The Sun

The Sun is a star. It is just like all the other stars that we see at night. It appears so big and bright because it lies nearby. It is only 93 million miles (150 million kilometers) away.

The Sun’s size

The Sun is the most important star for us because nothing could live on Earth without it. This huge star weighs more than 332,000 Earths would weigh. There’s no surface to stand on at the Sun. It’s a hot, glowing ball of gases about 865,000 miles (1,400,000 kilometers) across. It is so huge that a million Earths could fit inside it.

The Sun’s gravity keeps everything in the solar system orbiting it. It also controls the climate and weather of our entire planet.

Solar flares are a sudden release of the Sun’s energy. They can have an effect on Earth. Solar flares can break up radio communications and cell-phone calls. They can cause electrical power to go out. And they can cause beautiful colored lights to appear in the Arctic skies of our planet. These are known as auroras.

Observing the Sun

The brilliant disk we see from Earth is known as the Sun’s photosphere. This means “sphere of light.” It is the Sun’s inner atmosphere. The temperature there is nearly 10,000°F (5,500°C). Bubbles as large as England rise from the photosphere. They carry heat into space.

The Sun has an outer atmosphere as well. It is called the corona. We cannot normally see it because our daytime sky is much too bright. But when the Moon blocks the Sun in a total solar eclipse, the corona shines brightly in a dark sky. Astronomers can study the corona anytime with a special instrument called a coronagraph.

Surface of the Sun

Astronomers use special telescopes to study the Sun’s face. They can see areas called sunspots. Sometimes there are many sunspots; other times there are only a few. Sunspots appear in a cycle that takes about 11 years to complete. Sunspots are large magnetic storms in the Sun’s atmosphere. Some are much bigger than Earth; they can blast powerful jets of hot material into space. Scientists are interested in studying this material because it can sometimes bump into our planet and cause problems.

The Sun’s interior

Long ago people thought that the Sun might be burning fuel like coal or wood. But today we know that the Sun is a giant nuclear furnace. At its core the temperatures rise to about 25,000,000°F (14,000,000°C). There atoms of hydrogen race around at high speeds. Some collide and combine to form helium. This emits a huge amount of energy. When this energy escapes the Sun, we see it as light and feel it as heat. No life could exist on Earth if it were not for the Sun.

Solar activities in detail

Sunspots
As early as 28 B.C., Chinese annals carried records of dark areas observed on the sun. However, they were attributed to flying birds. In 1610, while surveying the wonders of the heavens revealed by his newly perfected telescope, Galileo observed the same small, dark spots on the face of the sun. By watching these spots appear on the western edge of the sun and then drift across the solar disk to disappear off the eastern edge, Galileo determined that the spots were in fact on the sun and not shadows cast by some other object or objects.

It has since been learned that the average lifetime for a given sunspot is about one week. However, some spots live for only a few hours, while others may live as long as 18 months. The spots range in size from the smallest observable objects, about 500 miles (800 km) in diameter, to huge areas that are over 50,000 miles (80,000 km) in diameter. The spots are related to magnetic activity in the sun’s outer layers, but the processes are not yet fully understood. They appear as dark areas simply because they are somewhat cooler than the surrounding gases.

Spots may occur singly or in groups of two to more than 100. Records of sunspots show that there is no strict periodicity either in the time when maximum or minimum numbers occur, or in the total number of spots. The average period between minimal is 11.08 years, but individual sunspot cycles have varied in length from 8 to 13 years. The number of sunspots at maximum has varied from 46 to 154. At the beginning of a new sunspot cycle, there are usually remnant spots of the preceding cycle at latitudes 5° to 10° from the solar equator. The spots of the new cycle begin at latitudes 25° to 40° from the equator, in both hemispheres. As the cycle progresses, the newer spots tend to form closer and closer to the equator. There is also a reversal in the magnetic polarities of spot groups in both hemispheres as a new cycle begins. That is, if the “leader” spots were of north polarity in the preceding cycle, they change to south polarity for the new cycle.

Active regions
In the photosphere surrounding sunspots there are usually bright vein like features known as faculae, a word derived from a Latin word meaning “torch.” Faculae are observed in normal white light without the aid of special devices. However, they are seen only near the edge of the sun, where they stand out in greater contrast with the photosphere. They are much longer-lived than sunspots.

The chromosphere over sunspot regions also shows bright areas of longer lifetime and greater extent than the spots themselves. These areas are called plages, from a French word meaning “beach,” but they may also be known as bright flocculi, from a Latin word meaning “flock of wool.” Above the plages, the chromosphere is mottled by alternately bright and dark features of fainter contrast than the plages. Not uncommonly, dark ribbon like features known as dark filaments are scattered over the solar surface in association with the plages, although other filaments may be at high solar latitudes outside of the sunspot zone. At the edge of the sun’s disk the filaments show up as prominences elevated above the chromosphere.

The corona itself is both hotter and denser than average above the active regions. Temperatures of several million degrees are not uncommon there, and coronal structures above such regions display the intricate patterns of magnetic fields that are associated with sunspots and their surroundings. Most of the coronal radiations -including X-rays and radio waves- are more intense in the active regions, and the lines in their spectra are stronger.

Prominences that are associated with active regions are characteristically short-lived. They often appear as closed loops or as violent ejections of matter in sprays and surges.

Flares
The most spectacular and in many ways the most interesting phenomena of active regions are flares. Flares are observed in the chromosphere as short-lived explosive brightening of plages. They begin with a sudden brightening in part of the plage, usually near sunspots, and increase to maximum brightness in 5 to 10 minutes. During this time the flare spreads to surrounding areas of the plage. The most favorable times for flares to occur are when sunspot groups are decaying or growing. Large bright flares usually last an hour or more. Smaller ones may disappear in a few minutes.

Intense bursts of X-rays and radio signals emitted from the corona are associated with many flares. These radiations are observed simultaneously with the flare, but following intense flares there are often greatly enhanced fluxes of energetic protons and alpha particles in a half hour to a few days, as recorded by particle detectors near the earth. Such an event was recorded in December 2006, when a shockwave spread across the surface of the sun about 20 minutes after a planet-sized sunspot erupted into a colossal flare. The protons and alpha particles from flares are accelerated from the corona above chromospheric flare regions at the time of the visual burst. Their arrival at the earth is delayed because their velocity is lower than the velocity of electromagnetic radiations, and sometimes by their complicated trajectories in the solar magnetic fields.

The Sun makes enough power to light 2,600 Earths filled with 200-watt light bulbs, and the Sun has been doing this every day for about 5 billion years.