May 5th, 2001
Astronomy HWK #4
1. There is evidence that supports the hypothesis that the Milky Way Galaxy has a massive black hole at its core. At the center of our very own galaxy is a mysterious source of energy. Vast amounts of radiation pour from this compact source which may be a Supermassive Black Hole. Astronomers found an intense radio source with strings of other radio sources clustered about it in the direction of the galactic center. The intense source was named Sagittarius A because the center of the galaxy lies in the direction of the constellation Sagittarius. The position of Sagittarius A as has been observed through radio telescopes around the world seems to be very near the dynamical and gravitational center of the galaxy. Based on its high luminosity and radio spectrum Sagittarius A is neither a star nor a pulsar. It has a luminosity of 5 stars but is smaller than our solar system. Also, it can’t be a supernova remnant since it is not expanding. The strongest evidence that it is a Supermassive Black Hole comes from Doppler shift of ionized gas in its vicinity. Their observed speeds, as high as 300 km/s, can be explained as an orbital and spiral motion around the center. Therefore, a source of gravity is required to hold these high-speed gases in orbit. Such a massive object, this compact, can only be a Supermassive Black Hole.
2. On the website, there is much clear evidence of a black hole in the central region of the Virgo cluster galaxy M 87. The first picture shows a spiral shaped hot gas at the core of our active galaxy. The gas is rotating so rapidly that there must be a black hole at the center. The region M 87 fits the description of a black hole. It weights as much as three billion suns, but is concentrated into a space no larger than our solar system. This strong gravitational pull towards the center is clear evidence of a black hole. Also, a jet of high-speed electrons which are emitted from the nucleus are predicted to be produced by the black hole “engine”. In the second picture, they measured the light from the disc and studied when it was red-shifted and blue-shifted. The gas on the side that is spinning away from the earth is moving 1.2 million miles per hour. The gas on the opposite side that is moving towards the earth is moving in the same direction. These high velocities are clear evidence that there is a strong gravitational pull towards the center of our galaxy. Also, since it is a black hole, not even light can escape its extremely strong gravitational pull.
3. On the website, I saw pictures of galaxies colliding. NASA’s Hubble Space Telescope has uncovered two galaxies colliding. It was described as “more than 1,000 bright, young star clusters bursting to life in a brief but intense fireworks show”. The pictures also provide evidence of what would happen if two galaxies were to crash into each other. They support the theory that if two spiral galaxies were to collide into each other, the result is an elliptical galaxy with far more stars that either of the original galaxies and also including many star clusters. The Hubble pictures were only able to see the brightest clusters of stars created by the crash. The photographs also tell scientists what would happen if a collision between our galaxy and the Andromeda galaxy, which is supposed to happen in several billion years and alter both of the systems. The two galaxies are closing in on each other at a speed of 300,000 miles per hour. The Andromeda is 2.2 million light years away and the collision should happen no sooner than 5 billion years in the future. This may be when our sun burns up which at the point would reduce Earth to a lump of rock. The crash also would not occur spontaneously but would take several billion years to run its course. During this processes, molecular hydrogen clouds will compress and explode, forming thousands of new star clusters. The gravitational pull between the two galaxies would cause them to form an elliptical galaxy with many new stars.
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