Stellar Anatomy and Longevity Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 17, 2011

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.”

Red-Giant Phase

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.

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