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The Sun

The Sun is the closest star to Earth and is the center of our solar system. A giant, spinning ball of very hot gas, the Sun is fueled by nuclear fusion reactions. The light from the Sun heats our world and makes life possible. The Sun is also an active star that displays sunspots, solar flares, erupting prominences, and coronal mass ejections. These phenomena impact our near-Earth space environment and determine our "space weather."

To understand how our Sun works, it helps to imagine that the inside of the Sun is made up of different layers, one inside the other. The core, or the center of the Sun, is the region where the energy of the Sun is produced. Even on Earth we know that the Sun produces energy because we see sunlight and we feel hot on a summer day.

The Sun's energy, which is produced in the core, travels outwards. The energy travels first through the radiative zone, where particles of light (photons) carry the energy. It actually takes millions of years for a photon to move to the next layer, the convection zone.

At the convection zone, energy is transferred more rapidly. This time it is the motion of the gases in the Sun that transfers the energy outwards. The gas at this layer mixes and bubbles, like the motion in a pot of boiling water.This bubbling effect is seen on the surface of the Sun, and is called granulation.

We can't see inside the Sun. So scientists use other diagnostics. These diagnostics help us know what is inside the Sun.

The Solar Atmosphere
The visible solar atmosphere consists of three regions: the photosphere, the chromosphere, and the solar corona. Most of the visible (white) light comes from the photosphere, this is the part of the Sun we actually see. The chromosphere and corona also emit white light, and can be seen when the light from the photosphere is blocked out, as occurs in a solar eclipse. The sun emits electromagnetic radiation at many other wavelengths as well. Different types of radiation (such as radio, ultraviolet, X-rays, and gamma rays) originate from different parts of the sun. Scientists use special instruments to detect this radiation and study different parts of the solar atmosphere.

The solar atmosphere is so hot that the gas is primarily in a plasma state: electrons are no longer bound to atomic nuclei, and the gas is made up of charged particles (mostly protons and electrons). In this charged state, the solar atmosphere is greatly influenced by the strong solar magnetic fields that thread through it. These magnetic fields, and the outer solar atmosphere (the corona) extend out into interplanetary space as part of the solar wind.

Solar Activity
The Sun is not a quiet place, but one that exhibits sudden releases of energy. One of the most frequently observed events are solar flares: sudden, localized, transient increases in brightness that occur in active regions near sunspots. They are usually most easily seen in H-alpha and X-rays, but may have effects in the entire elecromagnetic spectrum. The X-ray brightness from a large flare often exceeds the X-ray output from the rest of the Sun. Another type of event, the coronal mass ejection, typically disrupt helmet streamers in the solar corona. As much as 1e13 (10,000,000,000,000) kilograms of material can be ejected into the solar wind. Coronal mass ejections propagate out in the solar wind, where they may encounter the Earth and influence geomagnetic activity. Coronal mass ejections are often (but not always) accompanied by prominence eruptions, where the cool, dense prominence material also erupts outward.

All of these forms of solar activity are believed to be driven by energy release from the solar magnetic field. How this energy release occurs, and the relationship between different types of solar activity, is one of the many puzzles facing solar physicists today. The amount of solar activity on the Sun is not constant, and is closely related to the typical number of sunspots that are visible. The number of sunspots and the levels of solar activity vary with an 11 year period known as the solar cycle.

The Fate of the Sun
In about 5 billion years, the hydrogen in the center of the Sun will start to run out. The helium will get squeezed. This will speed up the hydrogen burning. Our star will slowly puff into a red giant. It will eat all of the inner planets, even the Earth.

As the helium gets squeezed, it will soon get hot enough to burn into carbon. At the same time, the carbon can also join helium to form oxygen. The Sun is not very big compared to some stars. It will never get hot enough in the center to burn carbon and oxygen. These elements will collect in the center of the star. Later it will shed most of its outer layers, creating a planetary nebula, and reveal a hot white dwarf star.

Nearly 99 percent of all stars in the galaxy will end their lives as white dwarfs. By studying the stars that have already changed, we can learn about the fate of our own Sun.


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