Our Star : The Sun

Our solar system is composed of the Sun and all things which orbit around it: the Earth, the other eight planets, asteroids, and comets. The Sun is 150 million kilometers (93 million miles) away from the Earth (this distance varies slightly throughout the year, because the Earth's orbit is an ellipse and not a perfect circle).

The Sun is an average star - there are other stars which are much hotter or much cooler, and intrinsically much brighter or fainter. However, since it is by far the closest star to the Earth, it looks bigger and brighter in our sky than any other star. With a diameter of about 1.4 million kilometers (860,000 miles) it would take 110 Earths strung together to be as long as the diameter of the Sun. The Sun is mostly made up of hydrogen (about 92.1% of the number of atoms, 75% of the mass). Helium can also be found in the Sun (7.8% of the number of atoms and 25% of the mass). The other 0.1% is made up of heavier elements, mainly carbon, nitrogen, oxygen, neon, magnesium, silicon and iron. The Sun is neither a solid nor a gas but is actually plasma. This plasma is tenuous and gaseous near the surface, but gets denser down towards the Sun's fusion core.

The Sun, as shown by the illustration to the left, can be divided into six layers. From the center out, the layers of the Sun are as follows: the solar interior composed of the core (which occupies the innermost quarter or so of the Sun's radius), the radiative zone, and the the convective zone, then there is the visible surface known as the photosphere, the chromosphere, and finally the outermost layer, the corona.
The energy produced through fusion in the Sun's core powers the Sun and produces all of the heat and light that we receive here on Earth. The process by which energy escapes from the Sun is very complex. Since we can't see inside the Sun, most of what astronomers know about this subject comes from combining theoretical models of the Sun's interior with observational facts such as the Sun's mass, surface temperature, and luminosity (total amount of energy output from the surface).

All of the energy that we detect as light and heat originates from nuclear reactions deep inside the Sun's high-temperature "core." This core extends about one quarter of the way from the center of Sun (where the temperature is around 15.7 million kelvin (K), or 28 million degrees Fahrenheit) to its surface, which is only 5778 K "cool".

Above this core, we can think of the Sun's interior as being like two nested spherical shells that surround the core. In the innermost shell, right above the core, energy is carried outwards by radiation. This "radiative zone" extends about three quarters of the way to the surface. The radiation does not travel directly outwards - in this part of the Sun's interior, the plasma density is very high, and the radiation gets bounced around countless numbers of times, following a zig-zag path outward.

It takes several houndred thousand years for radiation to make its way from the core to the top of the radiative zone! In the outermost of the two shells, where the temperature drops below 2,000,000 K (3.5 million degrees F) the plasma in the Sun's interior is too cool and opaque to allow radiation to pass. Instead, huge convection currents form and large bubbles of hot plasma move up towards the surface (similar to a boiling pot of water that is heated at the bottom by a stove). Compared to the amount of time it takes to get through the radiative zone, energy is transported very quickly through the outer convective zone.

The Sun's visible surface the photosphere is "only" about 5,800 K (10,000 degrees F). Just above the photosphere is a thin layer called the chromosphere. The name chromosphere is derived from the word chromos, the Greek word for color. It can be detected in red hydrogen-alpha light meaning that it appears bright red. Above the surface is a region of hot plasma called the corona. The corona is about 2 million K (3.6 million degrees F), much hotter than the visible surface, and it is even hotter in a flare. Why the atmosphere gets so hot has been a mystery for decades; SOHO's observations are helping to solve this mystery.

The Sun is not just a big bright ball. It has a complicated and changing magnetic field, which forms things like sunspots and active regions. The magnetic field sometimes changes explosively, spiting out clouds of plasma and energetic particles into space and sometimes even towards Earth. The solar magnetic field changes on an 11 year cycle. Every solar cycle, the number of sunspots, flares, and solar storms increases to a peak, which is known as the solar maximum. Then, after a few years of high activity, the Sun will ramp down to a few years of low activity, known as the solar minimum. This pattern is called the "sunspot cycle", the "solar cycle", or the "activity cycle".
Stars like the Sun shine for nine to ten billion years. The Sun is about 4.5 billion years old, judging by the age of moon rocks. Based on this information, current astrophysical theory predicts that the Sun will become a red giant in about five billion (5,000,000,000) years.

Source : SOHO Project