Stellar Anatomy and Longevity Help (page 3)
Stellar Anatomy—Model Stars
Astronomers have deduced certain things about the interiors of stars, including the Sun, just as they have been able to gather ideas about the interiors of the planets, including the Earth. Different types of stars have different internal structures. We also know that different types of stars have much different “atmospheres.” The “atmosphere” surrounding a star can extend outward for millions of kilometers and is called the corona .
Computers have been used to construct model stars using programs that simulate events believed to take place in the depths of these fusion furnaces. Known physical laws are applied to denote what happens inside a star with a given mass and material composition. Stars are considered to have concentric layers or “shells,” each with a certain temperature, mass, and combination of elements. The whole star is “constructed” by putting the shells together. This can be done outward from the center toward the surface and then to the corona or inward from the corona to the surface and ultimately to the center. Sometimes, when the two modeling schemes (inside-out versus outside-in) are both applied to objects having the same mass and material composition, the results are different.
In the depths of a large main-sequence star such as a blue supergiant, radiation and convection are believed to be mutually responsible for the transfer of energy from the core of the star into space. Near the surface, radiation predominates. Near the center, convection is thought to play a more important role because the matter deep inside stars is so dense that it is opaque to radiation. This model of a large star is shown in Fig. 13-5 a . In a much less massive main-sequence star, the reverse is believed to be the case. A red-dwarf star apparently has a convective outer region and a more transparent core. Radiation should predominate deep inside such a star. This model of a red dwarf is shown in Fig. 13-5 b .
Both these models are largely speculative. Computers can work only with the data that they get, and these data are based on educated guesswork. Nature is not a computer program. We can’t be sure that the models we invent are perfect representations of what really goes on in the Universe. In fact, people ought to know by now, after decades of working with computers, that there are always some differences between the “mind” of nature and the “mind” of a computer program! As observational techniques improve and computers become more enlightened about nature, star modeling should get better and better. We’ll never know for sure, however, what the inside of a star is like unless and until we can send some sort of probe down inside one. The Sun is the logical candidate for a first try, but no one has yet figured out how a navigable space vessel could be designed to survive the extreme temperature and pressure.
Interior Of The Sun
The Sun is an average star in terms of its location on the H-R diagram. It is neither a blue giant nor a red dwarf. What is the interior of the Sun like? Astronomers have one great advantage when they look at the Sun as a star and try to evaluate its internal and external characteristics. This is, of course, the fact that it is much closer than other stars. Telescopes can resolve surface details. Several different theoretical models have been constructed for our parent star.
All the models suggest a solar core temperature of several tens of millions of degrees Celsius. The surface is not nearly as hot, though, only around 5,000 to 6,000°C. Above the visible surface, the temperature rises as the altitude increases, up to a maximum of 500,000 to 600,000°C in the corona. Then the temperature begins to decrease and keeps on dropping as the distance reaches the orbital radius of Mercury, Venus, and so on through the rest of the Solar System. Some solar effects continue on out past the orbit of Pluto, in particular the solar wind, consisting of a continuous outward barrage of subatomic particles. Eventually, the temperature reaches the general level of interstellar space, and the solar wind is overcome by the effects of interstellar winds. This transition zone, where the Sun’s dominion ends, is called the heliopause .
The interior of the Sun is thought to more closely resemble that of a red dwarf (see Fig. 13-5 b ) than that of a blue supergiant (see Fig. 13-5 a ). This has led to the belief that the Sun’s future is bound to be more like that of a small star than a big one. We need not fear that our parent star will ever go supernova on us. But the Sun’s lifespan will not be infinite. There will come what has been called a last perfect day , after which the Sun will undergo a gradual but radical change. If human beings or other intelligent beings still populate the Earth on the last perfect day, they won’t go to bed one night after a benevolent dusk and wake up the next morning to a hostile dawn, but the time of change will be at hand. Someone somewhere will declare: “It is time for us to find a new star system. Everybody start packing.”
According to scientists, our Sun has plenty of hydrogen fuel left. Our parent star should keep on shining, just as it does today, for many millions of years to come. On that last perfect day, however, the hydrogen fuel will be depleted to the point that changes begin to take place inside the Sun. The metamorphosis will occur at first in the core and then will migrate outward toward the surface.
At some distant future time, the Sun’s core will start to contract because of the depletion of hydrogen fuel. Gravitation will take over for a while, having patiently waited all those billions of years since the hydrogen-fusion process began. As the core contracts, it will heat up again and eventually will become so hot that helium, the material by-product of hydrogen fusion, will begin to fuse. This will generate a new source of energy. The outer layers will continue to burn hydrogen into helium and energy. The core will attain a superhot surface that will cause the outer regions to expand; the result will be an ever-increasing overall solar diameter. The helium in the core will fuse into carbon.
As the Sun expands from continued pressure deep within itself, the surface will cool, but the surface area will increase greatly. The Sun’s apparent diameter in the sky, as seen from Earth, will double, then triple, then quadruple. The color will shift from the so-called yellow part of the spectrum into the orange and then into the red. Our parent star will leave the main sequence, migrating upward and to the right as plotted on an H-R diagram. Temperatures on the Earth will increase. First the inland ponds and lakes will boil. Then the rivers and large inland seas will boil. Then the oceans will boil completely away, the atmosphere will be driven off into space, and Earth will become a barren, scorching desert similar to Mercury’s current state. Some astronomers think that the Sun will expand so much that its radius will exceed 1 astronomical unit (AU). In this case, our planet will cease to exist.
Death Of A Star
Eventually, all the helium fuel in the core of the Sun will be spent. The core will again begin to collapse under the force of gravitation as the interior pressure drops. The rest of the Sun will follow, and the red giant will grow smaller. The internal and surface temperatures will rise, but not enough to cause further fusion reactions. The only thing stopping the inward drive of gravitation in the end will be the forces inside subatomic particles. It is believed that the Sun will come to rest at a diameter roughly the same as that of today’s Earth, and its energy output will fade away like the glow of an ember in a forgotten fire. This fate awaits all stars whose masses are less than, equal to, or somewhat greater than that of the Sun.
Stars that are much more massive than the Sun have more interesting deaths. They produce atoms heavier than carbon, including oxygen, neon, magnesium, silicon, sulfur, and iron. However, there is a limit to this. When iron is formed from fusion, it is the last hurrah. Iron will not fuse. When the iron is spent at the core, it collapses almost instantly and then rebounds, causing an outward shock wave that blows the star up. Then, in the mass that remains, gravitation can become so powerful that it overcomes everything else. The end products of some large stars are thought to be black holes , where matter has been crushed by gravitational forces so strong that time and space are altered and nothing can escape, not even the starlight. Black holes were once regarded as the figments of theorists’ imaginations, too weird to exist in the real Cosmos. Nowadays, though, there is credible evidence that they indeed exist. We’ll delve into the mysteries of these and other “stellar ghosts” in the next chapter.
Practice problems of this concept can be found at: Stars and Nebulae Practice Problems
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