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next closest proton orbit in order to satisfy then should form. the exclusion principle. Since the work of Chandrasekhar, the Nonetheless, the exclusion principle question of whether or not black holes cannot be an infinite repulsive force. This existed in nature remained controversial was first realized in 1928 by Subrahmanyan until the mid 1990s. Most scientists, Chandrasekhar, at the time a graduate including myself, became convinced of the student at Cambridge University in existence of black holes after seeing precise England. He pointed out that since nothing measurements on the motion of stars and is allowed to move faster than the speed of interstellar gas. The first very compelling light, the repulsion force of the exclusion case was a discovery in 1994 made with the principle should not impart a speed to the Very Long Baseline Array radio telescope. electrons higher than the speed of light. The astronomers William Watson and Chandrasekhar showed that under this Bradley Wallin at the Department of Physics assumption, a star that was heavier than of the University of Illinois at Urbanaabout one and a half times the mass of Champaign, showed that a gas cloud in a Figure 5: A photograph of Messier 106. The motion of a gas cloud in the center of this galaxy was the first the Sun would suffer a gravitational force nearby galaxy, known as Messier 106, was strong evidence for the existence black holes (© Sloan Digital Survey). 106. The Figure 5: A of photograph of Sky Messier motion of a gas cloud in the center of this galaxy created by its own mass bigger than the moving in an elliptical orbit around a small was the first strong evidence for the existence of maximum force of the exclusion principle. black void. Because the orbit of the cloud black holes (© Sloan Digital Sky Survey). This mass of one and a half times the mass was so precisely elliptical, it could only be be able to escape the Sun! The star would of the Sun is now called the Chandrasekhar that it was moving around an object that then be a black hole. The effect goes beyond limit. While the star is burning hydrogen was about the size of a star or smaller. But 7 the darkness of visible light: radio signals, nuclei to form heavier elements, the energy given the mass of the cloud and its orbit, microwaves and so forth, also propagate released by nuclear fusion keeps it from the mass of the object that was exerting at the speed of light and hence would be being smashed by gravity. However, as the gravitational force was calculated to trapped in the surface of the black hole. A soon as all the nuclear combustible is used, be about 40 million times the mass of the person inside a black hole would not be able gravity will act alone against the exclusion Sun! Such an object is safely above the to send any signals of his or her existence to principle and will win, resulting in an object Chandrasekhar limit. Moreover, the object the outside world. He or she would also not that can shrink to a point! But if a star with was roughly spherical and did not emit light. be capable of escaping: since nothing moves a mass bigger than the Chandrasekhar The conclusion was inevitable that Messier faster than light in the known universe, limit shrinks to a radius of about 4.5 km, 106 has a black hole in its core. The most direct evidence of a black nothing can escape the gravitational force the escape velocity near its surface will be bigger than the speed of light. A black hole hole to this day is the observation of the of this object. Everything that falls into a black hole is prevented by gravity from ever leaving. That such an object could exist was thought to be a mild amusement by many scientists for a long time. The story started to become more serious when physicists unveiled the mechanism that keeps two objects from occupying the same place: the exclusion principle. In general, it was thought, electrons in atoms will relax to the smallest orbit allowed near the protons of the nucleus. However, if all electrons could do that, then their electric repulsion resulting from being in the same orbit would make many-electrons atoms unstable. To circumvent this issue, the Austrian physicist Wolfgang Pauli postulated that two electrons in the universe could not have the same state of motion. An electron can be moving around a proton in the closest orbit allowed and then it can spin around its own axis. But an electron can only spin clockwise or counter-clockwise, therefore only two electrons can be placed in an atom moving both in the path closest to the proton (each spinning in opposite photograph of the center of our galaxy made with the W. M. Keck Telescope. There is an directions). A third electron added a Figure 6: A of Figure 6: to A photograph the made withbythe W. M.ItKeck Telescope. There is a object labeled Sgr A*center that doesof notour emitgalaxy light but is surrounded many stars. is the supermassive black two-electron atom then haslabeled to moveSgr to the hole that lives in the center of the Milky Way (© W. M. Keck Observatory / UCLA). A* that does not emit light but is surrounded by many stars. It is the supermassive bl Image courtesy of Leonardo Motta

Image courtesy of Leonardo Motta

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that lives in the center of the Milky Way (© W. M. Keck Observatory / UCLA). JOURNAL OF SCIENCE DARTMOUTH UNDERGRADUATE

DUJS 12S  

The Spring 2012 edition of the Dartmouth Undergraduate Journal of Science

DUJS 12S  

The Spring 2012 edition of the Dartmouth Undergraduate Journal of Science

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