Sun

The Sun is about 90 million mi (150 million km) away from Earth. It is modest by stellar standards, although it is over 100 times larger than the earth and over 300,000 times more massive. Although it consumes more than half a billion tons of its nuclear fuel each second, it has been shining continuously for five billion years, and it will continue to shine for another five billion.

To the ancient Egyptians the Sun was a god, for anyone who felt its warmth and watched the renewal of crops it brought in the spring realized it was a bringer of life. Greek mythology recounts the tale of Icarus, who died in a brazen attempt to fly too close to the Sun.

Ancient dogma held that the Sun orbited Earth. It was not until 1543 that Nikolaus Copernicus (1473-1543) published the heliocentric theory of the Universe, in which the Sun lay at the center and Earth was relegated to a circular orbit around it. Copernicus refused to have this magnificent work published until near his death. This was a wise decision, since earlier publication would have resulted in his equally earlier death at the hand of Church authorities who regarded his work as heresy.

Around 1610, the newly invented telescope was trained on the Sun. The early solar observers, including Galileo (1564-1642), noticed that the surface of the Sun had dark spots on it. Not only that, the spots moved across the solar surface, leading Galileo to conclude that A photograph of the Sun taken with a coronagraph, a telescope that creates an artificial eclipse, so the solar corona is visible. The shape of the corona varies with time and in some areas is nearly absent. Much of its radiation is in the x-ray and extreme UV range, and the corona is many times hotter than the surface of the Sun itself. U.S. National Aeronautics and Space Administration (NASA). the Sun rotated. This sent further shock waves through the religious world. The poor cardinals had hardly recovered from Copernicus's heresy, and now along came Galileo telling them that the backup embodiment of celestial perfection, the Sun, had ugly blemishes on its surface. Galileo got into serious trouble with the Church for this and other statements he made, and it would be 380 years before Pope John Paul II exonerated him.

In the nineteenth century an amateur German astronomer named Heinrich Schwabe (1789-1875) did some of the first scientific analysis of solar data. Observers had kept records over the years of the number of sunspots visible, and Schwabe noticed that the sunspot number rose and fell in a cyclic fashion, with a period of about 11 years. Later workers confirmed this activity cycle, and the sunspot number remains today an important piece of information about our star.

The nature of sunspots was first investigated by George Ellery Hale (1868-1938), who devoted his life to studying the Sun from the observatory he founded on Mt. Wilson near Los Angeles. Hale discovered that sunspots were cooler than the surrounding solar surface—which is why they appear dark—and that sunspots are associated with strong magnetic fields. Hale also discovered that the sun's magnetic field reverses itself with each 11-year cycle, so that there is a more fundamental 22-year magnetic activity cycle behind the sunspot cycle.

Since Hale's time, solar research has developed along two great lines of investigation. First, a huge observational database has been created. The national solar observing facilities of the United States are located at Sacramento Peak just east of Alamogordo, New Mexico, and at Kitt Peak, 30 mi (48 km) west of Tucson, Arizona. In addition to sunspots, these observatories have monitored prominences and flares, which are spectacular, eruptive events in the solar atmosphere, the solar magnetic field itself, the solar granulation, which is a result of the turbulent movement of the gas beneath the solar surface, and the total energy output of the Sun. Space-based satellites have studied the Sun in wavelength regimes, such as the ultraviolet and the x ray, not accessible from the ground. The result of these investigations is a detailed set of observations about the phenomena that occur on our star.

Illustration by Argosy. The Gale Group.

The other line of investigation is the theoretical analysis of processes within the Sun and in its atmosphere. Scientists have developed models to explain what is observed; if a model fails to fit the observations, it must be discarded. The goal of this work is to explain why the various features seen on the Sun appear. Why is there an 11-year-long solar activity cycle? What do the forces producing granulation have to do with the nature of the solar magnetic field? Where does the magnetic field originate? These questions, and many others, are only partially answered.

A fruitful line of research toward answering these questions involves observing not just the sun, but many other stars like the Sun. You would have a difficult time understanding what the human race was like by observing only one person; similarly, it is hopeless to try to understand the complex nature of the stars by observing only one of them. Long-term observations of solar-like stars have been carried out at the Mount Wilson Observatory near Los Angeles and at Lowell Observatory in Flagstaff, Arizona. Understanding the so-called "solar-stellar connection," derived from observations of other stars, may yield important clues about the processes at work in our own star.

Two million years after our earliest ancestors looked up and shielded their eyes from the brilliant, life-giving object in the sky, we still have only a rudimentary understanding of it.