The Sun

[Key Words] [General] [Eclipses] [Looking]

Key Words

General
Sun Prominence
This image was acquired from NASA's Skylab space station on December 19, 1973. It shows one of the most spectacular solar flares ever recorded, propelled by magnetic forces, lifting off from the Sun. It spans more than 588,000 km (365,000 miles) of the solar surface. In this photograph, the solar poles are distinguished by a relative absence of supergranulation network, and a much darker tone than the central portions of the disk. (Courtesy NASA)

Comet SOHO-6 and Solar Polar Plumes
This image of the solar corona was acquired on 23 December 1996 by the LASCO instrument on the SOHO spacecraft. It shows the inner streamer belt along the Sun's equator, where the low latitude solar wind originates and is accelerated. Over the polar regions, one sees the polar plumes all the way out to the edge of the field of view. The field of view of this coronagraph encompasses 8.4 million kilometers (5.25 million miles) of the inner heliosphere. The frame was selected to show Comet SOHO-6, one of seven sungrazers discovered so far by LASCO, as its head enters the equatorial solar wind region. It eventually plunged into the Sun. (Courtesy ESA/NASA)

Sources of the Solar Wind?
"Plumes" of outward flowing, hot gas in the Sun's atmosphere may be one source of the solar "wind" of charged particles. These images, taken March 7, 1996, by the Solar and Heliospheric Observatory (SOHO), show (top) magnetic fields on the sun's surface near the south solar pole; (middle) an ultraviolet image of the 1 million degree plumes from the same region; and (bottom) an ultraviolet image of the "quiet" solar atmosphere closer to the surface. (Courtesy ESA/NASA)

The Unquiet Sun
This sequence of images of the the Sun in ultraviolet light was taken by the Solar and Heliospheric Observatory (SOHO) spacecraft on February 11, 1996 from its unique vantage point at the "L1" gravity neutral point 1 million miles sunward from the Earth. An "eruptive prominence" or blob of 60,000°C gas, over 80,000 miles long, was ejected at a speed of at least 15,000 miles per hour. The gaseous blob is shown to the left in each image. These eruptions occur when a significant amount of cool dense plasma or ionized gas escapes from the normally closed, confining, low-level magnetic fields of the Sun's atmosphere to streak out into the interplanetary medium, or heliosphere. Eruptions of this sort can produce major disruptions in the near Earth environment, affecting communications, navigation systems and even power grids. (Courtesy ESA/NASA)

A New Look at the Sun
This image of 1,500,000°C gas in the Sun's thin, outer atmosphere (corona) was taken March 13, 1996 by the Extreme Ultraviolet Imaging Telescope onboard the Solar and Heliospheric Observatory (SOHO) spacecraft. Every feature in the image traces magnetic field structures. Because of the high quality instrument, more of the subtle and detail magnetic features can be seen than ever before. (Courtesy ESA/NASA)

The Sun is the most prominent feature in our solar system. It is the largest object and contains approximately 98% of the total solar system mass. One hundred and nine Earths would be required to fit across the Sun's disk, and its interior could hold over 1.3 million Earths. The Sun's outer visible layer is called the photosphere and has a temperature of 6,000°C (11,000°F). This layer has a mottled appearance due to the turbulent eruptions of energy at the surface.

Solar energy is created deep within the core of the Sun. It is here that the temperature (15,000,000° C; 27,000,000° F) and pressure (340 billion times Earth's air pressure at sea level) is so intense that nuclear reactions take place. This reaction causes four protons or hydrogen nuclei to fuse together to form one alpha particle or helium nucleus. The alpha particle is about .7 percent less massive than the four protons. The difference in mass is expelled as energy and is carried to the surface of the Sun, through a process known as convection, where it is released as light and heat. Energy generated in the Sun's core takes a million years to reach its surface. Every second 700 million tons of hydrogen are converted into helium ashes. In the process 5 million tons of pure energy is released; therefore, as time goes on the Sun is becoming lighter.

The chromosphere is above the photosphere. Solar energy passes through this region on its way out from the center of the Sun. Faculae and flares arise in the chromosphere. Faculae are bright luminous hydrogen clouds which form above regions where sunspots are about to form. Flares are bright filaments of hot gas emerging from sunspot regions. Sunspots are dark depressions on the photosphere with a typical temperature of 4,000°C (7,000°F).

The corona is the outer part of the Sun's atmosphere. It is in this region that prominences appears. Prominences are immense clouds of glowing gas that erupt from the upper chromosphere. The outer region of the corona stretches far into space and consists of particles traveling slowly away from the Sun. The corona can only be seen during total solar eclipses.

The Sun appears to have been active for 4.6 billion years and has enough fuel to go on for another five billion years or so. At the end of its life, the Sun will start to fuse helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow the Earth. After a billion years as a red giant, it will suddenly collapse into a white dwarf -- the final end product of a star like ours. It may take a trillion years to cool off completely.

* The Sun's period of rotation at the surface varies from approximately 25 days at the equator to 36 days at the poles. Deep down, below the convective zone, everything appears to rotate with a period of 27 days.

Eclipses

Eclipses have long been a source of mystery and spectacle. These events were viewed with fear and dread in the past and, even today, still thrill.

There is a lot of special vocabulary involved in eclipses but there is a way to keep from being confused. The eclipse is named for the object that is being eclipsed, or obscured. In a solar eclipse you observe the Sun (using only safe methods, of course). You will see the Sun with a piece apparently cut out of it. In a lunar eclipse you observe the Moon. A portion of its surface will be obscured.

Another way to avoid confusion is to consider the time at which you will be viewing the eclipse. Because of the geometry described below, you can only view a solar eclipse when the Sun is up, and the Moon is nowhere to be seen. You view lunar eclipses when the Moon is up.

Eclipses occur when the Sun, Earth and Moon line up. They are rare because the Moon usually passes above or below the imaginary line connecting Earth and the Sun. In a solar eclipse the Moon passes directly in front of the Sun. This can only happen when the phase of the Moon is "new." That occurs because, for Earth-based observers, the far side of the Moon is illuminated while the side facing Earth is in darkness. The Moon, like any sphere, casts a shadow. A solar eclipse occurs when that shadow sweeps across Earth. The black cone is called the umbra, as in umbrella. An observer anywhere in that region is completely in shade. None of the Sun is visible from there.

Surrounding the umbra is the penumbra. An observer there will see some, but not all, of the Sun. Outside of these regions, all of the Sun is visible. Note that the tip of the umbra barely touches Earth. At the current time the position of the Moon relative to the Sun is such that the Moon, which is 400 times smaller that the Sun, is 400 times closer! This means that the two objects appear to be the same size in the sky. Only observers at the tip of the umbral cone will see a total solar eclipse. A large number of observers across the globe will see a partial solar eclipse if they are in the penumbra.

An annular eclipse is a special partial solar eclipse. Because the Moon's orbit around Earth is an ellipse, not a circle, the Moon's distance from Earth varies. When the Moon is far from Earth it appears slightly smaller in the sky. (Earth's orbit around the Sun is also an ellipse, and during January, Earth is at its closest point to the Sun. The Sun's size is slightly larger than during the rest of the year.) With a "small" Moon and a "large" Sun the Moon will not completely block out the Sun. The umbra does not touch Earth. An observer would have to be above the surface of Earth to see a total eclipse. For individuals in just the right location, the Sun appears as a ring (annulus) around the silhouetted Moon.

In a lunar eclipse the Moon moves into Earth's shadow. They can only occur when the moon is "full." Observers on the night side of Earth see the Moon take on a reddish hue as it moves into Earth's umbra. If the entire disk of the Moon falls into the umbra it is total lunar eclipse. If only a portion does, then it is a partial lunar eclipse. Penumbral lunar eclipses are very difficult to detect because the Moon dims only slightly while moving through that region. Lunar eclipses are more common than solar eclipses. Total eclipses of the Sun and Moon are partial before and after totality.

Popular astronomy magazines, available on many news stands, always give timely eclipse details.

Looking

    So you want to see the Sun? Of course, there is that problem that you'll go BLIND if you look at the Sun. Take Galileo, for example. He stared at the Sun so long he started going blind . . . and it is known that it happens to everyone. The blindnes occurs because of the huge amount of light (visible and invisible) that is coming from the Sun.

    So, how can you see the Sun? Sunglasses don't do the trick for looking directly at it. Same with looking through the clouds at it. The best may to see the Sun is to look at an image that is projected onto a surface. One method is to use binoculars (but of course not directly). You mount the binoculars on a tripod and put a piece of cardboard (8 x 10 inches minimum) over the binoculars with a hole that lets one ocular of the binoculars through. Also, put a white piece of paper on the ground. Then aim the binoculars at the Sun, and the image will show up on the paper. The size will change with distance between the paper and the binoculars. Also, you can focus it by using the focus tools on the binoculars.

Spotting Sunspots

Sunspots are appropriately named. They appear as spots on the disk of the Sun. A sunspot will have a very dark central region known as the umbra. It is often surrounded by a less dark halo known as the penumbra. Think about the root and prefix of this word for a good language lesson. The umbra is dark because it is cooler (around 3,500°C/6,300°F) than the surrounding sunscape (around 5,500°C/10,000°F).

Spots change over a period of several days. Penciling in the detailed appearance and location of sun-spots on a fresh piece of paper over several days can give a clear illustration of this.

They also move across the Sun as the Sun spins on its axis. Because the Sun is fluid it does not spin as a rigid body. A spot near the equator will take about 25 days to complete one rotation. A spot near a pole, if there were ever one there, will take over a month to make the trip. Collections of sketches over a period of several years will also reveal the 11 year cycle of sunspots. Over that period the numbers of spots goes from a maximum to a minimum and back.

As with any experiment, follow good scientific procedures. Keep appropriate records by being sure that all papers have the date, time and appropriate viewing conditions written in the margins or on the back of the drawings. Read more about sunspots and their interesting behavior.

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