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Transcript
Stars: The world's second-oldest profession might be that of astronomer; it probably precedes even that of farmer (which extends roughly about 10,000+ years ago). The stars have been a source of regularity, relating the passage of years far more reliably then changes in weather. For instance, the ancient Egyptians used the appearance of the star Sirius (seemingly the brightest star in the sky, aside from the Sun) to let them know when the Nile would start flooding, allowing the planting of crops. Astrology developed in tandem, providing the stars and planets with special significance to mankind. The number of stars in the sky that we can see on crystal clear nights with just our eyes is maybe 4000. The number of stars in the universe is estimated to be in the area of 100,000,000,000,000,000 (100 quadrillion) or more. Birth of Stars: The universe is not a perfect vacuum dotted with stars and planets here and there. Instead, the universe has gas (about 90% hydrogen, virtually all of the remainder helium) dispersed about its volume. On average, this dispersion allows for about 1 hydrogen atom per volume the size of a grape. This rarefaction is significantly less than any vacuum we have achieved on Earth. However, there are some portions that have decidedly greater numbers of atoms than that within their volume. It's within these pockets that stars can come forth and they are sometimes known as "stellar nurseries". A clump of gas that will become a star typically initially occupies a volume that's on the order of one light-year x one light-year x one light-year (one light-year is the distance traveled by light in one year; since light travels about 300,000,000 meters/second, one light-year is on the order of 6,000,000,000,000 miles). Within this inconceivably huge volume of gas, there is a battle between mutual gravitational attraction, which wants to bring this whole mass together, and mutual repulsion that is exactly analogous to what happens to air molecules when a sound wave passes through--they bounce off each other. On rare occasions, however, a shock wave passes through the cloud (perhaps these shock waves are most often caused by supernovas). And on those occasions, that shock wave will draw atoms together more closely than they would otherwise be. This allows gravity to gain the upper hand and more and more atoms are drawn into the denser region before they can bounce away. This process is also assisted by having a rotation set up in the cloud. This introduces a centripetal force that will tend to draw the atoms in the cloud together. Thus, a protostar is formed, an object several times heavier than the Sun in mass and much, much greater in volume. Over time, gravity will force many of the atoms making up the protostar closer and closer together. Eventually, the atoms will overcome their mutual electrostatic repulsion and start undergoing fusion of hydrogen nuclei to helium nuclei. At this point, we now have a star. Likely, this happens hundreds of times a day, but as this is throughout the entire universe, the odds of one happening nearby are very small. After this point, the life of the star becomes a battle between internal thermonuclear fusion, which tries to push the star apart, and gravity, which tries to pull the star together. Most stars seem to come in pairs. This is almost certainly as a result of the nature of their birth, for the clouds from which stars are born are typically much, much larger than even the gigantic protostars. Thus, several stars are typically forming simultaneously. Life of Stars: The great bulk of a star's life is a relatively stable equilibrium between thermonuclear fusion and gravity. In the very center of the star, hydrogen nuclei are being fused into helium nuclei. The energy from these processes seeps outward from the core, greatly warming the external portion of the star (which does not undergo thermonuclear processes itself). The outer portion of a star is alive with convectional currents and magnetic anomalies like sunspots. A star can live billions of years this way; the Sun is believed to be approximately halfway through its estimated 10 billion-year life. However, it's lifetime is determined by its size at the outset--the larger the star, the shorter its life. However, the larger the star, the brighter it is. The Sun is relatively small. Very large stars will have lifetimes of "only" millions of years. A barometer often used in the study of stellar phenomena is the "Hertzsprung-Russell Diagram". This allows an astronomer to determine how bright a distant star is and thus have some indication as to its size and lifespan. The Death of Stars: All stars start their death throes in the same manner. During their long period of equilibrium between gravitational contraction and the explosive outburst of fusion processes, stars are using hydrogen as fuel to create helium. Eventually, their amount of hydrogen falls low and the fusion processes slow. This allows gravity to gain the upper hand and the star shrinks. This, though, raises the temperature in the core of the star. When the temperature gets high enough, the fusion processes start again. This time, however, helium is fused to create lithium, beryllium and other elements. Since the core of the star is hotter, the outer shell of the star expands. This expansion is actually great enough that although the star is emitting more energy, it's through a greater surface area and the star's surface gets a little cooler--hence, the star gets a little redder. Eventually, the helium runs low, though, and gravity gains the upper hand--the star shrinks. But when it shrinks enough, the temperature in the core is raised yet further and fusion processes again starts up, using the "ashes" of the previous fusion cycle. The star swells further and further, becoming a red supergiant. This will someday happen to the Sun, but don't buy insurance just yet. There's still about 5 billion years to go. Depending upon the mass of the star, this process could keep going until fusion processes are producing iron. What happens after that depends upon the star. Small stars will never reach the point where they're producing iron--the gravitational collapse of such relatively little mass is insufficient to heat the interior to that point. Thus, the star collapses down to the point where the atoms are essentially rubbing up against each other, much more close than in even the densest terrestrial material. This is called a "white dwarf" and is what will happen to the Sun after it leaves its red giant phase. A star can remain in the white dwarf phase for billions of years; eventually, the star will burn out, ultimately becoming a black dwarf. If the white dwarf is part of a system of two or more stars, it can lead to an interesting effect. As the non-white dwarf star produces energy, it is also emitting mass. This mass can accumulate on the surface of the white dwarf. If enough mass accumulates, it can lead to fusion processes within it. The onset of these fusion processes, though, is very swift--the whole thing violently explodes. This is called a "nova" in the textbook but is more properly called a "Type I Supernova". The explosion causes the white dwarf to briefly shine about as brightly as any star can. This process can repeat itself many times for the same binary system. A more cataclysmic fate awaits stars whose mass exceeds the Sun's by a factor of 1.4 or more. In those stars, it turns out that there is enough mass to allow the creation of iron in fusion processes. Iron, though, is a very special element. It represents the most optimum balance between two basic forces in nature, the electromagnetic force and the strong nuclear force. Without getting into details, the more protons and neutrons there are in an atomic nucleus, the greater the presence of the strong force, which wants to bring these particles together. However, the more protons there are in a nucleus, the greater the repulsion due to the electromagnetic forces. Therefore, nuclei smaller than iron don't have as much of the strong nuclear force binding it together, but nuclei larger than iron have a greater repulsive electromagnetic presence. In short, fusion processes lose energy when they try to fuse iron into heavier nuclei. Thus, fusion is no longer an option when the gravitational collapse continues. In fact, there is no option. In sufficiently massive stars, when a great deal of iron has accumulated in the core, the fusion processes shut themselves off almost like a light switch. This temporarily removes any obstruction from gravity's relentless desire to contract the star. In literally seconds, the star collapses. However, the core collapses much more quickly than the outer portion. The core effectively bounces, releasing a tremendous amount of energy that passes through the outer portion of the star. This huge amount of energy is sufficient to trigger fusion processes in that entire portion--it's as if the whole star is now undergoing fusion instead of just the core. The unimaginably gigantic explosion that results is called a "supernova". It is during this process that elements heavier than iron are created--there's so much energy running around that it can happen. Supernovae are not common events since so few of all stars have sufficient mass. A supernova "only" 100,000 light-years away occurred in January, 1987. It's the closest supernova to us in the last 400 years. In fact, prior to the supernova in 1987, all of our knowledge about this phenomenon had been gathered from observing supernovae in other galaxies. Within the category of stars that undergo a supernova are two possible ultimate demises. If the star has a mass less than three times the Sun's, the core that remains will have collapsed down so much that even the electrons making up the constituent atoms will be forced into their respective nuclei. The whole star will almost be like a giant, uncharged nucleus. This is called a "neutron star". The only reason a neutron star doesn't collapse further is that the neutrons are basically shoulder-to-shoulder with each other. Such stars have been observed, so this isn't speculation. If you look at the constellation of Orion, the middle star in the short sword handing down from his waist is not a star--it's a cloud of dust. It is what remains of a supernova that was observed on Earth in 1054. At the center of the cloud, observable in a wavelength of light that we can't see with our eyes, is the neutron star that was left behind. This neutron star, and many others, spin around very rapidly. They also have very concentrated magnetic fields. Because of these two effects, it's almost as if there's a searchlight rotating around and around with incredible regularity. In 1967, when these objects were first discovered, this regularity was so incredible that it was initially thought the object could be a beacon left by an extraterrestrial civilization--the object found was initially denoted as "LGM-1", where the LGM stands for "Little Green Men". Black Holes: For stars with a mass 3 or more times greater than the Sun's, even the rubbing together of neutrons isn't enough to slow down the gravitational collapse. In fact, nothing is. The star collapses down to a point-size particle, i.e., zero length, zero width and zero height; this object is called a "black hole". However, it still leaves behind an effect, that being its gravitational field. From far away, the black hole's gravitational field is just like that of any other star or other massive object. But the source of its gravitational field is concentrated at a single point, unlike any other object. Thus, when you get really close, you get significant effects. For instance, suppose the Sun were collapsed to a black hole and you were 3.1 km away with your feet pointing down towards the surface. Thus, your feet are about 2999 m away from the black hole while your head is about 3001 m away. The gravitational force at your feet is so much greater than the gravitational force at your head that you would easily be pulled apart like a cheap rubber band. The gravitational pull of a black hole is so strong that not even light can escape. That light would even be influenced by gravity was a concept that wasn't even suggested until Albert Einstein published his General Theory of Relativity in 1916. However, in 1919 scientists went to a region in Africa that would be subject to a solar eclipse. There, they were able to see that stars whose light was passing close to the Sun had that light deflected by the Sun by exactly the amount Einstein had predicted. The Sun, though, obviously won't prevent light from escaping. It takes a greater gravitational pull to do that. And this is why it is impossible to directly observe a black hole. We can only deduce its existence from indirect effects. If we sent a probe towards a black hole, what would happen to it? Assuming it survives, we would find that the frequency at which it is emitting its signal is getting longer and longer, i.e., what might have originally been a transmission rate of 50,000 bits/second is, to us, continuously decreasing as the probe gets closer and closer. In addition, the probe even appears to be slowing down. This is not because anything is happening to the probe, though, it's because of something happening to the light waves being emitted or reflected by the probe. Eventually, the probe will appear to come to a dead stop, finally vanishing as the last bit of light left it. In fact, though, the probe has passed through the black hole's "event horizon", the point at which no signals can escape a black hole. The event horizon's distance from the center of the black hole depends upon the mass that was used in the creation of the black hole in the first place. For a typical, new black hole, this event horizon will have a radius of about 10 km. Anything closer means that it has been sucked into the black hole, forever gone. What is it like inside a black hole? We can use our theories to speculate. For instance, it has not been ruled out that black holes act as conduits to other parts of the universe or even to entirely different universes. But a basic theory seems to uphold a bizarre intuition. Consider what life is like outside a black hole--theoretically, there are no constraints on our motion in space, but we are constrained to move in a very particular direction in time. The reverse might possibly be true in a black hole, i.e., you are constrained to move in a particular direction in space (toward the exact center of the black hole, of course), but you are as free to move about in time in a black hole as you are free to move about in space outside a black hole.