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14.1: Interior of the Sun

Difficulty Level: At Grade Created by: CK-12
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Can you visit the Sun?

Of course not. In Greek mythology, Icarus, got too close and his wax wings melted. Today, we have other ways to see the Sun. Spacecraft take photos and some have instruments that allow us to study the interior. Unlike Icarus, we don't need to worry about our wax wings melting.

Layers of the Sun

The Sun is a sphere, composed almost entirely of the elements hydrogen and helium. The Sun is not solid, nor is it a typical gas. Most atoms in the Sun exist as plasma, a fourth state of matter made up of superheated gas with a positive electrical charge.

Internal Structure

Because the Sun is not solid, it does not have a defined outer boundary. It does, however, have a definite internal structure with identifiable layers (Figure below). From inward to outward they are:

The layers of the Sun.

  • The Sun’s central core is plasma with a temperature is around 27 millionoC. At such high temperatures hydrogen combines to form helium by nuclear fusion, a process that releases vast amounts of energy. This energy moves outward, towards the outer layers of the Sun.
  • The radiative zone, just outside the core, has a temperature of about 7 millionoC. The energy released in the core travels extremely slowly through the radiative zone. A particle of light, called a photon, travels only a few millimeters before it hits another particle. The photon is absorbed and then released again. A photon may take as long as 50 million years to travel all the way through the radiative zone.
  • In the convection zone, hot material from near the radiative zone rises, cools at the Sun’s surface, and then plunges back downward to the radiative zone. Convective movement helps to create solar flares and sunspots.

The first video describes the basics of our Sun, including how it is powered by nuclear reactions: http://www.youtube.com/watch?feature=player_profilepage&v=JHf3dG0Bx7I (8:34).

The second video discusses what powers the sun and what is its influence on Earth and the rest of the solar system: http://www.youtube.com/watch?v=S6VRKKh6gyA&feature=related (8:25).

The Outer Layers

The next three layers make up the Sun’s atmosphere. Since there are no solid layers to any part of the Sun, these boundaries are fuzzy and indistinct.

  • The photosphere is the visible surface of the Sun, the region that emits sunlight. The photosphere is relatively cool — only about 6,700oC. The photosphere has several different colors, including oranges, yellow and reds. This characteristic gives it a grainy appearance.
  • The chromosphere is a thin zone, about 2,000 km thick, that glows red as it is heated by energy from the photosphere (Figure below). Temperatures in the chromosphere range from about 4,000oC to about 10,000oC. Jets of gas fire up through the chromosphere at speeds up to 72,000 km per hour, reaching heights as high as 10,000 km.

The chromosphere.

  • The corona is the outermost plasma layer. It is the Sun’s halo or "crown." The corona’s temperature of 2 to 5 millionoC is much hotter than the photosphere (Figure below).

(a) During a solar eclipse, the Sun’s corona is visible extending millions of kilometers into space. (b) The corona and coronal loops in the lower solar atmosphere taken by the TRACE space telescope.

The movie "Seeing a Star in a New Light" can be seen here: http://sdo.gsfc.nasa.gov/gallery/youtube.php.


  • The Sun is made mostly of plasma, a fourth state of matter made up of superheated gas with a positive electrical charge.
  • At the Sun's center is plasma, where nuclear fusion takes place. The radiative zone is outside the core. The convection zone, where convection takes place, is located outward from that.
  • The photosphere is the visible surface of the Sun, where sunlight is emitted from. The reddish chromosphere is heated by the photosphere and the outer corona is the Sun's crown.


Use this resource to answer the questions that follow.


1. What two elements make up the Sun?

2. What is our Sun?

3. What is the Sun's surface temperature?

4. What process powers the Sun?

5. What state occurs in the core of the Sun?

6. How much energy is produced by the Sun?


1. The Sun is very dense, so is there solid matter at the center? Why or why not?

2. What are the layers of the Sun and what are their characteristics?

3. What powers the Sun?

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chromosphere Thin layer of the Sun’s atmosphere that lies directly above the photosphere; glows red.
convection zone Layer of the Sun that surrounds the radiative zone where energy moves as flowing cells of gas.
corona Outermost layer of the Sun; a plasma that extends millions of kilometers into space.
nuclear fusion The merging together of the nuclei of atoms to form new, heavier chemical elements; huge amounts of nuclear energy are released in the process.
photon A particle of light.
photosphere The visible surface of the Sun.
plasma A high energy, high temperature form of matter. Electrons are removed from atoms, leaving each atom with a positive electrical charge.
radiative zone Layer of the Sun immediately surrounding the core where energy moves atom to atom as electromagnetic waves.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 15, 2016
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