Lesson Objectives

• Describe the layers of the Sun.
• Describe the surface features of the Sun.

Vocabulary

• chromosphere
• convection zone
• corona
• nuclear fusion
• photon
• photosphere
• plasma
• radiative zone
• solar flare
• solar prominence

Introduction

Consider Earth, the Moon, and all the other planets and satellites in the solar system. The mass of all of those objects together accounts for only 0.2% of the total mass of the solar system. The rest, 99.8% of all the mass in the solar system, is the Sun!

The Sun (Figure below) is the center of the solar system and the largest object in the solar system. This nearby star provides light and heat and supports almost all life on Earth.

Layers of the Sun

The Sun is a sphere, composed almost entirely of the elements hydrogen and helium. The Sun is not solid or 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 Sun’s central core is plasma with a temperature of around 27 million^oC. 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. Nuclear fusion in stars is discussed more in the Stars, Galaxies, and the Universe chapter.
• The radiative zone, just outside the core, has a temperature of about 7 million^oC. 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 (1e): http://www.youtube.com/watch?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 (1e): http://www.youtube.com/watch?v=S6VRKKh6gyA (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 6,700°C. The photosphere has several different colors; oranges, yellow and reds, giving 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 4,000°C to about 10,000°C. 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 corona is the outermost plasma layer — It is the Sun’s halo or ‘crown.’ The corona’s temperature of 2 to 5 million°C is much hotter than the photosphere (Figure below).

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

Surface Features

The Sun’s surface features are quite visible, but only with special equipment. For example, sunspots are only visible with special light-filtering lenses.

Sunspots

The most noticeable surface feature of the Sun are cooler, darker areas known as sunspots (Figure below). Sunspots are located where loops of the Sun’s magnetic field break through the surface and disrupt the smooth transfer of heat from lower layers of the Sun, making them cooler and darker and marked by intense magnetic activity. Sunspots usually occur in pairs. When a loop of the Sun’s magnetic field breaks through the surface, a sunspot is created where the loop comes out and where it goes back in again.

Solar Flares

There are other types of interruptions of the Sun’s magnetic energy. If a loop of the sun’s magnetic field snaps and breaks, it creates solar flares, which are violent explosions that release huge amounts of energy (Figure below).

A movie of the flare is seen here: http://www.youtube.com/watch?v=MDacxUQWeRw.

A strong solar flare can turn into a coronal mass ejection (Figure below).

A solar flare or coronal mass ejection releases streams of highly energetic particles that make up the solar wind. The solar wind can be dangerous to spacecraft and astronauts because it sends out large amounts of radiation that can harm the human body. Solar flares have knocked out entire power grids and disturbed radio, satellite, and cell phone communications.

KQED: Journey Into the Sun

The Solar Dynamics Observatory is a NASA spacecraft launched in early 2010 is obtaining IMAX-like images of the sun every second of the day, generating more data than any NASA mission in history. The data will allow researchers to learn about solar storms and other phenomena that can cause blackouts and harm astronauts. Learn more at: http://science.kqed.org/quest/video/quest-quiz-the-sun/.

Solar Prominences

Another highly visible feature on the Sun is solar prominences. If plasma flows along a loop of the Sun’s magnetic field from sunspot to sunspot, it forms a glowing arch that reaches thousands of kilometers into the Sun’s atmosphere. Prominences can last for a day to several months. Prominences are also visible during a total solar eclipse.

Solar prominences are displayed in this video from NASA’s Solar Dynamics Observatory (SDO): http://www.youtube.com/watch?v=QrmUUcr4HXg.

Most of the imagery comes from SDO’s AIA instrument; different colors represent different temperatures, a common technique for observing solar features. SDO sees the entire disk of the Sun in extremely high spatial and temporal resolution, allowing scientists to zoom in on notable events such as flares, waves, and sunspots.

Solar Dynamics Observatory

The video above was taken from the SDO, the most advanced spacecraft ever designed to study the Sun. During its five-year mission, SDO will examine the Sun’s magnetic field and also provide a better understanding of the role the Sun plays in Earth’s atmospheric chemistry and climate. Since just after its launch on February 11, 2010, SDO is providing images with clarity 10 times better than high-definition television and will return more comprehensive science data faster than any other solar observing spacecraft.

Lesson Summary

• The mass of the Sun is 99.8% of the mass of our solar system.
• The Sun is mostly made of hydrogen with smaller amounts of helium in the form of plasma.
• The main part of the Sun has three layers: the core, radiative zone, and convection zone.
• The Sun’s atmosphere also has three layers: the photosphere, the chromosphere, and the corona.
• Nuclear fusion of hydrogen in the core of the Sun produces tremendous amounts of energy that radiate out from the Sun.
• Some features of the Sun’s surface include sunspots, solar flares, and prominences.

Review Questions

1. In what way does the Sun support all life on Earth?

2. Which two elements make up the Sun almost in entirety?

3. Which process is the source of heat in the Sun and where does it take place?

4. Why would human astronauts on a trip to Mars need to be concerned about solar wind? What is solar wind?

5. Describe how movements in the convection zone contribute to solar flares.

6. Do you think fusion reactions in the Sun’s core will continue forever and go on with no end? Explain your answer.

Further Reading / Supplemental Links

• To find these videos for download, check out: http://www.nasa.gov/mission_pages/sdo/news/briefing-materials-20100421.html and http://svs.gsfc.nasa.gov/Gallery/SDOFirstLight.html.
• Subscribe to NASA’s Goddard Shorts HD podcast: http://svs.gsfc.nasa.gov/vis/iTunes/f0004_index.html.
• To learn more about the SDO mission, visit: http://sdo.gsfc.nasa.gov/.
• To learn about an older solar mission, SOHO, see: http://sohowww.nascom.nasa.gov/.

Points to Consider

• If something were to suddenly cause nuclear fusion to stop in the Sun, how would we know? When would we know?
• Are there any types of dangerous energy from the Sun? What might be affected by them?
• If the Sun is made of gases such as hydrogen and helium, how can it have layers?

Going Further – Applying Math

Have would you measure something that you cannot reach? The answer is that you can use simple geometry. We can measure the diameter of the Sun, even though we cannot go to the Sun and even though the Sun is far too large for a human being to measure. To measure the Sun we use the rules of similar triangles. The sides of similar triangles are proportional to each other. By setting up one very small triangle that is proportional to another very large triangle, we can find an unknown distance or measurement as long as we know three out of four of the parts of the equation. If you make a pinhole in an index card and project an image of the Sun onto a clipboard held 1 meter from the index card, the diameter of our projected image of the Sun will be proportional to the true diameter of the Sun. Here’s the equation: s / d = S / D, where s = diameter of the projected image of the Sun, S = true diameter of the Sun. The calculation also requires you to know the true distance between the Earth and the Sun, D = 1.496 x 10⁸ km and the distance (d = 1 meter) between the clipboard and the index card. Before you can correctly solve this equation, you will need to be sure all of your measurements are in the same units – in this case, change all your measurements to km. Try this out and see how accurately you can measure the true diameter of the Sun.