luminosity of the sun. They are a million times as luminous as a red dwarf star. Given this, just a small percentage of high mass, blue stars will completely dominate the light coming from the disk. What about the galactic bulges? How can we explain those?
What about the galactic bulges? How can we explain their color? 1. The bulges are very blue because an enormous amount of star formation is occurring there. 2. The bulges are orange because there is no star formation there. 3. The high-mass blue stars orbit in the disk and the red stars fall to the center of the galaxy.
The orangish-red color suggests that there are extremely few, high mass, blue stars in the bulge. The light is mostly coming from redder stars. When we look at distant galaxies we are not seeing the light from individual stars. Instead, we see the integrated glow of all the stars. Look at the two H-R diagrams.
Which of the two H-R diagrams for star clusters do you expect would look more like the disk and more like the bulge, in integrated light?
So there is very little star formation occurring in the bulge of disk galaxies. We know that high mass stars live very short lives. Since there are high mass stars in the disk, there must be active star formation occurring in the disk. The primary mechanism for creating stars is the collision of large molecular clouds with the spiral density waves in the galaxy disks. But where do the spiral density waves come from?
Interactions between galaxies can create spiral density waves. But what is the physical mechanism? Let s look at the effect of tidal interactions.
The Earth-moon system.
As the moon orbits the Earth, the Earth slowly wobbles from the force exerted on it by the moon. We know that Newton s Law of gravity is: F = Gm 1 m 2 /r 2 But we have slightly different r for the oceans compared to the rocky center of the Earth.
The water nearer the Moon feels a greater pull of gravity, because it is slightly closer. The water on the far side of the Earth feels less gravity from the moon than the center of the earth.
So, as the Earth wobbles due to the gravitational effect of the Moon, the ocean water closer to the Moon is pulled upward, causing a high tide. The water on the far-side of the Earth is also bulged outward, because of the smaller force of gravity. That water is slightly left behind, as the rocky Earth wobbles. The result is two bulges of water on opposite sides of the Earth. These are the two high tides. The water that is at 90 degrees from the moon has to fill in the bulges, and there is low tide there.
The water nearer the Moon feels a greater pull of gravity, because it is slightly closer. The water on the far side of the Earth feels less gravity from the moon than the center of the earth.
As the bulges align with the position of the Moon. So as the Earth spins, the rocky part passes in and out of the bulges. That is why at the ocean shore you witness two high tides and two low tides every day. When the Moon is high in the sky, or when the Moon is directly under your feet. There will be a high tide on the ocean coast.
The Sun also effects tides on the Earth, but it only has 25% of the effect of the Moon. Sometimes the Moon and Sun work together to create even bigger tides than usual ( spring-tides ). And sometime they work against each other and the tides are smaller than normal ( neap-tides ). What phases of the Moon will give spring tides and which will give neap-tides?
Which Moon phases give spring-tides and which give neap-tides? 1. Full moon neap and New moon -spring 2. 1 st /3 rd Quarter neap, new/full - spring 3. 1 st /3 rd Quarter spring, new/full - neap 4. The phases of the moon have no influence on the tides. But the seasons do. Biggest tides are during spring time.
Spring tides occur during new/full moon and neap tides during the 1 st /3 rd Quarter Moon
What about galaxies orbiting around each other? Galaxies are huge, but they are not solid like the Earth. They are made up of individual stars, gas, and dust. What happens when a galaxy gets to close to another galaxy?
The result will be to stretch the galaxies. The stars, gas and dust on the near side will feel a greater pull than the center of the galaxies, and the far sides will feel less pull than the center of the galaxies.
The result is stars, gas and dust are pulled into a tidal stream. This stream has a higher density of gas and dust than the other regions. After the interaction the density wave doesn t move around the galaxy. Instead, stars and molecular clouds that are still in orbit around the galaxy s center, pass in and out of the density wave. Think of an accident that occurs on the highway. 30 minutes after the accident is cleared and the traffic begins to flow, there is still a remnant of the accident.
What you witness is traveling down the highway with big spaces in between the cars and trucks. All of a sudden you realize that there are trucks and cars all around you. Then a little bit later, everything is spread out again. The accident created a density wave. Even after the accident is removed, the effect is to move into the high density wave and back out of it again. The wave endures because there is always traffic moving in and out of the over-density. And this density wave doesn t move. It stays centered on the location of the original accident.
In the original interaction, star formation can be set off in the density wave because the gas and dust are compressed. After the interaction, there is continued star formation along the density wave as molecular clouds collide with the density wave and become compressed.
When interactions are not in the plane of the large galaxy, then warps in the disk ocurr.
The result of the creation of spiral density waves. After the interaction, there will be increased star formation for quite some time, as molecular clouds continue to collide with the spiral density waves. Over time, self-sustained star formation and disruption of the density wave, causes the S- shape density wave to fragment into spurs. Then there are multiple density wave fragments.
Over time, after several orbits of material, the spiral density wave is lost. The result is a slowing of star formation rates. Eventually, the disk galaxy has virtually no recognizable spiral pattern at all.
Quiz #10 What would really happen to you if you were to fall all the way into the Event Horizon of a black hole? We know the gravity is unbelievably strong. What would happen to your body and why? (Note: I am not looking for a relativity answer here. Just a gravity answer)