Sun image

THE SUN is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.

Pink Sun

Technically, the Sun is classified as a G2V star. Stars are classified according to their surface temperature and luminosity (brightness). The Sun appears very bright because it is much closer to us than other stars, but in fact it is not unusually luminous. If we put the Sun at the same distance as other "nearby" stars, it would look about the  same as many other stars. The main temperature classes are denoted (in decreasing temperature) by the letters O, B, A, F, G, K and M.

It is a huge, bright sphere of mostly ionized gas about 5 billion years old. The closest star to Earth, it is 145 million km distant (this distance is called an Astronomical Unit). The next closest star is 300,000 times further away. There are Space Shuttle going by the Sun probably millions of similar stars in the Milky Way galaxy (and even more galaxies in the Universe), but the Sun is the most important to us because it supports life on Earth. It powers photosynthesis in green plants and is ultimately the source of all food and fossil fuel. The Sun's power causes the seasons, the climate, the currents in the ocean, the circulation of the air, and the weather in the atmosphere.

The Sun is some 333,400 times more massive than Earth (mass= 1.99 x lO 30 kg), and contains 99.86% of the mass of the entire solar system. It is held together by gravitational attraction, producing immense pressure and temperature at its core (more than a billion times that of the atmosphere on Earth, and a density about 160 times that of water).

The Core

Sun with its arms of fire

The core is the hottest part of the Sun and of the Solar System.  It has the temperature is 16 million degrees K, and a density of up to 150 g/cm³ (150 times the density of liquid water) which is sufficient to sustain thermonuclear fusion reactions. The released energy prevents the collapse of the Sun and keeps it in gaseous form. The total energy radiated is 383 billion trillion kilowatts/ second, which is equivalent to that generated by 100 billion tons of TNT exploding each second.

The core produces almost all of the Sun's heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. The energy produced by fusion in the core, except a small part carried out by neutrinos, must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kineticenergy of particles.

In addition to the energy- producing solar core, the interior has two distinct regions: a radiative and a convective zone. From the edge of the core outward, first through the radiative and then through the convective zone, the temperature decreases from 8 million to 7,000 K, and density decreases from 20 gm/ cm 3 to 4 x l0 -7 gm/ m 3 . It takes about 10 million years for photons to escape from the dense core and reach the surface.

A corona is a type of plasma atmosphere of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. The Latin root of the word corona means crown.

The Power

The Sun's power (about 386 billion billion megaWatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma Corana image rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convectionthan by radiation.

Because the Sun is gaseous, it rotates faster at the equator (26.8 days) than at the poles (as long as 35 days). The Sun's "surface," known as the photosphere, is just the visible 500 km- thick layer from which most of the Sun's radiation and light finally escapes, and is the place where sunspots are found. Above the photosphere lies the chromosphere (" sphere of color") that may be seen briefly during total solar eclipses as a reddish rim, caused by hot hydrogen atoms, around the Sun. Temperature steadily increases with altitude up to 50,000 K, while density drops to 100,000 times less than in the photosphere. Above the chromosphere lies the corona (" crown"), extending outward from the Sun in the form of the "solar wind" to the edge of the solar system. The corona is extremely hot millions of degrees Kelvin. The process that heats the corona is very mysterious and poorly understood, since the laws of thermodynamics state that heat energy flows from a hotter to a cooler place. Mysterious phenomena, such as this, are studied by researchers in NASA'S Space Physics Division.

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Image of the Sun with sunspots

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature. Like magnets, they also have two poles.

Image of the Sun showing the heat from the sunspots

Although they are at temperatures of roughly 3000–4500 K (2727–4227 °C), the contrast with the surrounding material at about 5780 K (5500 °C) leaves them clearly visible as dark spots, as the luminous intensity of a heated black body (closely approximated by the photosphere) is a function of temperature to the fourth power.

If the sunspot were isolated from the surrounding photosphere it would be brighter than an electric arc. Sunspots expand and contract as they move across the surface of the Sun and can be as large as 80,000 kilometers (50,000 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope. They may also travel at relative speeds ("proper motions") of a few hundred m/s when they first emerge onto the solar photosphere.

Fast Facts

Spectral Type of Star

G2 V


4.5 Billion Years

Mean Distance to Earth

150 Million Kilometers

Rotation Period (at equator)

26.8 days


695,000 Kilometers


1.99 x 103 Kilograms


Hydrogen 71%, Helium 26.5%,
Other 2.5%

Effective Surface Temperature

5.770 K

Energy Output (Luminosity)

3.83 x 10~ ergs/ sec

Solar Constant

0.1368 Watts/ cm2

Inclination of Solar Equator to Ecliptic


The Sun's satellites

Pink Sun

There are eight planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should be classified as planets and which as "smaller objects" has been the source of some controversy, but in the end it is really only a matter of definition. Pluto is no longer officially a planet but we'll keep it here for history's sake.)

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Burst around the Corner

Arms of fire coming out of the Sun

The Sun erupted with a good-sized solar flare and a coronal mass ejection (CME) on its far-side beyond the view of SDO, but the resulting strands of particle clouds as seen in extreme ultraviolet light still made for quite a show that lasted about three hours (Jan. 2, 2012). Note how a portion of the strands fall back to the Sun. It appears the force of the blast was unable, for some portion of the material, to overcome the pull of the Sun's magnetic fields. This blast was not directed at Earth.

Blossoming Blast

Arms of fire coming out of the Sun

SDO observed a beautiful prominence eruption shot off the east limb (left side) of the Sun (April 16, 2012). Such eruptions are often associated with solar flares, and in this case an M1.7 class (medium-sized) flare did occur at the same time, though it was not aimed toward Earth. In the associated movie (in extreme ultraviolet light) covering four hours of activity, some of the charged particles do not have enough force behind them to break away and they can be seen streaming back into the Sun.

Flaring Active Region

Image of the Sun sunflares

Active Region 1514 just could not contain itself as it popped off over a dozen flashes, minor eruptions, and flares over almost two days (June 27-29, 2012). There is a larger blast near the end of the video clip. The jerking of the Sun also near the end was caused by some calibration testing by the spacecraft. We'll be keeping a close eye on this one as it rotates more towards facing Earth.

Partial Solar Eclipse from SDO

Solar Eclipse

The Moon came in between the Solar Dynamics Observatory (SDO) satellite and the Sun (seen here in extreme ultraviolet light) and produced a partial solar eclipse from space. For 1 hour and 41 minutes team SDO observed the lunar transit. This event only happens a few times a year, but it does give the SDO team an opportunity to better understand the AIA instrument on SDO and give it a fine-tuning. The sharp edge of the lunar limb helps researchers measure the in-orbit characteristics of the telescope, e.g., how light diffracts around the telescope's optics and filter support grids. Once these are calibrated, it is possible to correct SDO data for instrumental effects and sharpen the images even more than before.

Credit: NASA/SDO

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Sun Spots

Image of mottled appearance around the sunpots

This image (right) shows the region around a sunspot. Notice the mottled appearance. This granulation is the result of turbulent eruptions of energy at the surface.

(Courtesy National Solar Observatory/Sacramento Peak)

Coronal Hole on the Sun

Coronal Hole looks like Big Bird

This image of a coronal hole on the sun bears a remarkable resemblance to the 'Sesame Street' character Big Bird. Coronal holes are regions where the sun's corona is dark. These features were discovered when X-ray telescopes were first flown above the Earth's atmosphere to reveal the structure of the corona across the solar disc. Coronal holes are associated with 'open' magnetic field lines and are often found at the sun’s poles. The high-speed solar wind is known to originate in coronal holes. The solar wind escaping from this hole will reach Earth around June 5-7, 2012.

Image Credit: NASA/AIA

The parts of the Sun

Sun parts

This gives a basic overview of the Sun's parts. The three major interior zones are the core (the innermost part of the Sun where energy is generated by nuclear reactions), the radiative zone (where energy travels outward by radiation through about 70% of the Sun), and the convection zone (in which convection currents circulate the Sun's energy to the surface). The flare, sunspots and photosphere, chromosphere, and the prominence are all clipped from actual SOHO images of the Sun.

Sees Massive Filament Erupt on Sun

Image of a massive filament erupt on the son

On August 31, 2012 a long filament of solar material that had been hovering in the sun's atmosphere, the corona, erupted out into space at 4:36 p.m. EDT. The coronal mass ejection, or CME, traveled at over 900 miles per second. The CME did not travel directly toward Earth, but did connect with Earth's magnetic environment, or magnetosphere, with a glancing blow. causing aurora to appear on the night of Monday, September 3.

Solar Eruption

Image of a massive filament erupt on the son

A solar eruption gracefully rose up from the sun on Dec. 31, 2012, twisting and turning. Magnetic forces drove the flow of plasma, but without sufficient force to overcome the sun’s gravity much of the plasma fell back into the sun.

The length of the eruption extends about 160,000 miles out from the Sun. With Earth about 7,900 miles in diameter, this relatively minor eruption is about 20 times the diameter of our planet.

Image Credit: NASA/SDO

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