Astronomy

Pioneer 5 - USA Solar Monitor - (March 11, 1959)

Space probe is now in a solar orbit.

Pioneer 6 - USA Solar Probe - 63.4 kg - (December 16,1965 - Present)

The Probe is still transmitting from solar orbit.

Pioneer 7 - USA Solar Probe - 63 kg - (August 17, 1966- ?)

Solar-orbiting probe was recently turned off.

Pioneer 8 - USA Solar Probe - 63 kg - (December 13,1967 - Present)

Solar probe is still transmitting from solar orbit.

Pioneer 9 - USA Solar Probe - 63 kg - (November 8, 1968- March 3, 1987)

This probe stopped functioning on March 3, 1987. It is still in solar orbit.

Skylab - USA Space Station - 91,000 kg - (May 14, 1973)

Skylab, which was America’s first space station, was manned for 171 days by three crews during 1973 and 1974. The space station included the Apollo Telescope Mount (ATM), which astronauts used to take more than 150,000 images of the Sun. Skylab was abandoned in February 1974 and re-entered the Earth’s atmosphere in 1979.

Explorer 49 - USA Solar Probe - 328 kg - (June 10, 1973)

Solar physics probe placed in lunar orbit.

Helios 1 - USA & West Germany Solar Probe - 370kg - (December 10, 1974 - 1975)

Solar probe is in a solar orbit; came within 47 million kilometers of the Sun.

Solar Maximum Mission - USA Solar Probe - (February14, 1980)

The Solar Maximum Mission (SMM) was designed to provide coordinated observations of solar activity, in particular solar flares, during a period of maximum solar activity. The spacecraft suffered an on-orbit failure. A repair mission on STS-41C in 1984 , during which shuttle astronauts rendezvoused with SMM, was successful. SMM collected data until Nov. 24, 1989, and re-entered the Earth’s atmosphere on Dec. 2, 1989.

Helios 2 - USA & West Germany Solar Probe - (January16, 1976)

Solar probe came within 43 million kilometers of the Sun.

Ulysses - Europe & USA Solar Flyby - 370 kg - (October6, 1990)

The Ulysses spacecraft is an international project to study the poles of the Sun and interstellar space above and below the poles. It used Jupiter as a gravity assist to swing out of the ecliptic plane and onward to the poles of the Sun. The Jupiter flyby was on February 8, 1992. The first solar polar passage was in June 1994. The spacecraft passed the solar equator in February 1995 and passed over the north pole in June 1995.

Yohkoh - Japan Solar Probe - (August 31, 1991)

This spacecraft studied high-energy radiation from solar flares. The Japanese are undertaking this project in cooperation with the United States and Great Britain.

SOHO - Europe & USA Solar Probe - (December 12,1995)

The main scientific purpose of SOHO (Solar and Heliospheric Observatory) is to study the Sun’s internal structure, by observing velocity oscillations and radiance variations, and to look at the physical processes that form and heat the Sun’s corona and that give rise to the solar wind, using imaging and spectroscopic diagnosis of the plasma in the Sun’s outer regions coupled with in-situ measurements of the solar wind. SOHO will be put into a “halo orbit” around the L1 Lagrange point-the point 1.5 million kilometers away from us at which the gravitational pull of the Earth balances that of the Sun.

Spotting Sunspots

Naked eye observations of the Sun will result in blindness. The improper use of telescopes or binoculars will cause blindness much faster. Now, having said that, there are safe and easy ways to safely observe the Sun. Do not hesitate to use them.

The Sun and some of its behaviors are well know to youngsters. They know the difference between night and day and the realize that the Sun is up longer during the summer than in the winter. But few, if any, of your students have ever directly observed the surface of the Sun. They have no reason to expect that the Sun is spinning or that it has blemishes which evolve over time.

If these facts are covered in a textbook students are forced to accept these statements on faith alone. That is not a particularly compelling way to learn. It is possible for students to discover important details about the Sun themselves.

Incorrect observation of the Sun causes blindness because of the tremendous amount of visible and invisible light coming off of its surface. Sunglasses and other inappropriate filters may block visible light but can not cut out enough ultraviolet and infrared light, causing the eye’s retina to burn out.

Looking through thick clouds or with UV-blocking sunglasses is just as dangerous. Fortunately, two things prevent major problems. The first is that it is painful to look at the Sun. Eyes tear up and you have to look away. The second is that few youngsters bring sunglasses to class.

A projected image of the Sun is, however, perfectly safe to observe. Binoculars, with the front of one ocular covered, can be used to project a sharp image of the Sun on to a white piece of paper. The binoculars are attached on a tripod, with a mount or securely with tape. Place a piece of cardboard, at least 20 by 25 centimeters, with one hole the same size as the front of one ocular at the front of the binoculars. Fasten it securely with tape. It will cast a shadow on the paper.

Aim the binoculars by tilting them up and down and back and forth until the Sun shines through. Never look through the binoculars when they are pointing anywhere near the Sun! Sharpen the image of the Sun on the paper by using the binocular’s focusing knob or lever. The size of the image can be altered by moving the paper closer or farther from the binoculars.

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) than the surrounding sunscape (around 5,500°C).

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.

Astronomical Alignments

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.

 Image of a simulation of convective penetration in Stellar interiors.

The images represent computer-generated models of convection motions occurring in stars like the Sun. The work has been accomplished by Andrea Malagoli and Fausto Cattaneo at the University of Chicago under a NASA/ESS HPCC Grand Challenge Grant.

Capturing Convection

Sometimes a single physical process in nature can explain a variety of events. Convection is one such process. It functions because heated fluids, due to their lower density, rise and cooled fluids fall. A heated fluid will rise to the top of a column, radiate heat away and then fall to be re-heated, rise and so on. Gasses, like our atmosphere, are fluids, too. A packet of fluid can become trapped in this cycle. When it does, it becomes part of a convection cell.

Convection cells can form at all scales. They can be millimeters across or larger than Earth. They all work the same way. The convection that students are most likely to have observed is in cumulonimbus clouds or “thunderheads.” These towering vertical clouds can be seen to evolve over a few minutes. The tops of the clouds have a sort of cauliflower appearance as warm moist air rises through the center of the cloud. The moisture in the cloud condenses as it cools. The air gives up some of its heat to the cold high altitude air and begins to fall.

As the air falls along the exterior of the cloud, it returns to warmer low altitudes where it can be caught up in the rising column of air in the center of the cloud. This fountain-like cell can form alongside other cells, and a packet can move between cells. Hail forms when water droplets, carried by the strong updrafts, freeze, fall through the cloud and are caught in the updraft again. An additional layer of water freezes around the ice ball each time it makes a trip up through the cloud. Eventually, the hail becomes too heavy to be carried up anymore, so it falls to the ground. Large hailstones, when cut apart, show multiple layers, indicating the number of vertical trips the stone made while it was caught in the convection cell.

Convection also occurs on the Sun. A high resolution white light image of the Sun shows a pattern that looks something like rice grains. Very large convection cells cause this granulation. The bright center of each cell is the top of a rising column of hot gas. The dark edges of each grain are the cooled gas beginning its descent to be re-heated. These granules are the size of Earth and larger. They constantly evolve and change.

Thunderheads and granulation are large-scale examples of convection. Fortunately, there are examples of convection that fit into a classroom. An excellent example can be seen in hot Japanese Miso (soybean paste) soup.

The interior of the broth is hot. The surface of the soup is exposed to cool air. Hot packets of fluid rise out of the interior of the soup to the surface where they give off heat. Now cooled, they fall down into the bowl to be re-heated. Left alone, the soup will dissipate its heat in this way (and through conduction with the sides of the bowl) and reach room temperature.

The soybean paste granules and other ingredients will highlight the convection cells vividly. As students gaze into their soup, they will see the rising and descending columns of fluid. The cells will evolve and change their positions. Dark bottomed bowls show the effect best. If the soup is stirred up, students can observe the cells reform. Of course, the demonstration material can be consumed at the conclusion of the demonstration.

Convection acts as described in the examples above where gravity’s effects are present (so that warm, low density fluids can rise and cool, high density fluids can fall). What happens in the weightlessness of space where up (rise) and down (fall) have no meaning?

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 kilometers 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.

X-Ray Image

This is an X-ray image of the Sun obtained on February 21, 1994. The brighter regions are sources of increased X-ray emissions.

Solar Disk in H-Alpha

This is an image of the Sun as seen in H-Alpha. H-Alpha is a narrow wavelength of red light that is emitted and absorbed by the element hydrogen.

Solar Flare in H-Alpha

This is an image of a solar flare as seen in H-Alpha.

Solar Magnetic Fields

This image was acquired February 26, 1993. The dark regions are locations of positive magnetic polarity and the light regions are negative magnetic polarity.

Sun Spots

This image shows the region around a sunspot. Notice the mottled appearance. This granulation is the result of turbulent eruptions of energy at the surface.

1991 Solar Eclipse

This shows the total solar eclipse of July 11, 1991 as seen from Baja California. It is a digital mosaic is derived from five individual photographs, each exposed correctly for a different radius in the solar corona.

1994 Solar Eclipse

The following image was taken November 3, 1994, as observed by the High Altitude Observatory White Light Coronal camera from Chile.

* 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.

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. 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) 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.

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.