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Black Holes |
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A black hole is defined by the escape velocity that would have to be attained to escape from the gravitational pull exerted upon an object. For example, the escape velocity of earth is equal to 11 km/s. Anything that wants to escape earth's gravitational pull must go at least 11 km/s, no matter what the thing is � a rocket ship or a baseball. The escape velocity of an object depends on how compact it is; that is, the ratio of its mass to radius. A black hole is an object so compact that, within a certain distance of it, even the speed of light is not fast enough to escape. |
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A black hole itself is invisible because no light can escape from it. In fact, when black holes were first hypothesized they were called "invisible stars." If black holes are invisible, how do we know they exist? This is exactly why it is so difficult to find a black hole in space! However, a black hole can be found indirectly by observing its effect on the stars and gas close to it. For example, consider a double-star system in which the stars are very close. If one of the stars explodes as a supernova and creates a black hole, gas and dust from the companion star might be pulled toward the black hole if the companion wanders too close. In that case, the gas and dust are pulled toward the black hole and begin to orbit around the event horizon and then orbit the black hole. The gas becomes heavily compressed and the friction that develops among the atoms converts the kinetic energy of the gas and dust into heat, and x-rays are emitted. Using the radiation coming from the orbiting material, scientists can measure its heat and speed. From the motion and heat of the circulating matter, we can infer the presence of a black hole. The hot matter swirling near the event horizon of a black hole is called an accretion disk. |
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Only stars with very large masses can become black holes. Our Sun, for example, is not massive enough to become a black hole. Four billion years from now when the Sun runs out of the available nuclear fuel in its core, our Sun will die a quiet death. Stars of this type end their history as white dwarf stars. More massive stars, such as those with masses of over 20 times our Sun's mass, may eventually create a black hole. When a massive star runs out of nuclear fuel it can no longer sustain its own weight and begins to collapse. When this occurs the star heats up and some fraction of its outer layer, which often still contains some fresh nuclear fuel, activates the nuclear reaction again and explodes in what is called a supernova. The remaining innermost fraction of the star, the core, continues to collapse. Depending on how massive the core is, it may become either a neutron star and stop the collapse or it may continue to collapse into a black hole. The dividing mass of the core, which determines its fate, is about 2.5 solar masses. It is thought that to produce a core of 2.5 solar masses the ancestral star should begin with over 20 solar masses. A black hole formed from a star is called a stellar black hole |
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Astronomers have found convincing evidence for a supermassive black hole in the center of the giant elliptical galaxy M87, as well as in several other galaxies. The discovery is based on velocity measurements of a whirlpool of hot gas orbiting the black hole. In 1994, Hubble Space Telescope data produced an unprecedented measurement of the mass of an unseen object at the center of M87. Based on the kinetic energy of the material whirling about the center (as in Wheeler's dance, see Question 4 above), the object is about 3 billion times the mass of our Sun and appears to be concentrated into a space smaller than our solar system. |
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A few words from the scientist:
When we got together last summer it was a particularly exciting time to be creating a lesson about black holes. A new instrument on HST had just become the ideal science instrument to find and study supermassive black holes that reside in the center of galaxies. Among the remarkable things HST can accomplish with this instrument, the Imaging Spectrograph (STIS), will be a black hole survey. STIS is something like a "Cosmic Speed Gun" - when Hubble is pointed at a galaxy it can determine the speed of material that circulates the galactic center. The faster stuff moves around the center, the more massive that center must be. With high-school-level physics we can determine the mass of these supermassive black holes. Just as we know the mass of our Sun by observing the planets in their orbits about the Sun, we will know the mass of the black holes that reside in the center of each and every galaxy in which we point the STIS "speedgun." The real STIS image of the central region of galaxy M84 is used in the "Amazing Space Black Hole Activity." As more STIS results accumulate, it seems that many, if not most, galaxies have supermassive black holes at their center.
In my research I have studied gravity on a much smaller scale. I use the circulation of our oceans and atmosphere to test how this motion affects the gravitational field of the earth. For example, satellites orbiting the Earth exhibit measurable changes in their orbits as a result of El Ni�o. In fact, when you get up and leave the computer terminal you will be changing the Earth's gravitational field by a very small amount. Of course, your walking across the room is not measurable from space. However, it would be an interesting experiment to see how many teachers would have to get up and walk across their rooms to cause a measurable change in the Earth's gravitational field. The gravitational phenomena that occur on Earth and in our solar system are many times smaller in magnitude and scale and not nearly as bizarre as those that occur in and around a black hole. I hope that the activities developed in this lesson plan will capture students' imagination, provide them with a better understanding of our universe, and let them have fun all at the same time.
Daniel Steinberg |
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