THE SUN. Similar in many languages, "Sun," related to Latin's "sol," was represented in ancient Greece as Helios, god of the Sun. Giver of warmth and life, we hardly think of the Sun as a star, the term "Sun and stars" in constant use, though the surmise that it is a star goes back to ancient times. It is different because it is OUR star, the one that belongs to us, the one we can see most closely, and the one we know most about. (Though AT NO TIME ATTEMPT TO VIEW THE SUN, as it is so bright it can burn the eye; leave that to professionals.) With the Sun only 150 million kilometers away, astronomers can detect incredible detail. The next nearest of similar brightness, Alpha Centauri, is 271,000 times farther, each of its two components (it is a double) appearing as mere points. The Sun is the reference to which all other stars are compared, their diameters in solar diameters, their brightnesses in solar luminosities. To understand them better, to make sense of their real characteristics, we need to know the solar characteristics, which while the Sun is hardly the brightest and biggest in the sky, are still astounding. It is 1.4 million kilometers across, the equivalent of 109 Earths set side by side, and has a mass of two million trillion trillion kilograms, or 330,000 Earths. Most astonishing perhaps is its luminosity of 400 trillion trillion watts. To put that in perspective, it would cost the gross national product of the United States for millions of years for a local power company to run the Sun for one second. This immense energy, pouring from a body with a yellow-white "surface" (an opaque gas) of 5800 Kelvin, is generated by thermonuclear fusion in the Sun's deep core where the temperature reaches 15 million Kelvin and the density 14 times that of lead. No matter the density, however, the core, like the rest of the Sun, is entirely gaseous. The outer layers are made of 91 percent hydrogen, 9 percent helium (the element named after Helios, since helium was discovered there first), and a bit over a tenth of a percent of everything else, oxygen dominating these followed by carbon, neon, and nitrogen. In the heat and pressure of the core, the hydrogen is slowly being turned into helium, a small amount of mass lost and converted to energy in the process.
A model of the Sun shows its nuclear fusing core (where hydrogen is turned into helium by the conversion of mass into energy), an envelope where energy is transferred by radiation, and an outer layer where convection (the rising of hot gases, falling of cool gases) rules. At the surface (the photosphere), rising magnetic fields locally cool the solar gases to make sunspots. Surrounding the whole affair is the magnetically heated solar corona, which is confined by great magnetic loops. The corona is the seat of the solar wind Prominences are threads of cool gas that lie in the corona and are supported by magnetic fields. (From Stars, J. B. Kaler, Scientific American Library, Freeman, NY, 1992.)
After 4.5 billion years (as found from the ages of meteorites), the core of the Sun is now about half helium, and there remains enough hydrogen to last for another five or so billion years. When it runs out the Sun will become a giant like so many of those that populate the naked-eye sky. The Sun spins slowly with a period of 25 days at its equator, the spin and churning outer gases producing a magnetic field about five times the strength of Earth's. The rotation wraps the field up into powerful ropes that break through the surface to chill local areas and make sunspots. The magnetism heats a tenuous outer layer, the corona, to two million Kelvin. The corona's thinness makes it dim and visible to us only during a total eclipse (when the bright surface is blotted out by the Moon). From the surface, controlled by magnetism and luminosity, flows a thin but fast "wind" that flows past the Earth and makes comet tails point away from the Sun. Bubbles in the solar wind produced by collapsing solar magnetic fields produce the northern and southern lights (the aurora), and have even been know to affect power grids on the ground. Many of the same phenomena are being seen in the stars around us, the Sun providing a way to understand them, the stars in turn allowing us better to understand the Sun, our own personal star.
Written by Jim Kaler. Return to STARS.