The closest star around which the Earth revolves is the Sun. It is the gravitational center of the Solar System and lies about 270,000 times closer to us than the next nearest star (Proxima). The Sun is a solitary, yellow dwarf star of spectral type G2 that has been on the main sequence for about 4.6 billion years. The Sun consists largely of hydrogen (71% by mass) and helium (27%), with much smaller amounts of heavier elements. It puts out 400 trillion trillion watts of energy, produced by the fusion of hydrogen to helium by the carbon-nitrogen cycle in its core. The Sun is 109 times wider than the Earth. It spins on its axis with a period that varies from 25 days at the equator to 33.5 days near the poles.
(92,975,699 mi., 8.3 light-minutes)|
||1,392,000 km (865,000
||14 million ºC (25
(Earth = 1)
|Density (water = 1)
|Surface gravity (Earth =1 )
||617.5 km/s (383.8
The Sun is located in one of the branches of our galaxy Milky Way
Much like the Earth, the Sun has many different layers that define its structure. Unlike the Earth, the Sun is completely gaseous, there is no solid surface on the Sun. Although the Sun is completely made of gas, the density and temperature of the gas changes drastically as you travel from the center to the outermost regions. In the core of the Sun the density is as high as 150 grams per cubic centimeter. At the other extreme, near the base of the outermost layer, the corona, the density has dropped to about 1x10-15 grams per cubic centimeter. This value is close to laboratory vacuum densities here on Earth. The temperature patterns in the Sun are not well understood. The core has a very high temperature of more that 15 million degrees Kelvin. As you move away from the heat producing core the temperature drops to about 6000 degrees at the photosphere, the effective surface of the Sun. The puzzling thing is that the temperature then rises again to more than 2 million degrees in the corona which is the furthest layer from the core. Our Sun provides an exciting "laboratory" in which to do experiments. Because it is our nearest star scientists are able to investigate its behavior with various instruments from space as well as from Earth.
Surface and 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. 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.
Most of the energy we receive from the Sun is the visible (white) light emitted from the photosphere. The photosphere is one of the coolest regions of the Sun (6000 K), so only a small fraction (0.1% ) of the gas is ionized (in the plasma state). The photosphere is the densest part of the solar atmosphere, but is still tenuous compared to Earth's atmosphere (0.01% of the mass density of air at sea level). The photosphere looks somewhat boring at first glance: a disk with some dark spots. However, these sunspots are the site of strong magnetic fields. The solar magnetic field is believed to drive the complex activity seen on the Sun. Magnetographs measure the solar magnetic field at the photosphere. Because of the tremendous heat coming from the solar core, the solar interior below the photosphere (the convection zone) bubbles like a pot of boiling water. The bubbles of hot material welling up from below are seen at the photosphere, as slightly brighter regions. Darker regions occur where cooler plasma is sinking to the interior. This constantly churning pattern of convection is called the solar granulation pattern.
Above the photosphere is the chromosphere, a region about 2500 kilometers thick. Just prior to and just after the peak of a total solar eclipse, the chromosphere appears as a thin reddish ring. The conspicuous color of the chromosphere (compared to the mostly white corona) led to its name (meaning "color sphere") The chromosphere is most easily viewed in emission lines such as Hydrogen alpha, where bright regions known as plages, and dark features called filaments are visible.
Rising above the Sun's chromosphere, the temperature jumps sharply from a few tens of thousands degrees Kelvin to as much as a few million degrees in the Sun's outer atmosphere, the solar corona. Understanding the reason the Sun's corona is so hot is one of the many challenges facing solar physicists today. Because of the very high temperatures, the corona emits high energy radiation and can be observed in X-rays with instruments on satellites above the atmosphere.
Sunspots are a feature of the Sun that have been observed since ancient times. When viewed through a telescope, they have a dark central region known as the umbra, surrounded by a somewhat lighter region called the penumbra. Sunspots are dark because they are cooler than the surrounding photosphere. They are the site of strong magnetic fields. The sunspots come in many shapes and sizes; they often appear in groups. These spots are much bigger than the Earth; they can be over 10 times the diameter of the Earths. The reason sunspots are cool is not entirely understood, but one possibility is that the magnetic field in the spots inhibits convection underneath them. Sunspots typically grow over a few days and last anywhere from a few days to a few months. The number of sunspots on the Sun is not constant, but varies with an 11 year period known as the solar cycle. Solar activity is directly related to this cycle. Other forms of solar activity are: solar flares, coronal mass ejection, … All of these forms of solar activity are believed to be driven by energy release from the solar magnetic field.
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