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An Overview of Astronomy

A Concise Guide to the Universe


The Sun

A. The sun is the closest star to the Earth. It is a giant ball of gas without any solid surface. When we look at the sun we only see the surface. The processes that take place in its interior determine the various aspects of the sun's surface. There are three primary parts to the sun's surface: the photosphere, the chromosphere and the corona.

  1. The photosphere is the visible surface of the sun. It is not a solid surface, but made of a layer of glowing gas about 500 Km deep. It has a temperature of approximately 6000K. It is actually a collection of many small cells. This pattern of bright cells surrounded by darker regions is called granulation. The cells are caused by heated gases rising from the center of the sun, cooling off, then going back down. This type of motion is called a convention current.
  2. The chromosphere lies above the photosphere, and is a layer of gas approximately 10,000 Km deep. It is about 1000 times less bright than the photosphere, and so can only be seen during a solar eclipse. The chromosphere contains elongated flame-like structures called spicules, which last for 5 to 15 minutes. These spicules appear to be cool regions extending up into the much hotter corona.
  3. The sun's atmosphere above the chromosphere is called the corona, from the Greek word for crown. It extends as far as twelve solar radii from the surface of the sun. It has a temperature of between 500,000 and 2,000,000 K. The density of the gas in the corona must be very small for the atoms to attain such temperatures and not emit a great deal of blackbody radiation.

The outer corona is so hot and the particles move so fast that the sun cannot hold them in its orbit. The vast streams of particles that leave the surface of the sun are called the solar wind. It contains mostly protons and electrons, but also carries heavier particles. The particles of the solar wind that are caught in Earth's magnetic field cause the auroras that light up the northern and southern skies. The particles travel like beads on a wire, and converge where the magnetic field lines converge, at the north and south poles, to create disturbances in the atmosphere that then glow in brilliant colors.

B. Sunspots are regions of intense magnetic activity on the surface of the sun. The sunspots actually glow very brightly, but compared to the photosphere, they are cooler by 1000 K and are relatively dark. Sunspots form in pairs, one acting like the positive pole of a magnet, the other acting like the negative pole. Galileo, who saw sunspots traversing the surface, found the first evidence that the sun rotated on its axis. The sun rotates on its axis every 25 days. Sunspot activity peaks and ebbs on an eleven-year-cycle. The eleven-year cycle is explained as a result of the sun exhibiting differential rotation.. The equator rotates faster than the North or South Poles. Therefore, the magnetic fields get wound up and tangled. When the fields burst through the surface. they form sunspot pairs. Every eleven years. the sunspots reverse the direction of the magnetic points. This theory of how sunspots form is called the Babcock model.

C. A solar flare is a violent eruption on the surface of the sun. Flares can release as much energy as one billion atomic bombs, and can reach many times the Earth's radius out into the solar system. Their shape is determined by magnetic fields on the surface of the sun.

D. A solar prominence is a large archlike or eruptive stream of gas.


An Overview of Astronomy

A Concise Guide to the Universe



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