Stars and Galaxies The Sun

The Sun’s Layers More than 99% of all matter in the solar system is in the Sun. Within the universe, the Sun is just an ordinary star: an enormous ball of gas that produces energy by nuclear fusion. Hydrogen (H) is fused into helium (He) in its core. Energy produced in the core then travels outward through the radiation zone and the convection zone (where gases circulate in giant swirls). Finally, it passes into the Sun’s atmosphere.

The Sun’s Atmosphere The lowest layer of the Sun’s atmosphere and the layer that gives off light is called the photosphere. Often called the surface of the Sun, although the surface is not a smooth feature Temperatures in the photosphere are about 6,000 K.

Above the photosphere is the chromosphere. Extends upward to about 2,000 km above the photosphere. A transition zone occurs between 2,000 km and 10,000 km above the photosphere.

Above the transition zone is the Sun’s corona . The corona is the largest layer of the Sun’s atmosphere, and it extends millions of kilometers into space. Temperatures in the corona can reach as high as 2 million K. Charged particles escape the corona and move through space as the solar wind. During a total solar eclipse, the corona becomes visible as the moon blocks the Sun’s lower layers:

Tremendous pressures and extreme temperatures (10 million K) in the Sun’s core cause nuclear fusion to occur: hydrogen fuses into helium. Energy produced by fusion in the Sun’s core travels outward by radiation and convection. The Sun’s atmosphere (photosphere, chromosphere, and corona) shines by the energy produced in the core. The yellow appearance of the light shining from the surface of the Sun indicates the Sun’s surface temperature: about 6,000 K.

Surface Features Although the Sun looks to be a smooth object it has many different surface features.

Sun Spots Areas of the Sun’s surface that appear dark because they are cooler than the surrounding areas are called sunspots. We have been studying sunspots since Galileo first discovered them with his telescope. The frequency of sunspots varies over an 11-year cycle.

Sun’s Rotation Scientists observe that sunspots move, therefore they have concluded that the Sun rotates. It does not rotate as a solid object like the Earth, instead it rotates faster at the equator than at the poles. At the equator it sunspots take about 25 days to complete one rotation whereas at the poles they take about 33 days.

Prominences and Flares Sunspots are related to several features on the Sun’s surface. Intense magnetic fields associated with sunspots might cause prominences which are huge, arching columns of gas. Some prominences blast material from the sun into space at speeds ranging from 600 km/s to more than 1,000 km/s

When gases near the sunspots brighten suddenly, shooting outward at high speed these violent eruptions are called solar flares.

CMEs Coronal Mass Ejections (CMEs) are powerful eruptions on the Sun’s surface. They are huge bubbles of gas threaded with magnetic field lines that eject from the Sun over several hours (kind of like a Sun burp). They can disrupt the flow of the solar wind and produce disturbances that strike Earth. This can affect telecommunications systems when severe. These are associated with solar flares and prominences but can occur in the absence of these. The frequency of CMEs varies with the sun spot cycle.

By P. Charbonneau and O.R. White of the High Altitude Observatory (NCAR) in Boulder Colorado. This series of pictures below of a CME is taken from their slide show. Decaying sunspot 1060 delivered a parting shot on April 8th. The active region’s magnetic field erupted, sparking a B3-class solar flare and hurling a faint coronal mass ejection (CME) almost directly toward Earth. Geomagnetic disturbances are possible when the cloud arrives on April 11th or 12th. The Solar and Heliospheric Observatory (SOHO) captured this image of a coronal mass ejection (CME) -- defined by the thin circle of light around the left and bottom of the sun – on Nov. 21, 2012, at 1:42 p.m. EST as it began to move away from the sun into space. This image from SOHO is what's known as a coronagraph, in which the bright light of the sun is blocked in order to make the dimmer structures in the sun's atmosphere, or corona, visible. Credit: ESA&NASA/SOHO

An Average Star? The Sun is considered an average star: it is middle-aged, its absolute magnitude is typical, and it shines with a yellow light. What is unusual about the Sun is that it is not part of a system where two or more stars orbit each other. When two stars orbit each other they make up a binary system. Three or more stars can make up multiple star systems.

Binary star systems can be detected because one star occasionally eclipses the other. The total amount of light the system gives off varies, which changes the apparent magnitude. Similar techniques are being used by the Kepler mission to detect the transit of planets in front of their stars.

The closest star system to the Sun is Alpha Centauri, which is a triple star system. Alpha Centauri A and B are medium-sized stars similar to the Sun. The tiny third star of the system, Proxima Centauri, is actually the closest star to our solar system (4.24 ly).