The Sun shines due to the energy produced in its core through a reaction called nuclear fusion,
which requires extreme heat and pressure.
The Sun's temperature reaches 16 million degrees Kelvin and the pressure is so high that nuclear
fusion is possible.
In this process, energy is released when four hydrogen nuclei fuse
together to form one helium atom.
It takes thousands of years for this energy to reach the surface of the Sun and to be emitted into space.
Above the core is the radioactive zone, where the energy from nuclear fusion is transported up by photons
(small energy packets similar to elementary particles such as electrons).
The upper layer of the Sun's interior is called a convective zone because this is where convection takes place
in a process similar to convection in the Earth's mantle.
Huge hot gaseous bubbles rise and transport the energy up, while the cooler ones travel down.
The visible part of the Sun's surface is called the photosphere. Of course, it is not a solid surface;
it is simply the bottom layer of the Sun's atmosphere. It is still very hot, about 5,840 K.
If you photographed the photosphere, it would appear to be covered with rice grains, called granules.
These granules actually are the tops of convective bubbles that carry energy upwards. Each is about 1,000 km wide.
Sunspots are formed when the magnetic field of the sun gets twisted and pokes up through the surface.
The Sun's magnetic field is so strong that it suppresses the energy flow through the spots and lowers the temperature.
This makes sunspots look darker. Sun spots typically form in groups.
Studying the motion of the sunspots shows that the Sun rotates faster around its equator, at a rate of about 29 days,
and slower near the poles, which takes about 31 days.
The solar corona spreads above the photosphere up to 6 solar radii.
The temperature rises again and reaches about a million degrees.
However, the solar corona is less dense, so it shines with less intensity than the photosphere.
The only way to see the solar corona is during a total solar eclipse, when the moon is between the Earth and the Sun.
Since the Moon is about the same visible size as the Sun, during an eclipse the moon covers the Sun completely
and blocks the majority of solar light, allowing a magnificent view of the solar corona and solar prominences.
Solar prominences are spectacular feature in the solar corona, which
look like bright loops and arcs since they contain hot gasses that follow the curved lines of Sun's magnetic field.
Solar prominences can loop hundreds of thousands of miles into space.
Astronomers study the sun with specially-constructed telescopes, but because the Earth's atmosphere blocks many of the high-enery
rays from the Sun we need to use spacecraft if we want to take get a clearer look.
Early observations were made from the first space station, Skylab, in 1970.
In 1990, the Ulysses spacecraft was launched, tasked with studying the magnetic field of the sun.
The Solar & Heliospheric Observatory (SOHO), an international collaboration
between the European Space Association and NASA
was launched in 1995 to study the Sun from its deep core to the outer corona and the solar wind.
Every minute the Sun loses 4 million tons of its mass because of nuclear fusion!
However, it has enough supplies to shine at least 5 billion years more. This lost mass turns into energy.
The visible light is only part of the energy emitted from the sun. The Earth receives less than half of one billionth of this energy.
The Sun has an average 11-year cycle of activity. During solar minimum, there are very few sunspots,
and during solar maximum there are many. The last solar maximum was in 2000, and the next one will occur sometime during 2013.
Beside the sunspots, the number of solar flares and prominences also raises during the peak of the Solar Cycle.
Solar flares are the most powerful expression of solar activity.
They are giant bursts in the solar corona, equivalent to explosion of thousands of bombs.
The eruptions are so strong that the ejected material and highly energetic electromagnetic waves (X-rays and UV light) reach the Earth and beyond.
One of the main goals of the SOHO mission was to monitor solar flares, since they strongly influence the Earth's atmosphere.
Their highly energetic particles not only cause the spectacular polar lights, but could be quite dangerous for the astronauts in the space station.
They also cause magnetic storms that disturb satellite communication.