The Sun is our nearest star!

It took thousands of years for us to realize this simple fact, but now we can use it to our advantage as we study other stars in the universe. The Sun is our nearest star. Its radiant energies light up our daytime sky and make all life possible on this planet, even from as far away as 93 million miles. Like many things in nature, the Sun consists of many different parts that influence each other and exchange both energy and matter.

Interior: Deep within its core where gravitational pressures compress and heat its gases, atoms collide so furiously that some fuse together. At temperatures of 15 million degrees Centigrade, its abundant store of hydrogen turns to helium via thermonuclear fusion. Every second, over 4 million tons of matter are converted to pure, radiant energy. Some of this energy goes into creating pressure that literally holds up the Sun against gravity. The rest leaks out of the dense core in the form of light, and deposits huge amounts of energy throughout the inside of the star. It takes many thousands of years for this light energy to make it to the surface because there is so much matter in the way. [MORE ].

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 Activity: Sunspot Cycles

Solar Magnetism and Sunspots: Believe it or not, although you cannot hold in your hand a piece of the Sun, you can explore a model of the forces that control most of its active surface. The Sun has a magnetic personality. For over 100 years, astronomers have known from direct observations that the Sun's surface has a magnetic field that is about twice as strong as the Earth's, but spread out over 10,000 times the area. We don't exactly know where it comes from. It may have been left over from the interstellar cloud that created the Sun over 4.5 billion years ago. Some astronomers think it is actually generated by the Sun itself. Over all, the Sun's field looks a lot like a bar magnet. It has a north and south polarity as all magnets do. Much of its shape can be seen during a total solar eclipse as it leaves an imprint on the Sun's outer gases, just like iron filings outline the field of a bar magnet. But there is more to the Sun's magnetism than what you might find by just looking at a bar magnet. [MORE ]

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Solar Activity Cycle: In the mid-1800s astronomers discovered from thousands of sunspot sightings that, when they tabulated and graphed them, their numbers increased and decreased over time in a repeatable cycle. These extremes represent the amplitude of the cycle. We now call this the solar activity cycle or the sunspot cycle. During the last 200 years, the time between years of maximum activity, which is called the period of the cycle, has been about 11 years, but sunspot cycles can be as short as 9 or as long as 15 years. During sunspot minimum conditions, such as the year 1996, astronomers counted fewer than 5 sunspots on the surface of the Sun at any one time. During sunspot maximum conditions, as many as 250 could be seen. On September 20, 2000 one very large sunspot group could be seen with the naked eye with the proper safety precautions. (You should never look directly at the Sun without proper shielding to avoid eye damage!). [MORE ]

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Solar Flares: When electrical circuits get crossed, you often see sparks fly and lots of smoke as the wires become heated. The electricity, carried by electrons in one wire are trying to flow one way, while the electrons from another circuit are trying to flow the opposite way in the same material. This causes the electrons to collide, and instead of an organized smooth flow, it becomes disorganized. The energy of motion (kinetic energy) of the electrons in the currents is transformed into heat energy as nearby atoms in the wire are jostled about. A very similar thing happens on the Sun, but with dramatic consequences that extend clear across the entire solar system! [MORE ]

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Coronal Mass Ejections: A few times every day during solar maximum conditions, the Sun can let loose a titanic blast of material. For days, a heated cloud of plasma can be suspended by magnetic pressures just above the photosphere in a region called the chromosphere. Then, for reasons not fully understood, this billion-ton cloud can become unhinged and be propelled away from the Sun. The cloud may only have started off as a gentle puff of plasma. As it enters the lower reaches of the solar corona, the Sun's outer atmosphere, the cloud expands and accelerates enormously to speeds of millions of kilometers per hour. Within a few days, the cloud has reached the orbit of the Earth, while parts of the cloud itself still envelope the orbits of Venus and Mercury. In time, these coronal mass ejections, or CMEs as they are called, cause interplanetary space to be filled with a changing patina of cloud fragments and magnetic field blobs, millions of kilometers across, and flowing outward in a great pinwheeling pattern, out beyond the orbit of Pluto. [MORE ]

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Solar Activity Cycle: All you have to do is look at a picture of a total solar eclipse and it is pretty obvious that the Sun's influences do not stop at its surface. For much of the 20th century, astronomers have suspected that the outer atmosphere of the Sun, called its corona, is not the end of the line. By the 1950s, it was pretty clear from studying mathematical models of systems such as stars, that a corona is not stable. It has to be constantly leaking away into space like the steam rising from a pot of boiling water. The first convincing proof of such a solar wind was when astronomers discovered that the tails of comets didn't point exactly away from the Sun. They were cocked at a 5-10 degree angle away from this direction. Because comet tails are composed of gases boiled off from the comet, they acted like million kilometer-long windsocks in the solar system. There had to be a wind from the Sun pushing them ever so slightly out of kilter. It took the technological advances of the Space Age to confirm this flow of particles in the early 1960s. Since then, astronomers and space scientists have learned a lot about this wind, and the winds from other stars too! [MORE ]

 Watch a CME Movie

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 Activity: CME Activity

 Activity: How common are CMEs?

 Activity: Density and Mass of a CME

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