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--------------------- 2655 - MILKY WAY - our home in the cosmos?
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- We will probably never be able to send a probe outside our galaxy that can take a picture of our beautiful Milky Way. However, we can take pictures of other galaxies, thought to be similar, to imagine what our Milky Way looks like.
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- The Milky Way can be seen on a clear night aching across the sky. The center is off the tea cup spout of the Constellation Sagittarius. We believe that our Milky Way is about 100,000 lightyears across and 2000 lightyears thick. It is a relatively flat disk with a bulge in the middle.
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- There are several hundred billion stars in the Milky Way with the greatest concentration in the center bulge. Using radio wavelengths instead of visible wavelengths we can peer past the dust and see the complex structure of the core of our Milky Way. The core is 2000 lightyears in diameter.
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- The stars at the center are 2,000,000 times the mass of our Sun. They are 1000 AU, apart, only 1000 times the distance from Earth to Sun, 93,000,000,000 miles apart. And, they would likely collide on an average of every 1,000,000 years. An “AU” is an Astronomical Unit which is 93,000,000 miles, the average distance between Earth and he Sun.
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- The first galaxy to have its picture taken was the Needle Galaxy (NGC4565) in Coma Berenices constellation. The picture was taken in 1785 by Sir William Herschel, the same astronomer who discovered the planet Uranus. The Needle Galaxy is about 30,000,000 lightyears away.
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- We are seeing what it looked like that long ago in the past. At that time, 30 million years ago on Earth advanced primates, seals, dolphins, grass, daisies, asters, sunflowers, lettuce, giraffes, bears, hyenas first appeared. It was the Miocene Epoch. Humanoids would not appear for another 15,000,000 years.
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- We believe the southern Pinwheel Galaxy (NGC5236, M83) looks the most similar to our Milky Way. It is 15,000,000 lightyears away. So, we are seeing it as it appeared when humanoids first appeared on the Earth. Our Milky Way has 6 spiral arms and a central bar running through the center where most of the young, bright stars are. The star bar is 27,000 lightyears long.
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- There is a massive molecular ring of gas and dust that rotates around this star cluster at the center. This dust cloud extends from 5 to 25 lightyears out from the center. X-rays and gamma-rays emanating from the center indicate the presence of the annihilation of matter and antimatter positrons. The speed of some 20 stars orbiting close in to the center indicate the presence of a massive black hole there.
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- By studying the orbits of our Sun and other stars about the galactic center, we can calculate the mass of the Milky Way. The stars orbiting close in to the black hole are clocked at 1000 km/second, or 3,000,000 miles per hour. To hold these fast moving stars in their orbit’s the mass at the center would need to be 2,500,000 times the mass of our Sun.
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- The velocity of an object in circular orbit multiplied by itself is equal to the mass at the center times the Gravitational Constant divided by the distance from the center (the radius).
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---------------------------------- (Orbit velocity)^2 = G * M / radius
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- This formula assumes the orbits are perfect circles, which they are not, they are ellipsis, but the formula gives a good approximation.
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------------------------------ Mass = radius * velocity^2 / G
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------------------------------ G = gravitational constant of proportionality. The value of G depends on the units of measurement.
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------------------------------ G = 6.67*10^-11 m^3/(kg*sec^2)
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------------------------------ M = mass = (10^6 m/sec)^2 * 5 lightyears / G
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------------------------------ One lightyear is = 9.46*10^15 meters
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------------------------------ M = 47.3*10^27/ 6.67*10^-11
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------------------------------ M of the Black Hole = 7.09*10^38 kg
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------------------------------ Mass of our Sun = 2*10^30 kg
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------------------------- Mass of the black hole = 3.55*10^8 = 355,000,000 solar masses
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- Our Sun orbits this center, and us too, at 250 km/second, 560,000 miles per hour. It takes our Sun 225,000,000 years to complete one orbit. The Sun is 30,000 lightyears, or about 60%, out from the center. Since the Sun is about 10 billion years old it has completed 44 orbits around the galaxy.
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- Since our Earth is 4.5 billion years old, the Earth has made only 20 laps around the galaxy. Since humanoids have only existed for 15 million years, we humanoids have only gone 7% around the circuit.
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------------------------------ 10,000 million years / 225 million years per orbit = 44 orbits
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- The total time to complete one orbit traveling at 250 km/second would be equal to 2*pi*30,000 lightyears / 250 km/sec.
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----------------Time for one orbit = 2*pi*3*10^4 * 9.46*10^15 m / 250*10^5 m/second
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------------------------------ Time = 71.3*10^14 seconds
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------------------------------ One year = 3.16*10^7 seconds
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--------------------- Time = 71.3*10^14 seconds/ 3.16*10^7 seconds/year = 22.6*10^7
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------------------------------ Time = 226 million years
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To calculate the mass of the galaxy inside the Sun’s orbit we use the same formula for circular velocity:
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------------------------------ Orbit velocity^2 = G * M / radius
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------------------------------ M = (250 km/sec^2)^2 * 30,000 lightyears / G
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------------------------------ M = 6.25*10^21 * 28.38*10^19 / 6.67*10^-11
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------------------------------ M = 26.87*10^40 kg
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------------------------------ Mass of our Sun = 2*10^30 kg
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---------------------- Mass of the galaxy = 1.35*10^11 solar masses inside the Sun’s orbit.
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- Using the same calculations for stars orbiting the far perimeter of the galaxy where the stars are traveling 230 km/sec at distances of 52,200 light years.
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------------------------------ Orbit velocity^2 = G * M / radius
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------------------------------ M = (2.2*10^5 m/sec)^2 * 52,200 LY / 6.67*10^-11
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------------------------------ M = 5.29*10^10 * 5.22*10^4 * 9.46*10^15 / 6.67*10^-11
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------------------------------ M = 3.92*10^41 kg
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------------------------------ M = 1.96*10^11 solar masses
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- Taking a ratio of these two masses we can see that 69% of the galaxy’s total mass lies inside the Sun’s orbit which is 60% of the way out from the center. Obviously, the galaxy is much denser as we move toward the center.
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- Another way to calculate the Mass of the Milky Way is to use Kepler’s formula where the square of the period is proportional to the cube of the radius.
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------------------------------ Galaxy Mass / Sun Mass = (30,000 LY)^3 / (2.5^10^8)^3
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------------------------------ One lightyear = 206,265 AU
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------------------------------ Galaxy Mass = (3*10^4 * 2.06*10^5)^3 / 6.25^10^16
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------------------------------ Galaxy Mass = (6.188*10^9)^3 / 6.25^10^16
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------------------------------ Galaxy Mass = 236.9*10^27 / 6.25^10^16
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------------------------------ Galaxy Mass = .38*10^11 solar masses
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- Our simple calculations have resulted in only 20% of the astronomers’ recognized weight of the galaxy using the most sophisticated techniques available. The Milky Way galaxy is thought to be 1,000,000,000,000 times (10 * 10^11) the mass of our Sun.
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- This total mass is 10 times larger that the sum of all the visible stars we can count. 90% of the galactic mass is unaccounted for and we call it dark matter because we can not see it. When we look into the night sky there is something between all those spots of starlight and we do not know what it is.
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- The Milky Way contains 100,000,000,000 to 1,000,000,000,000 stars. If the average galaxy contains at least 100 billion stars how many stars are there out there?
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- Sixteen years ago NASA’s Hubble telescope took a picture that contained 10,000 galaxies. This was a single long time exposure like looking through a soda straw and counting the galaxies inside the straw.
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- If we could repeat this same picture enough times in every direction we could count the number of stars in the visible Universe. The problem is that we would need another 12,700,000 pictures and that would take Hubble 1,000,000 years to take them all.
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- If we counted all the galaxies in the pictures there would be 127,000,000,000 galaxies. If we multiply by 100 billion stars per galaxy ( The lower estimate for the number of stars in our galaxy) then there are 12,700,000,000,000,000,000,000 stars in the Universe.
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- In a dark night sky we can count only 2000 to 3000 stars with the naked eye. We can only see out to 3000 lightyears distance with the naked eye and every star we see is inside our Milky Way. There are a few visible extragalactic galaxies that are the exceptions such as the M31, Andromeda Galaxy.
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- 2000 out of 12.7 *10^21, you can see how many stars we are missing. It would be easier to count all the grains of sand on all the beaches and in all the deserts in the world.
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- Astronomers have discovered that the larger galaxies are filled with the older stars, 13,000,000,000 years old. The smaller, fainter, and younger (4,000,000,000 years old) galaxies have the hot star formations.
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- The smaller galaxies are about 10% the mass of the larger galaxies. We still do not know how galaxies form and how they evolve. For some reason star formation tends to stop in the older galaxies. Old age brings fewer stars. Funny how that happens! Old slows you down as well.
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- March 8, 2020 584 2655
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--------------------- Sunday, March 8, 2020 --------------------
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