Your Standard Solar Model

By combining theoretical modeling of the sun's (unobservable) interior with observations of the energy that the sun produces, astronomers have come to an agreement on what is called a standard solar model, a mathematically-based picture of the structure of the sun. The model seeks to explain the observable properties of the sun and also describe properties of its unobservable interior.

With the standard solar model, we can begin to describe some of the interior regions— regions hidden, beneath the photosphere, from direct observation. Below the photosphere is the convection zone, some 124,000 miles (200,000 km) thick. Below this is the radiation zone, 186,000 miles (300,000 km) thick, which surrounds a core with a radius of 124,000 miles (200,000 km).

The sun's core is tremendously dense (150,000 kg/m3) and tremendously hot: some 15,000,000 K. We can't stick a thermometer in the sun's core, so how do we know it's that hot? If we look at the energy emerging from the sun's surface, we can work backward to the conditions that must prevail at the sun's core. At this density and temperature, nuclear fusion is continuous, with particles always in violent motion. The sun's core is a giant nuclear fusion reactor.

At the very high temperatures of the core, all matter is completely ionized—stripped of its negatively charged electrons. As a result, photons (packets of electromagnetic energy) move slowly out of the core into the next layer of the sun's interior, the radiation zone.

Here the temperature is lower, and photons emitted from the core of the sun interact continuously with the charged particles located there, being absorbed and re-emitted. While the photons remain in the radiation zone, heating it and losing energy, some of their energy escapes into the convection zone, which in effect, boils like water on a stove so that hot gases rise to the photosphere and cool gases sink back into the convection zone. Convective cells become smaller and smaller, eventually becoming visible as granules at the solar surface. Thus, by convection, huge amounts of energy reach the surface of the sun. At the sun's surface, a variety of processes give rise to the electromagnetic radiation that we detect from the earth. Atoms and molecules in the sun's photosphere absorb some of the photons at particular wavelengths, giving rise to the sun's absorption-line spectrum. Most of the radiation from a star that has the surface temperature of the sun is emitted in the visible part of the spectrum.

Close Encounter

Astronomers use a technique called helioseismology to study the sun's interior. By measuring the frequencies of various regions on the surface of the sun, conclusions can be drawn about the sun's internal structure. This process is analogous to the way in which seismologists monitor and measure the waves generated by earthquakes in order to draw conclusions about the unseen internal structures of our own planet.

The Least You Need to Know

V Staggering as the sun's dimensions and energy output are, the sun is no more nor less than a very average star.

V The sun is a complex, layered object with a natural nuclear fusion reactor at its core.

V The gamma rays generated by the fusion reactions in the core of the sun are converted to optical and infrared radiation by the time the energy emerges from the sun's photosphere.

V Never look directly at the sun, especially not through binoculars or a telescope. The safest way to observe the sun is by projecting its image, either through binoculars or a telescope or simply through a pinhole punched in cardboard.

V Although the sun has been a dependable source of energy for the last 4 billion years, its atmosphere is frequently rocked by such disturbances as sunspots, prominences, and solar flares, peaking every 11 years.

Of Giants and Dwarfs: Stepping Out into the Stars

In This Chapter

V Which stars are nearest and farthest?

V Observing and calculating stellar movement

V Measuring the size of stars

V Classifying stars according to stellar temperatures and chemical composition

V Determining the mass of a star

V Stellar biographies

Our own star, the sun, is relatively accessible. We can make out features on its surface and can track the periodic flares and prominences that jut from its surface. But our sun is only one star of over 100 billion in our Galaxy alone, and all the other stars that exist are much farther away, parts of other distant galaxies.

Earlier, we said that if the earth were a golf ball, the planet Pluto would be a chickpea eight miles away. But even distant Pluto is very close in comparison to our stellar neighbors. On the same scale, the nearest star (in the Alpha Centauri system) would be 50,000 miles away. That distance wraps around the earth's equator twice. The other stars are even farther. Much farther.

In this chapter, we reach out to the myriad cousins of our sun.

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