The Sun

Did you know that the nearest star is only 149,600 km from us? It's a huge ball of hydrogen gas, more than 300,000 times more massive than our Earth; and without it life could never have existed on our planet. This star is our Sun. It's almost 5 billion years old, which may seem really old to the rest of us, but for a star it's only about middle-aged. But what, you may ask, does it look like? What's it made of?
The Sun as seen in X-rays (courtesy of Imagine the Universe).

Well, the Sun is a rather ordinary star. It's not very large or small, old or young. It's your basic G2 V star, like a vast amount of other stars in our galaxy. The Sun has many layers. The first is the core at the center of the Sun. The temperature at the core is on the order of 16 million degrees Kelvin, which is hot enough to maintain nuclear fusion. The vast amount of energy released at the core is enough to keep the Sun from collapsing, as well as keeping the Sun hot enough to remain gaseous. This amount of energy is equivalent to exploding 100 billion tons of TNT each second! Between the core and the surface of the Sun are two other regions, the radiative and con-vective zones. Traveling outward from the core between these two regions, the temperature drops to 8 million degrees Kelvin, and then to a mere 7,000 degrees Kelvin (but still nice and toasty!).
The Parts of a Star (courtesy of Imagine the Universe).

This leads us to the surface of the Sun, also known as the photosphere. This is only the visible layer from which most of the radiation and light escape from the Sun. The photosphere is only 500 km thick. The photosphere is also where sunspots are located, what appear to black spots blemishing the otherwise yellow-orange surface of the Sun. Above the photosphere lies the chromosphere, or sphere of color. This is the part that can be seen briefly during a solar eclipse. It appears as a reddish rim, and that's caused by all the hot hydrogen atoms surrounding the Sun. Moving outward, beyond the chromosphere is the corona. The corona can also be described as the solar wind that is nothing more than a stream of charged particles racing away from the Sun. It extends out from the sun towards the outer reaches of our solar system. It is very hot, but the process that heats it is still mysterious and not understood. We have often hear of solar flares, although most of us don't understand what they are or what causes them. A large flare is the effect of a huge ejection of mass from the corona of the Sun. Why this happens, astronomers are still not certain.
This image shows a coronal mass ejection taken by SOHO/LASCO on April 7, 1997. (courtesy of NASA)

Coronal mass ejections are balloon-sped bursts of the solar wind rising above the corona. Thatąs what the bright spots are in the picture above. The solar plasma is heated to tens of millions of degrees, and the charged particles are accelerated to near light speed, faster than the solar wind. Each of these coronal mass ejections releases up to 220 billion pounds of material, and the speed at which the mass is ejected can reach 2 million miles per hour. To date, solar flares and coronal mass ejections are the largest explosions in our solar system, close to the power of one billion hydrogen bombs. (http://helios.gsfc.nasa.gov/cme.html)

Because of the explosive nature of our Sun, scientists are interested to learn more about it. Large explosions, such as solar flares and coronal mass ejections, can cause a great deal of damage here on Earth. These explosions can cause power blackouts, disruptions in communications, radiation danger to astronauts, and destruction of satellites in orbit. Due to the potential danger and lack of understanding of such events, a team of UK, US and Japanese scientists are building a new satellite, Solar-B, to be launched into orbit in the autumn of 2005. The satellite will include an optical telescope to measure the strength and direction of the magnetic field, as well as determining the evolution of magnetic features over time. There will also be an X-ray telescope attached to provide temperature images of the Sun's corona. The final instrument is an extreme-ultraviolet spectrometer that will image explosions in the Sun's atmosphere and gather data on the velocities, temperatures, and densities of the gas. (SSE Press Release April 5, 2001: http://sse.jpl.nasa.gov/whatsnew/pr/010405G.html)

For more information about the Sun, visit www.nasa.gov on the web. For more pictures of the Sun, visit the photo gallery at http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-solar.html

Laura Guirl -- TA Fall 2001
Last Edited -- 9/15/2001

Did you know that the nearest star is only 149,600 km from us? It's a huge ball of hydrogen gas, more than 300,000 times more massive than our Earth; and without it life could never have existed on our planet. This star is our Sun. It's almost 5 billion years old, which may seem really old to the rest of us, but for a star it's only about middle-aged. But what, you may ask, does it look like? What's it made of?	The Sun as seen in X-rays (courtesy of Imagine the Universe).
Well, the Sun is a rather ordinary star. It's not very large or small, old or young. It's your basic G2 V star, like a vast amount of other stars in our galaxy. The Sun has many layers. The first is the core at the center of the Sun. The temperature at the core is on the order of 16 million degrees Kelvin, which is hot enough to maintain nuclear fusion. The vast amount of energy released at the core is enough to keep the Sun from collapsing, as well as keeping the Sun hot enough to remain gaseous. This amount of energy is equivalent to exploding 100 billion tons of TNT each second! Between the core and the surface of the Sun are two other regions, the radiative and con-vective zones. Traveling outward from the core between these two regions, the temperature drops to 8 million degrees Kelvin, and then to a mere 7,000 degrees Kelvin (but still nice and toasty!).	The Parts of a Star (courtesy of Imagine the Universe).
This leads us to the surface of the Sun, also known as the photosphere. This is only the visible layer from which most of the radiation and light escape from the Sun. The photosphere is only 500 km thick. The photosphere is also where sunspots are located, what appear to black spots blemishing the otherwise yellow-orange surface of the Sun. Above the photosphere lies the chromosphere, or sphere of color. This is the part that can be seen briefly during a solar eclipse. It appears as a reddish rim, and that's caused by all the hot hydrogen atoms surrounding the Sun. Moving outward, beyond the chromosphere is the corona. The corona can also be described as the solar wind that is nothing more than a stream of charged particles racing away from the Sun. It extends out from the sun towards the outer reaches of our solar system. It is very hot, but the process that heats it is still mysterious and not understood. We have often hear of solar flares, although most of us don't understand what they are or what causes them. A large flare is the effect of a huge ejection of mass from the corona of the Sun. Why this happens, astronomers are still not certain.	This image shows a coronal mass ejection taken by SOHO/LASCO on April 7, 1997. (courtesy of NASA)
Coronal mass ejections are balloon-sped bursts of the solar wind rising above the corona. Thatąs what the bright spots are in the picture above. The solar plasma is heated to tens of millions of degrees, and the charged particles are accelerated to near light speed, faster than the solar wind. Each of these coronal mass ejections releases up to 220 billion pounds of material, and the speed at which the mass is ejected can reach 2 million miles per hour. To date, solar flares and coronal mass ejections are the largest explosions in our solar system, close to the power of one billion hydrogen bombs. (http://helios.gsfc.nasa.gov/cme.html) Because of the explosive nature of our Sun, scientists are interested to learn more about it. Large explosions, such as solar flares and coronal mass ejections, can cause a great deal of damage here on Earth. These explosions can cause power blackouts, disruptions in communications, radiation danger to astronauts, and destruction of satellites in orbit. Due to the potential danger and lack of understanding of such events, a team of UK, US and Japanese scientists are building a new satellite, Solar-B, to be launched into orbit in the autumn of 2005. The satellite will include an optical telescope to measure the strength and direction of the magnetic field, as well as determining the evolution of magnetic features over time. There will also be an X-ray telescope attached to provide temperature images of the Sun's corona. The final instrument is an extreme-ultraviolet spectrometer that will image explosions in the Sun's atmosphere and gather data on the velocities, temperatures, and densities of the gas. (SSE Press Release April 5, 2001: http://sse.jpl.nasa.gov/whatsnew/pr/010405G.html) For more information about the Sun, visit www.nasa.gov on the web. For more pictures of the Sun, visit the photo gallery at http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-solar.html