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The Sun

radius: 6.96 x10*8 meters (100 times Earth's radius)
mass: 1.99 x10*30 kg (330,000 times Earth's mass)
average density: 1410 kg/m*3 (about the density of water)
surface temperature: 5780K (about 10,000 degrees F)
luminosity: 3.86 x 10*26 W (4-trillion-trillion 100W light bulbs)
angular size: 0.5' (about the size of the tip of the little finger held at arm's length)
average distance from Earth: 8.3 light minutes (the distance light travels in 8.3 minutes)
or 1 Astronomical Unit
or 93 million miles
or 150 million kilometers

The Sun is the star closest to us. It is the center of our solar system; all the planets orbit the Sun in elliptical paths. The Sun is the powerhouse of energy that keeps the solar system running, and it produces the source for order according to the laws of physics.

As with all stars, the Sun is, in general, a big, burning ball of gas. The gas our Sun burns is hydrogen. It is a G-type star on the Hertzsprung-Russell scale of spectra versus luminosity. The Sun's luminosity is about the same as 4-trillion-trillion 100 Watt light bulbs buming each second. That figures to be an apparent magnitude of -26.8 and an absolute magnitude of 4.8.

Since the Sun is the star we know best, it is the star we use as a standard for study of other stars. We can directly observe what is going on with the Sun and can apply what we see to other stars, giving us ideas of stellar processes and evolution.

How do we know what we know about the Sun? We cannot go to the Sun. Even if we could, its temperature is about 6000 Kelvin (about 10,000 degrees Fahrenheit) and it has no solid surface. We would not survive. Instead, we have to determine what is going on in the Sun using indirect means.

The Standard Solar Model

We have the Standard Solar Model. Examining the Sun layer by layer, we develop theories as to what is happening, We observe what is going on on the outside of the top layer, and using those observations, the laws of physics, and a bit of common sense, we can determine what conditions must be on the bottom.

Once we determine what the conditions must be on the bottom of the first layer, we go to the next layer, since the bottom of the first layer is the top of the second layer. Then we go to the next layer, and on and on, until we get to the core.

The Solar Core

The core of the Sun consists of fusing hydrogen. The fusion process is what gives the Sun its energy. It fuses 600 million tons of hydrogen into 595 million tons of helium per second. That's a lot of energy!

The energy starts its million-year trek from the core to the surface through a series of processes. First it is radiated. The energy transfer through the radiation zone is fast, so fast that it begins to build up to the point where it becomes opaque to radiation. The electrons created from the fusion process become so dense they have to use another means to be transported to the "surface" from that point.

The next energy transport is that of convection. Convection is the transfer of energy through the movement of mass. The mass in this case is the dense amounts of electrons. It is hotter on the side toward the core, and cooler toward the "surface." Heat tends to rise to a cooler region, where it becomes cooled and sinks. Watch water boil; it has the same effect. The convection zone gathers the hot energy from the radiation zone and "boils" it to the cooler outside layers of the Sun.

The Photosphere

Once the hot energy is taken to the top of the convection zone, the energy is given off to the photosphere. This is the layer which represents the "surface" from our vantage point of Earth. Energy is released in many forms from this point: heat, light at various wavelengths, and particles. The photosphere is the layer of the Sun which contains visible markings such as sunspots and granulation. The photosphere is cooler than the convection zone, but not as cool as the next layer.

Sun photo shot with 100 ASA Focal Color film at 1/125s, C-8,
aluminized mylar solar filter, Mayetta, Kans. 05/18/99.

The Chromosphere

The chromosphere is the next layer of the Sun. This is the layer close to the solar "surface" that we see during an eclipse. We consider the chromosphere part of the solar atmosphere. It, too, has many features we like to look at, such as spicules.

Next is the transition zone. We do not fully understand what is happening in this layer of the solar atmosphere, but temperatures rise drastically in this zone. High amounts of energy are released from this zone into the next layer.

The Corona

The corona is the next layer of the solar atmosphere. During solar eclipses, the corona illuminates the area around the Sun as an eerie glow. The corona can restrict the energy flow from the Sun, or allow free-flowing particles to escape through coronal holes at one million tons per second.

The ftee-flowing particles coming from the coronal holes go into the solar wind, which sends the energy and particles out into the solar system, During high solar activity, many coronal holes are detected. Particles the Sun emits interact with the geomagnetic fields of planets. The interaction sometimes causes aurora.

The Solar Wind

The solar wind is the last part of the solar atmosphere. It extends past the farthest reaches of the solar system and is the result of the high temperature of the corona.
Escaping electromagnetic radiation (light) travels at the speed of about 500 km/s. At this speed they reach Earth in just a few days.

The Sun is the powerhouse of our solar system. Without it, we would not exist.

Brenda Culbertson
Mayetta, KS 66509

Earn The Astronomical League's Award
for Observing the Solar System

Planetary Club Rules and Regulations

About 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.
  4. 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.

F. Planetary Configurations

  1. An inferior planet is one with an orbit inside that of Earth, i.e., a planet that orbits the sun with a radius less than that of Earth. A planet is in inferior conjunction when it is directly between the Earth and the sun.
  2. A superior planet is one with an orbit outside of Earth. A planet is in superior conjunction when it is directly on the opposite side of the sun from the Earth. A planet is in superior opposition when the Earth is directly in-between the planet and the sun.
  3. An inferior planet will experience points on its trajectory that take it as far from the sun as can be seen from an Earth viewer's perspective. These points are known as the greatest eastern and western elongations.

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