- 3130 - MILKY WAY - mysteries? The Dark Matter halo far outside and surrounding the Galaxy is the greatest mystery. We know the Dark Matter is there because we see the effect of its gravity. We see a constant orbital velocity of all the stars with ever increasing radius. Rotational velocity should fall off with the square root of velocity, if the mass is inside the radius.
- ----------------------- 3130 - MILKY WAY - mysteries?
- Review 3129 covered the basics of what we think we know about our galaxy. Now we will highlight some of what we don’t know, enter the mysterious.
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- The Spiral Arms look like a swirling mass of stars in a spin wheel. But, actually the arms are almost stationary structures. We learned that the Sun’s orbit is 200,000,000 years at a velocity of 220,000 meters/sec, 490,000 miles per hour. I’ll bet you did not know you were moving that fast!
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- The stars halfway to the center of the Galaxy have about the same velocity, 200,000 m/sec. and have a period of orbit half as long. It the spiral pattern were moving with the stars the pattern would have wound up over 50 times over the last 10 billion years. Instead the spirals have only wound up 3 or 4 times, at most. the arms are not spiraling but are almost stationary.
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- The Spiral Arms are Density Waves through which groups of stars flow in and out . The best way to visualize this is to be in an airplane looking down at Bay Area traffic flow. The traffic jams are areas of high density through which the cars pass at slower speeds. The traffic jam appears stationary, or barely moves forward or backwards, and the speed of the jam wave is unrelated to the speed of the cars going through it.
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- The Bulge at the center of the Galaxy is a sphere with a high density of stars out to 3,000 lightyears. A less dense spheroid of stars and gas are a halo out to 163,000 lightyears. There are some 200 globular clusters of stars in the halo. The stars in these clusters are old stars because the ratio of heavy iron to hydrogen to our Sun’s ratio is 1/100,000. Younger 3rd and 4th generation stars have a higher ration of heavy elements.
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- The center of the Bulge has recently opened up to astronomers with new technology in Infrared, Radio, X-ray and Gamma Ray telescopes. Visible light telescopes see mostly the dust that is between us and the center.
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- By tracking stars orbiting the center astronomers calculate the mass to be 4,000,000 Solar Mass within a radius of less than 45 Astronomical Units. Pluto is out 40 AU so this is like having 4 million Suns between our Sun and Pluto. That would be some oven we would be in. This much mass in this little space by deduction leads to the center being a super massive Black Hole.
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- Cosmic Rays are ionized sub-atomic particles, electrons, protons and heavier nuclei. These are all ionized meaning they carry an electrical charge. They appear to be accelerating across the Galaxy by magnetic lines of force. They appear to originate from supernovae explosions.
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- Cosmic Rays reach enormous energies before they plow into our upper atmosphere. How these particles reach these high energies is a mystery.
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- The Dark Matter halo far outside and surrounding the Galaxy is the greatest mystery. We know the Dark Matter is there because we see the effect of its gravity. We see a constant orbital velocity of all the stars with ever increasing radius. Rotational velocity should fall off with the square root of velocity, if the mass is inside the radius.
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----------------- Mass = (velocity)2 * radius / G
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- If the velocity with respect to radius is constant, then the mass must be proportional to radius. The further from the center the more mass exists. However, what we see is the opposite. The Bulge is the biggest mass we see at the center and the Disk and Spiral Arms fade out to nothing as we get 50,000 lightyears from the center. The Galaxy does not obey this formula. The nine planets orbiting the Sun obey this formula exactly with Mercury having the fastest velocity and Neptune having the slowest velocity.
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- The Galaxy’s flat rotational curve extends out to the furthest stars 100,000 lightyears radius. The Disk we see is only out to 50,000 lightyears radius. Therefore the total mass is 10 times larger. 80 to 90% of the Galaxy mass must be in unseeable Dark Matter. 1,000,000,000,000 Solar Mass is Dark! The luminous part of the Galaxy that we see is just the tip of the iceberg.
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- What could Dark Matter possibly be?
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----------- Interstellar gas? no, because we could detect its radiation.
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----------- Dust? no, it would emit infrared radiation that we could detect.
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----------- Gravitationally bound star-like objects? Neutron stars, no. Black Holes, no. White Dwarfs, maybe but doubtful. Brown Dwarfs, no because we could detect them with gravitational lensing.
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--------- Elementary atomic particles? Electrons or protons, no. Neutrons, no, they would decay in 15 minutes. Massive neutrinos, possibly. Cold Dark Matter consisting of heavy particles not yet discovered, very possibly. Radiation detection and gravitational lens testing have ruled out everything except Cold Dark Matter, or something else we have not yet imagined.
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- There are still a lot of mysteries out there to be solved in our home in the Milky Way.
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- April 20, 2021 MILKY WAY - mysteries? 867 3130
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--------------------- --- Wednesday, April 21, 2021 ---------------------------
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