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The Sun is a very common G2V type main sequence dwarf star. As it has been conceived from antiquity as all stars it is a sphere made of hot gases. It is luminous as it produces energy in its core with thermonuclear reactions. The Sun belongs to the Population I stars, or the so called heavy element-rich stars. It is a star made of material that partly comes from stars of previous generations which died with an explosion that ejected mater in the interstellar medium. The Sun has been formed approximately 4.57 billion years ago by a huge hydrogen molecular cloud that collapsed and formed our star and the planetary system.

The power of the Sun comes from thermonuclear reactions and as it belongs to the main sequence stars it burns hydrogen to helium which will be burnt at a later stage too. At present it has reached the middle of its life and its effective temperature is around 5770 K, while the temperature at the center is estimated around 15x10⁶ K. The Sun is in some aspects the most important astronomical body of our solar system Practically it gives all the energy of our solar system and dominates with its mass almost 98% of the system's total mass.

Since the era of the ancient Greek philosophers and especially when Galileo started the Sun's study with a telescope and up to the space age, the Sun became subject of scientific observation, exploration and discovery.

The chemical composition of the Sun is primarily hydrogen and helium as well as some other heavier elements, such as oxygen, carbon and iron. The latter and all the other heavier chemical elements exist in very small quantities, occupying less than 2% of the star (see table). The solar material is ionized in the form of plasma, neutral on the average. The Sun rotates around its axis with a variable rotational speed. This effect is called the differential rotation of the Sun. The differential rotation plays a very important role in the life and activity of the Sun, as, together with the magnetic field, it contributes to the development of sunspots and solar activity cycle of 11 years (see below). The sidereal period of rotation (I.e. The rotation of the Sun with respect to the stars) varies substantially from ~24.5 days near the equator to 35 or more days near the poles. The average synodic rotation period with respect to the Earth is about 26.2 days and for an observer in another planet or spacecraft it varies with its rotational speed around the Sun. Two periods frequently used are the Carrington rotation which is an average synodic period of 27.2753 days determined by Carrington based on sunspot position reappearance, and the Bartels synodic period which lasts 27 days. On the Sun we observe sunspots, which are dominant lower temperature and intense magnetic field regions that live several days or weeks and their number, size and position varies periodically with an ~11 year period (see below).

http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html

Bulk parameters

Sun
Earth
Ratio (Sun/Earth)



Mass (1024 kg)
1,989,100.
5.9736
333,000.



GM (x 106 km3/s2)
132,712.
0.3986
333,000.



Volume (1012 km3)
1,412,000.
1.083
1,304,000.



Volumetric mean radius (km)
696,000.
6371.
109.2



Mean density (kg/m3)
1408.
5515.
0.255



Surface gravity (eq.) (m/s2)
274.0
9.78
28.0



Escape velocity (km/s)
617.6
11.19
55.2



Ellipticity
0.00005
0.0034
0.015



Moment of inertia (I/MR2)
0.059
0.3308
0.178



Visual magnitude V(1,0)
-26.74
-3.86
-



Absolute magnitude
+4.83





Luminosity (1024 J/s)
384.6





Mass conversion rate (106 kg/s)
4300.





Mean energy production (10-3 J/kg)
0.1937





Surface emission (106 J/m2s)
63.29





Spectral type
G2 V





Model values at center of Sun:
Central pressure:     2.477 x 1011 bar                 
Central temperature:  1.571 x 107 K
Central density:      1.622 x 105 kg/m3

TABLE:  Elemental composition of the Photosphere

from http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961112a.html

TABLE:  Elemental composition of the Photosphere [1^st and 3^rd column present the logarithm of the element compared to the logarithmic abundance of hydrogen that is customarily set to 12] and comparison with Meteorites Elem. Photosphere Meteorites [2nd and 4th column] form Martin Asplund, Nicolas Grevesse, A. Jacques Sauval, Pat Scott, The chemical composition of the Sun, Annual Reviews of Astronomy and Astrophysics, 2009, 47, 481:522 and arXiv:0909.0948v1 [astro-ph.SR]

The Sun is permeated by an overall dipole magnetic field, caused by a magneto-hydrodynamic dynamo mechanism. In regions where the magnetic field is very intense the magnetic buoyancy (B²/2µ₀) leads to the formation of sunspots as a previously submerged magnetic tube emerges above the surface (photosphere). Sunspots are usually formed in pairs as a magnetic tube emerges and have opposite magnetic polarities as they are parts of an emerged magnetic flux tube. The magnetic field in and around the sunspots is very complex and spectacular. Magnetic field filaments and tubes rise to its surface and exhibit complex behavior. Charged particles, electrons and protons mainly, are trapped in all flux tubes that constitute a magnetic trap in the form of a magnetic bottle. In the strong and frequently complex magnetic fields of the sunspots many particles are trapped at times. When instability occurs, as a result of magnetic reconnection in a system of flux tubes, it leads to an explosive event giving rise to a solar flare or a coronal mass ejection. These explosive phenomena result in

(a) increase of electromagnetic radiation in almost all frequencies (from radio to X and gamma rays),

(b) ejection of energetic particles (low energy cosmic rays, high energy electrons and protons and heavier ions like helium and iron, as ions are accelerated in a different way),

(c) fast plasma of the solar wind leads to the formation of shock waves which travel in the interplanetary medium,

(d) geomagnetic activity,

(e) ionospheric disturbances and other phenomena.

Filament eruptions, microflares and nanoflares are other types of solar activity.

Internally the Sun is divided in three main parts defined by concentric spheres, followed at the top by some tenuous layers, the photosphere, chromosphere and the corona.

•  The core is the central region where all the power is generated by thermonuclear reactions that fuse hydrogen to helium. The core has extremely high density and temperature that are necessary for the thermonuclear reactions which produce energy, gamma ray photos and neutrinos, during the formation of a nucleus of helium from four very fast protons. The core has a radius of ¼th to 1/5th solar radii. The density at the center is around 150 times the density of water and the temperature around 15 million Kelvin, i.e. some 2500 times higher than the temperature of the photosphere, the visible part of the Sun.

•  the radiative zone is the concentric shell that covers the core where  photons that come from the core are scattered continuously and eventually they move outwards randomly taking extremely long times of the order of 17,000 years to 50 million years with an average of 170,000 years to finally appear on the photosphere.

• the convective zone, is the region of the Sun where energy is transferred with convection, with rotating vortices of various sizes, as in a saucepan full of boiling water.

• the photosphere is the visible part of the Sun and the base of the atmosphere. In the photosphere we observe the sunspots, granules, supergranules, faculae and other features. The temperature drops with height from 6000 to 4500 K, the pressure between 7x10^-3 to 1.6x10-1 bars and the effective temperature is around 5770 K, while the density is around 2×10-4  kg/m^-3.

• the chromosphere is a very thin layer 2000 to 3000 km that extends above the photosphere where the temperature increases from 4500 K to 20,000 K. The chromosphere is very faint and only visible either with special filters (with Halpha filter as it emits strongly in the Ha hydrogen line of the Balmer series) or during solar eclipses. Note that the chemical element helium (He) has been discovered in spectra of emission lines of the chromosphere during the solar eclipse of 1868. Spicules are numerous spike like features of upwards rising jets of gas with velocities up to 30 km/s, a lifetime of 5 to 10 minutes, that are visible at all times in the chromosphere.  Spicules extend upwards, up to 7000 km, like grass and cover all the  available area of the Sun.

  

figure of the temperature change in the solar atmosphere by Dr Eugene Avrett, Smithsonian Astrophysical Observatory

• The solar transition region is the part of the solar atmosphere where the temperature increases enormously, the atmosphere heats up suddenly from 4000 K and becomes the hot corona at 1,000,000 to 2,000,000 K.

• the corona is the hot and tenuous extension of solar atmosphere outwards that is visible during solar eclipses or with special instruments that cover the disk of the Sun and enable the observer to see the corona. It is not yet known how exactly the temperature increases from 4000 K to 1,000,000 K in the corona as highly ionized iron line observations show (FeXIV 530.3 nm, FeX at 637.4 nm). It is generally believed that the extreme temperatures of the corona are the result of Alfven waves travelling in the corona that heat it up and produce the solar wind by cascade effect. 

Figure of the speed dependence upon distance, Courtesy of the SOHO UVCS consortium. SOHO is a project of international collaboration between ESA and NASA.

• The solar wind is solar plasma that accelerates gradually as the solar atmosphere expands radially and the solar wind becomes extremely fast, supersonic or superalfvenic to be more precise. It becomes faster than the Alfven velocity which is the speed of Alfven waves in magnetized plasma. 

• The heliosphere is formed as the solar wind expands continuously radially outwards and eventually fills up a huge region in the Galaxy that is called the heliosphere in which all planets orbit and interact with the solar wind.

The major characteristics on the Sun are the sunspots, the granules, the prominences and the coronal holes. 

Sunspots from the Solar Dynamics Observatory (NASA/Goddard Space Flight Center)

1. Sunspots are prominent features of the Sun seen from antiquity (Theophrastos and Chinese astronomers) that are caused by strong magnetic fields on the Sun. They are darker than the surrounding Sun. In a sunspot we see the umbra in the middle, surrounded by the penumbra, as their temperature are respectively 2000 and 1000 degrees lower than the temperature of the rest of the Sun. The number, extend and position of sunspots vary substantially with time quasiperiodically. The same periodicity applies for solar flares, coronal mass ejections, radio flux from the Sun, cosmic rays from the Sun or the Galaxy (with different phase effect).  The timeseries of sunspots observed since the time of Galileo exhibit a prominent periodicity of approximately 11 years that in effect is 22 years if we take into account the changing magnetic field of the Sun that varies with the same period. All these effects we call solar activity.
2. Granules are small photospheric structures vortex convection cells (~1000 kilometers in size), which are irregularly shaped convective cells of hot rising gas in the center and cooler gas falling back to the Sun at the edges of the granule that live around 5 to 8 minutes. Granules are grouped to super granules around 10,000 to 30,000 km in size with lifetime of several hours to one day.

   

   Granules from the Swedish Vacuum Solar Telescope

3. Prominences are magnetic clouds that contain solar material and are seen dark on the solar disk or bright when seen above the limb of the Sun. 
4. Faculae are bright regions usually around sunspots, better observed near the limb of the solar disk.
5. Coronal holes are regions with outward directed open magnetic field lines that are not connected with nearby regions. Coronal holes are the sources of solar wind, or at least the sources of fast solar wind. During low solar activity, when the magnetic field of the Sun is almost dipolar, coronal holes are near the poles of the Sun and the two fast solar wind streams originating from them dominate all the heliosphere.

A prominence observed from Skylab (NASA)

The solar wind is a continuous stream of solar plasma (ionized particles, mainly protons and electrons, neutral on the average) that as it is highly conductive carries frozen-in the magnetic field. The solar wind that stems from the Sun accelerates gradually to supersonic, or more properly superalfvenic, i.e. faster than the speed of Alfven waves (a kind of magnetohydrodynamic wave) that generate and propagate in magnetized plasmas.

The interaction of the magnetized plasma of the superalfvenic solar wind with the planets forms the magnetospheres around them. The form of magnetospheres of the planets with intrinsic dipolar magnetic field, like the Earth, Jupiter and Saturn are different than the magnetospheres of Venus and Mars or the comets. The solar wind with its variable conditions influences all the Solar System planets varying their magnetospheres and in absence of a strong magnetic field their atmospheres.

Figure caption: elemental composition of the solar wind

[http://sohowww.nascom.nasa.gov/explore/lessons/compos9_12.html#Activity 1]

image caption: calculated magnetic field of the Sun using magnetograms

[from www.lmsal.com/solarsoft/latest_events/ ]

Solar explosive phenomena like flares, coronal mass ejections affect, sometimes drastically, planetary environments, especially when the planet is within the affected region of the heliosphere. It is worth noting that the ionosphere of the Earth has been occasionally affected by low energy gamma ray bursts coming from distant magnetars.

The sunspots, surface areas of lower temperature than its surroundings, appearing almost as black structured spots on the Sun's surface, undergo a periodic variation of some 11 years. The sunspot cycle, as it is called, influences prominently the space weather, the conditions around the Earth and even Earth's climate as solar power varies with time and perhaps because cosmic rays that are greatly affected by the solar activity (the extend of the heliosphere and the magnetic fields in the heliosphere) are believed by some scientists that influence substantially the formation of clouds and hence weather. The sunspot magnetic activity cycle is one of the most important solar phenomena, as it affects the Earth and the planets.

The number of sunspots is considered to be a very good index of strength of the solar activity. It is mainly used because there are almost continuous and to an extend reliable measurements for centuries and even since the time of Galileo.

Among the unsolved and interesting scientific issues of Solar Physics we can mention the solar neutrino problem, the coronal heating problem, and the faint young Sun problem.

Humans observed the Sun for millenia. One of the greatest achievements of the 20th century is solar observations using experiment onboard spacecraft. The first spacecraft to measure the inerplanetary medium and solar environment were NASA’s Pioneers 5, 6, 7, 8 and 9, and OGOs that have been launched between 1959 and 1968. The German Helios 1 and 2 spacecraft probes that gave invaluable and till today unique information about the inner heliosphere followed in the 1970s and the Skylab Apollo Telescope Mount, followed later by  NASA’s Solar Maximum Mission in 1980 gave extremely important results in the understanding of our nearest star. Japan's Yohkoh launched in 1991, the Solar and Heliospheric Observatory (SOHO) by the European Space Agency and NASA (1995) and  NASA’s Solar Terrestrial Relations Observatory (STEREO) in 2006 that enable humans to have a complete view of all the solar surface using two spacecraft, one going faster than the Earth and the other slower than our planet in its orbit around our star that are now almost behind the Sun and more recently the more advanced Solar Dynamics Observatory (2010). The Genesis  spacecraft was a solar wind sample suitable to directly measure the properties of solar material returned damaged by inappropriate landing to Earth (2004) and samples are undergoing analysis.

Links:

For the present status of the Sun and excellent collection of all observed solar explosions see

http://www.lmsal.com/solarsoft/latest_events/

Solar and Heliospheric Observatory:

http://sohowww.nascom.nasa.gov/

National Solar Observatory:

http://www.nso.edu/

The Ulysses Mission:

http://ulysses.jpl.nasa.gov/

The Solar Maximum Mission:

http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/solarmax.html

SDO | Solar Dynamics Observatory:

http://sdo.gsfc.nasa.gov/

STEREO:

http://stereo.gsfc.nasa.gov/

See also the HINODE mission:

http://www.nasa.gov/mission_pages/hinode/index.html

Solar pictures:

1. SOHO-Gallery: Best Of SOHO:

2. The Sun from STEREO spacecraft:

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