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Transcript
Biography of a Star: Our Sun's Birth, Life, and Death Depending on the size of the original lump of gas and dust, the process of stellar birth can give rise to different sorts of stars. A small lump never develops high enough pressures and temperatures to start nuclear fusion. It is doomed to remain a dark, dismal stellar wanna-be -- a so-called brown dwarf. A larger lump becomes a large star, so hot and bright that it burns itself out in a few tens of millions of years. A lump in the middle, not too small and not too large, becomes a middling star such as the Sun. Which is good: If the Sun had been much smaller, Earth would have been a dark, dead world; much larger, and Earth would have been broiled. In its early years, the Sun went through a tempestuous youth, whipping up strong winds that cleared the solar system of whatever gas had not been incorporated into a planet. But then the Sun settled down. From studying rocks, fossils, and Antarctic ice, scientists think the Sun has been brightening over time, but only slightly. And how much longer will it continue to shine? For an idea of the Sun's life expectancy, astronomers look to clusters of stars, such as one named Messier 67, which is about the same age as our Sun. By simulating the life cycles of these stars on a computer, astronomers have ascertained how long stars live. They predict that the Sun will be able to fuse hydrogen into helium in its core at about the same rate for another 5 billion years. (What a relief!) If the Sun were a car, the gas tank would now be half full. What will happen when the Sun does run out of gas? (Hydrogen gas, that is.) Fortunately, the Sun will still have reserves of hydrogen in the layers that surround the core. The core will heat up this shell of hydrogen. When the shell gets hot enough to fuse hydrogen to helium, the release of energy will carry on there. It is as if the driver of the car poured an extra few gallons into the fuel tank. But this trick has a price. The source of energy will no longer be the dense, massive core, but rather a shell closer to the surface -- and that will make a big (so to speak) difference in the structure of the Sun. The Sun will puff up until its radius is 30 times greater. It will become a red giant, similar to the star Arcturus, though much smaller than a supergiant such as Betelgeuse (see photo on p. 3). A red giant is red because its exterior cooled from 9,000 to 3,000 degrees Fahrenheit as it expanded; for a star, red means cool. This red-giant stage will last for about 2 billion years. That Time Bomb in the Middle The striking but now-outdated video Universe, produced by NASA in the 1970s, shows the red-giant Sun engulfing the Earth. Though certainly dramatic, this is now thought to be incorrect. Astronomers have had to scale down their estimates of the size of red giants based on data from the satellite Hipparcos and from the new optical and infrared interferometers -- networks of telescopes which can take images of large, nearby stars. Now we think the Sun will not engulf us when it becomes a red giant. But that is small comfort. In its retirement from normal core fusion, our previously nurturing star will care little for its planetary children. It will be pumping out a thousand times more energy, making Earth a good approximation to hell. To add insult to injury, the solar wind -- a stream of particles which now gives us fun things such as the aurora borealis -- will become a cyclone that will make radio communication impossible and perhaps evaporate the atmosphere altogether. Looking on the bright side, the red-giant Sun may be warm enough to melt the water-rich but now-frozen moons of Jupiter and Saturn. Humanity, if it is still around, might relocate there. Meanwhile, what happens to all that helium being produced in the shell? It gently rains onto the dead, but still toasty, core of the Sun, making the core more massive and more compressed. This raises the temperature of the core until suddenly -- and I really do mean suddenly, as in seconds -the helium in the core fires up and begins to fuse itself into carbon. Using the fuel-tank analogy, this is as if the exhaust itself starts to burn. The end is drawing near. Now the Sun has to rearrange its internal structure all over again, as its source of energy is once again the central core. The Sun will contract back to a bit larger than its original radius and will give off 10 times as much energy as what we are used to now. This phase only lasts another 500 million years, as there are a lot fewer helium nuclei (it took four hydrogen nuclei to make one helium nucleus, and three heliums to make one carbon) and the energy production is much less efficient. As the Sun exhausts the helium in the core, it desperately staves off the inevitable by resorting again to those reserves in its outer layers. Again the Sun expands. This time, it grows so large that its outer edge is only weakly gravitationally bound to the core. The Sun barely holds itself together anymore. This eleventh-hour attempt at life-support is pitifully ineffective; the final red-giant stage can be maintained for only 100 million years. At this point, things will really start falling apart. The Sun's outer layers, freed from the gravitational clutches of the core, will waft away. Over the course of about 10,000 years, these layers will spread out into space as an enormous sphere of gas lit up by the now-naked hot core. These layers constitute a "planetary nebula," so called because in a small telescope the gas cloud looks a bit like the disc of a planet (see photo on p. 3). The hot core is now a "white dwarf," a stellar cinder. As a white dwarf, the ex-Sun will glow white-hot for a near-eternity. Alas, there will be no dramatic explosions to entertain our distant descendants: The Sun would have had to start with at least eight times more mass to die the spectacular death of a supernova. The Sun, modest in life, is subdued in death. After the planetary nebula fades, there is no nuclear fusion at all (no extra fuel, no fuel tank, not even the trunk is left), just a lump of hot carbon and some happy memories. The Sun will be well and truly dead. The sphere of gas drifts off and eventually is gathered up in a new cloud, and become part of the next generation of star formation. Perhaps one day, the ashes of the Sun will throw their lot in with another star to be born, live, die, and, perhaps, give sustenance to other warm little planets. BETH HUFNAGEL is a postdoctoral researcher at Michigan State University in East Lansing. As an auditor, she used to ferret out the secrets of corporate finance -- talents now applied to the evolution of Sun-like stars. Her email address is [email protected]. George Musser contributed to this article. Earth Earth is the third planet from the Sun and the fifth largest: orbit: 149,600,000 km (1.00 AU) from Sun diameter: 12,756.3 km mass: 5.972e24 kg Planet Earth Amazing pictures of Earth from space combine useful science and artistic beauty. Orbit : Nasa Astronauts Photograph the Earth A beautiful coffee table book. Kids often ask me which is my favorite planet. My answer is always "Earth". This book shows why. Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth). It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet. Mir space station and Earth's limb Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful. The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km): 0- 40 Crust 40- 400 Upper mantle 400- 650 Transition region 650-2700 Lower mantle 2700-2890 D'' layer 2890-5150 Outer core 5150-6378 Inner core The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle. Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms): atmosphere oceans crust mantle outer core inner core = 0.0000051 = 0.0014 = 0.026 = 4.043 = 1.835 = 0.09675 The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is: 34.6% Iron 29.5% Oxygen 15.2% Silicon 12.7% Magnesium 2.4% Nickel 1.9% Sulfur 0.05% Titanium The Earth is the densest major body in the solar system. The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth. Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates: North American Plate - North America, western North Atlantic and Greenland Earth's Plate Boundaries delineated by earthquake epicenters South American Plate - South America and western South Atlantic Antarctic Plate - Antarctica and the "Southern Ocean" Eurasian Plate - eastern North Atlantic, Europe and Asia except for India African Plate - Africa, eastern South Atlantic and western Indian Ocean Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean Nazca Plate - eastern Pacific Ocean adjacent to South America Pacific Plate - most of the Pacific Ocean (and the southern coast of California!) There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries. The Earth's surface is very young. In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old. There is no record of the critical period when life was first getting started. Space Shuttle view of the Strait of Gibraltar 71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past). Earth's atmosphere seen at the limb The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible. (Water vapor is also an important greenhouse gas.) View from Apollo 11 The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen. The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year. Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras (see the Interplanetary Medium). Irregularities in these factors cause the magnetic poles to move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The "geomagnetic north pole" is the position on the Earth's surface directly above the south pole of the Earth's field; see this diagram.) The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.