- 3129 - - MILKY WAY GALAXY - When astronomers add up all the ordinary matter detectable around us, such as in galaxies, stars and planets, they find only half the amount expected to exist, based on predictions. This normal matter is "baryonic," which means it's made up of baryon particles such as protons and neutrons.
----------------------- 3129 - MILKY WAY GALAXY -
- Half of this “ordinary matter” in our galaxy is too dark to be detected by even the most powerful telescopes. It takes the form of cold, dark clumps of gas. In this dark gas is the Milky Way's "missing" baryonic matter.
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- Stars twinkle because of turbulence in our atmosphere. When their light reaches Earth, it gets bent as it bounces through different layers of the atmosphere. Rarely, galaxies can twinkle too, due to the turbulence of gas in the Milky Way. We see this twinkling because of the luminous cores of distant galaxies named "quasars."
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- Astronomers can use quasars a bit like backlights, to reveal the presence of clumps of gas around us that would otherwise be impossible to see.
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- The Australian Square Kilometre Array Pathfinder (ASKAP) is a highly sensitive telescope that can view an area about the size of the Southern Cross and detect tens of thousands of distant galaxies, including quasars, in a single observation.
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- Of the 30,000 galaxies we could see, six were twinkling strongly. Surprisingly, five of these were arranged in a long, thin straight line. Analysis showed that ADKAP had captured an invisible clump of gas between us and the galaxies. As light from the galaxies passed through the gas cloud, they appeared to twinkle.
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- The cloud of gas we detected was inside the Milky Way, about 10 light-years away from Earth. That means light from those twinkling galaxies traveled billions of light-years towards Earth, only to be disrupted by the cloud during the last ten years of its journey.
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- A hydrogen "snow cloud" was disrupted and stretched out by gravitational forces from a nearby star, turning into a long thin gas cloud. Snow clouds have only been studied as theoretical possibilities and are almost impossible to detect. But they would be so cold that droplets of hydrogen gas within them could freeze solid.
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- In many ways our Milky Way is harder to study than the Andromeda Galaxy that is
2,500,000 lightyears away. We can see the Andromeda, even with the naked eye. We are living inside the Milky Way , an inside-out perspective.
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- The intergalactic dust hides much of our internal view from us. However, the Sloan Digital Sky Survey that is measuring the motion and chemical composition of 250,000 stars has already made some amazing discoveries with only 20,000 stars surveyed so far.
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- Our Milky Way was made from the collisions of hundreds of smaller galaxies. How did astronomers reached this conclusion? Much of this we learned be studying our neighbor galaxy, Andromeda.
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- The Milky Way is one of the larger galaxies. It contains about 2*10^11 stars, that is 200,000,000,000 that are orbiting in this spiral galaxy. The spiral has a stellar disk that is 100,000 lightyears across.
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- It has a stellar bulge, or spheroid in the center that holds a super massive Black Hole. It has an extended spheroid halo of gas, dust, and globular clusters of stars. And, extended farther it has a spheroid halo of Dark Matter that we cannot see.
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-------------- Center - a super massive Black Hole
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-------------- Bulge - spheroid
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-------------- Disk - 50,000 LY radius, and 1,000 LY thickness
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-------------- Inner Star Halo
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-------------- Outer Star Halo
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-------------- Outer Dark Matter halo.
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- The Survey discoveries tell us the inner halo formed first from collision of smaller but massive galaxies that rotated in the same direction as our Galaxy. The outer halo of stars formed later from smaller galaxies orbiting in the reverse direction of our Galaxy.
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- Our Sun is orbiting around the center of the Galaxy at 493,000 miles per hour. It is about half way out the Disk. Outside the Disk is the halo of stars that was once considered a single structure, but is now certain to be divided into two broadly overlapping, but different structures.
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- The inner halo located well outside the Disk is made up of of stars with heavy elements, such as iron and calcium. These stars are 2nd, 3rd, 4th generation stars made from the interstellar gas from supernovae explosions. These stars are orbiting in the same direction but more slowly, at 50,000 miles per hour.
- The outer halo still further outside the Disk is orbiting in the opposite direction at 100,000 miles per hour. The outer halo of stars do not contain the heavier elements indicating they are 1st or 2nd generation stars. These stars definitely came from different galaxies that evolved in a different location in the Universe. They merged with the Milky Way at different times than the inner halo stars.
- The Sun is located 26,000 lightyears from the center Black Hole. That is about halfway out the Disk. Since we cannot see the center, due to the intergalactic dust, astronomers had to determine this by measuring distances and motions of globular clusters in the halo and calculating where the center of their distribution would be.
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-------------- The orbital velocity of the Sun around the center is 492,000 miles per hour.
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-------------- The orbital period of the Sun around the center is 2*pi*r / velocity
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-------------- Period = 6.28*26,000 LY / 2.2*10^5 m / second
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-------------- where: lightyear = LY = 9.46*10^15 meters
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-------------- where: year = 3.16*10^7 seconds ( note that 3.16 is pi)
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--------------- Period = 220,000,000 years.
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- So, the last time we got around our orbit of the Milky Way the dinosaur era was just starting to walk the Earth.
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- The mass inside the radius of the Sun:
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---------- Force of Gravity = Centripetal Force
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---------- G*M*m / r^2 = m*v^2 / r
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---------- M = r*v^2 / G
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---------- where: r = radius = 26,000 LY
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---------- where: v = velocity = 220,000 meters/second (492,000 mph)
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--------- where: G = force of gravity = 6.67*10^-11 m^3/kg*sec^2
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---------- M = Mass = 18*10^40 kilograms
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--------- where: Ms = Solar Mass = 2*10^30 kilograms
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---------- M = 10^11 Solar Mass
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---------- Mass = 10^11 Stars the size of our sun
- Therefore, there are 10^11 stars inside the Sun’s orbit around the galaxy. Or, astronomers estimate that the average star is 0.5 Solar Mass and half the mass is interstellar gas, not stars, so, it works out that there are still 10^11 stars.
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- 100,000,000,000 stars, roaming around at random velocities of 20,000 meters / second, or 44,700 miles per hour. What are the chances of them colliding into each other?
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- The length of the Galactic Disk is 11,400 lightyears. It is 2000 LY thick near the bulge and tapers off to 1000 LY thick near the edge. The mass of the Disk is 10^10 Solar Mass. The mass of the Galactic Center is 10^6 Solar Mass. About 10% of the Disk is gas and the remainder stars.
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- The average density for the number of stars is 2*10^10 / (volume of the Disk).
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- The volume of the Disk is pi*r^2 * thickness.
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----------- Volume = pi*(11,400 LY)^2 * 2000 LY
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----------- Volume = 8*10^11 LY^3
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----------- where : LY = 9.46*10^15 meters
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----------- Volume = 7*10^59 meters^3
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----------- The Density = 2*10^10 stars / 7*10^59 m^3
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----------- The Density = 3*10^-50 stars / m^3
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----------- Cross-section area of a star = pi*r^2 = pi * (2*7*10^7m)^2 = 6*10^18 m^2
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----------- Distance between stars^3 = 1 / number density
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----------- Distance^3 = meters^3 / 3*10^-50 stars/m
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----------- Distance = (33*10^48)^.333
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----------- Distance = 3.2*10^16 meters
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----------- Distance = 3.4 lightyears, on average
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----------- Our closest star is 4.2 lightyears, Proxima Centauri.
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- Number of collisions = (cross-section area)*(number density)*(distance between collisions)
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- For the first collision, where the Number of collisions = 1, what is the (distance) = dx?
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----------- 1 = (6*10^18 m^2)*(3*10^-50 stars / m^3)*(dx)
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------------ dx = 56*10^29 meters
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------------ dx = 6*10^14 lightyears
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----------- The Milky is only 10^5 lightyears across, so it is very unlikely that two stars will collide if they have to travel 6*10^14 lightyears between collisions. How long would it likely take between collisions?
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- The average random velocity of stars in the Disk is 20,000 m/sec, or , 44,700 miles per hour. Therefore, the average time between collisions is Distance between collisions / random velocity.
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----------- Time between Collisions = 56*10^29 meters / 20*10^3 m/sec = 2.8*10^26 sec. /3.16*10^7 sec/yr = .89*10^19 years
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---------- Time between Collisions = 9*10^18 years.
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The time between collisions is 10^8 times greater than the age of the Universe, however we need to consider the force of gravity pulling the stars together even when they physically will not collide.
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- The force of gravity effectively increases the cross section of the star’s influence. See note (1) for the derivation of this formula, ( 1+Ve^2 / Vran^2):
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----------- Gravity Cross-section = Physical cross-section * ( 1+Ve^2 / Vran^2)
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------------ where: Ve = the escape velocity for Solar Mass = 620,000 m/sec.
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------------ where: Vran = random velocity of 20,000 m/sec.
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------------ ( 1+Ve^2 / Vran^2) = 1+( 6.2*10^5)^2 / (2>10^4)^2 = 1000
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- The effective cross-section due to gravity is 1000 times greater. Repeating the same calculations as above we see the effect of gravity on star collisions:
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- Number of collisions = (cross-section area)*(number density)*(distance between collisions)
----------------- Distance between collisions = dx:
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---------------- 1 = (6*10^21 m^2)*(3*10^-50 stars / m^3)*(dx)
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---------------- dx = 6*10^11 lightyears = 600 billion lightyears , which is still 100 times more distance between average collisions than there is in the Observable Universe.
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---------- Time between Collisions = 56*10^26 meters / 20*10^3 m/sec = 2.8*10^23 sec. /3.16*10^7 sec/yr = .89*10^16 years
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---------- Time between Collisions = 10^16 years, and the Universe is only 10^10 years old
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- The mathematical conclusion is that stars rarely collide. However, in the center of the Galaxy where the density of stars is much, much higher star collisions do occur. The estimate is that overall 1% of the stars are disrupted by a close passing star during their lifetime.
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- There is more to learn about the Milky Way’s spiral arms, density waves, the spheroid bulge, the Black Hole, Cosmic Rays, and the Dark Matter Halo to be saved for the next
review.
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-------- Note (1) The effective cross-section of a star involved in a collision increases when we include the effect of gravity. The factor of increase is (1+ Ve^2 / Vran^2) which we learned was a factor of 1000:
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---------- where: Ve is the escape velocity
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---------- where: Ve^2 = 2*G*M/r = 620,000 m/sec
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---------- Ve^2 = 2*6.7*10^-11*2*10^30 /7*10^8 m^2 / sec^2
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----------- Ve =6.2 *10^5 m/sec
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---------- where: V random = 20,000 m/ sec
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- To derive the formula we use the Conservation of Kinetic Energy, KE and the Conservation of Angular Momentum, AM:
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--------- where: KE = ½ M*v^2
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--------- where: AM = M*v*r
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--------- where: “v” is the orbital velocity at radius “r”
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- We assume two situations. One where the stars pass at a distance, D, and there motion is not affected. Their velocity remains, Vran. The second situation is where the stars are close enough for gravity to pull them together at a maximum velocity of Vmax, just before they collide. As the stars approach the velocities increase and their distance separation decreases but the Angular Momentum must remain the same:
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--------------- M* Vran*D = M*Vmax*r
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---------------- Vmax = Vran * D/r
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- The Kinetic Energy must also be the same for both situations:
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-------------- ½ * M * Vran^2 = ½ * M * Vmax^2 - ½ * M * Ve^2
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-------------- where: Ve is the escape velocity at the surface of the star.
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-------------- Vran^2 = Vmax^2 - Ve^2
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-------------- Vran^2 = (Vran * D/r)^2 - Ve^2
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-------------- (D/r)^2 = (1 + Ve^2/Vran^2)
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-------------- where D^2/r^2 = the ratio of the gravity cross-section area to the physical cross-section area.
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- April 20, 2021 MILKY WAY GALAXY 866 3129
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----- Comments appreciated and Pass it on to whomever is interested. ----
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for copies or Index of all reviews
--- to: ------ jamesdetrick@comcast.net ------ “Jim Detrick” -----------
--------------------- --- Tuesday, April 20, 2021 ---------------------------
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