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The Amazing Structure of the Sun

Statistics on the Sun stretch the imagination. Sizes, distances and temperatures are far beyond our experience.

The diameter of the Sun is about 1,390,000 km. The average diameter of the Earth is 12,740 km. About 109 Earths could be placed side-by-side along the diameter of the Sun. Students can model this size comparison The Athena Space Curriculum.

Sun-Earth size comparison illustration.

The volume of the Sun is unimaginable. The volume is 1.406 x 1018 km3. Approximately 1,300,000 Earths could fit inside the Sun. Its mass is about 1.989 x 1030 kg. This is the mass of about 300,000 Earths.

The structure of the Sun is separated into several regions: Interior, Photosphere, Chromosphere, Transition Region, Corona, and Solar Wind.

Courtesy of SOHO

For a simpler structure see the Athena Solar Anatomy. Examine the Sun From Core to Corona. William Shatner narrates a short movie on the Sun (5 megabyte file).

The Interior: The Core and the Radiative Layer

The core occupies the first 25% of the distance from the center.

  • The temperature at the core is enormous. The temperatures range between 15,000,000°C (27,000,000°F) at the center to 7,000,000°C (12,600,000°F) at the outer edge of the core. This is very hot. Metal that is hot enough to glow white is only around 6000° C.
  • The density changes dramatically. It is 8 times the density of gold (160g/cm3) at the center. At the outer edge of the core the density is about the density of gold (20g/cm3).

At these temperatures and pressures the hydrogen is fused into helium and elements are in the plasma state. (For a FLASH animation of this process visit Basic Solar Facts and find the Proton-Proton Cycle).

Surrounding the core is the radiative zone.

  • The radiation that escapes the core is mostly x-rays. The Sun's energy output is about 4 x 1026 Watts of energy. For comparison, the average U.S. power consumption for 2001 was estimated at 1012 Watts.
  • According to Einstein's equation, E=mc2, this amount of energy is equal to about 4.4 billion kilograms of matter being completely changed to energy every second. On Earth that amount of matter would weigh about 1 million tons.
  • This radiation takes about 1 million years to find its way out of the radiative layer, even traveling at the speed of light, due to collisions between light and matter within the radiative layer.

Between the Radiative zone and the Convective zone is an Interface layer. It is now believed that there is a magnetic dynamo in this layer that generates the Sun's magnetic field. (see also Why Do Sunspots and CMEs Occur)

In the final 200,000 km (124,000mi.) to the Photosphere, energy is carried by convection in the Convective zone.

  • The temperature drops from 2,000,000°C (3,600,000°F) to 5,700°C (10,000°F).
  • The density drops from 0.2 g/cm3 to 0.0000002 g/cm3.
  • Hot plasma rises and cooler plasma sinks, creating 'cyclones' as the Sun rotates. This animation (3 megabyte file) of convection within the interior of the Sun shows hotter plasma moving upward and cooler (darker) plasma sinkin.

The Photosphere

As these bubbles of upwelling, hot plasma reach the surface of the Photosphere, bright spots or granules are created.

Close-up of a Sunspot
Click the image above to see an animation of this process. The brighter spots are Solar granules and the large, dark spots are Sunspots.

These granules and sunspots are features of the Photosphere which is a thin layer only 100 km (62 miles) thick. We are most familiar with this layer because it is the visible surface of the Sun. It produces most of the white light we see.

The Chromosphere

Above the Photosphere is the Chromosphere. The temperature rises from about 6000°C (10,800°F) to about 20,000°C (36,000°F). At this temperature hydrogen emits a reddish light. Solar flares and eruptions are common in this region.

Transition Region

Between the Chromosphere and the Corona is a thin, irregular layer that is poorly understood. This layer, called the Transition Region, is being examined by TRACE (Transition Region And Coronal Explorer). Within this region the temperature rapidly increases from 20,000°C to 1,000,000°C. Scientists are studying this region to increase their understanding of the processes that cause this temperature increase.

The Corona

Above the Chromosphere is the Corona. The temperature in the Corona is about 1,000,000°C (1,800,000°F). Hydrogen and other elements are ionized and blown into space as a continuous outpouring of plasma known as the Solar Wind.

The image below captures a sweeping prominence as it erupts from the Sun. Prominences are huge arcs of gas injected into the corona. They can reach 200,000 to 300,000 km into the corona. Prominences can be quite stable and last for days. When they erupt they contribute additional energy to the solar wind. The Solar and Heliospheric Observatory (SOHO) took the image below using the Extreme ultraviolet Imaging Telescope (EIT). This instrument "looks" at the Sun at four different wavelengths of light. All of these wavelengths are in the ultraviolet region of the Electromagnetic Spectrum. The four wavelengths in angstroms (10-8cm) are 304Å, 284Å, 195Å and 171Å. The image below shows the Sun at 304Å. Emission in this wavelength shows the upper chromosphere at a temperature of about 60,000°C. The hottest areas appear almost white, while the darker red areas indicate cooler temperatures.

Image of the Sun taken bye the Extreme Imaging Telescope in the SOHO spacecraft.
Source/Credits: Solar & Heliospheric Observatory (SOHO). SOHO is a project of international cooperation between ESA and NASA.

Sometimes eruptions are very large and earn the name Coronal Mass Ejection (CME). SOHO captured these images of a CME. The central disk of the Sun is covered to screen the instrument from the intense radiation of the Sun and allow the instrument to detect the less intense corona and CMEs. The white circle shows the size of the Sun.

Image of a CME taken by the SOHO spacecraft.
Courtesy of SOHO/LASCO consortium. SOHO is a project of international cooperation between ESA and NASA.

The bright white region coming from the central disk toward the right is a Coronal Mass Ejection. The two, smaller white lines coming from the bottom right are sun grazing comets that were quickly annihilated by the heat of the Sun. Click on the above image to see a short movie of the CME and the comets.

An additional tutorial on the Sun is available.

Solar Wind and the more energetic Coronal Mass Ejections are extremely important to the Earth and to our life and society. It is time to learn more about the Solar Wind and CMEs and Why Sunspots and CMEs Occur.

Next Step: Solar Wind and CMEs »

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