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Sun - A Small Blue Planet

solar —the photosphere earth

Eight minutes after a photon leaves the Sun's photosphere, it reaches Earth. Along with countless billions of other photons, it streams through Earth's atmosphere. The photon, a product of a hydrogen fusion reaction 30,000 years ago, has finished its trip. Obviously the Sun has a profound impact on doings here on Earth, but recent research suggests the connections may run deeper than initially thought.

The link may lie in the solar activity cycle, which is the periodic variation in active features such as sunspots, prominences, and flares, in the Sun's atmosphere and on its visible surface. The cause of the activity cycle is not well understood, but astronomers generally agree that the Sun's differential rotation, combined with the turbulent motions in its convection zone, create a magnetic dynamo that results in a perpetual tangling and rearrangement of the Sun's magnetic field. When magnetic field lines, which normally lie below the photosphere, become tangled and burst into the Sun's atmosphere, active features such as sunspots and prominences invariably form. When the magnetic field becomes tangled to a critical level, it rearranges and simplifies its configuration, and the amount of solar activity decreases correspondingly. The sunspot cycle typically has a length of about 11 years, but there is compelling circumstantial evidence that variations in the length of the solar activity cycle are closely related to changes in the global temperature, with shorter solar cycles corresponding to warmer temperatures on Earth.

And in the end, the sun will have its final and greatest impact on Earth. What of the core, which our photon left a million years ago? Five billion years from now, the seemingly countless hydrogen nuclei will all have been converted into helium "ash"—the Sun's fuel will be gone. To stave off destruction by the inexorable force of gravity, the sun's core will contract and heat to the point that the helium will ignite. In the process, the sun will expand into a red giant star, swallowing the innermost planet, Mercury, and turning Earth into a charred wasteland. But the helium is the last fuel reserve the sun will be able to use, and it will eject its outer layers, leaving behind only its collapsed core, a small, dying white dwarf.



Seeds, M.A. Horizons, Discovering the Universe. New York. Wiley, 1991.


Giampapa, Mark S. "The Solar-Stellar Connection." Sky & Telescope (August 1987): 142.

Pasachoff, J. "The Sun: A Star Close Up," Mercury (May/June 1991): 66.

Jeffrey C. Hall


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—The narrow middle layer of the Sun's atmosphere. It is about 17,492°F; (9,700°C) and is very faint relative to the photosphere.

Convection zone

—The outermost third of the solar interior. Here heat is transported to the surface in giant convective bubbles of gas, which rise to the surface, release their heat, and then sink back into the interior to pick up more heat.


—The central region of the Sun, where thermonuclear fusion reactions take place.


—The highest and hottest layer of the solar atmosphere. Matter in the corona may have a temperature of 3,599,541°F (1,999,727°C) and may be several million miles above the photosphere.


—The lowest layer of the solar atmosphere, where most of the visible light is emitted. Because this is the layer we see in white light photographs, it is often called the solar "surface," even though it is a very thin gas.


—A large region of glowing gas suspended in magnetic fields, often arching far above the photosphere. Some prominences are quiescent, remaining for days, while others are eruptive, and dissipate violently.

Radiative zone

—The central two-thirds of the solar interior. Here energy is transported by the flow of photons, or light waves, through the matter.

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