SP-401 Skylab, Classroom in Space

[94] Part II - Student Experiments

Chapter 8: Exploring the Stars and Planets.

picture of an  astronaut performing a spacewalk outside Skylab

 

[95] Astronomy is man's oldest science. What he learned from it influenced him in many ways. The length of the day, the phases of the Moon, and the seasons of the year are governed by the orbital mechanics of the solar system. Ancient man observed the heavens to determine the best time for planting crops. Nomads and seafarers learned to use the stars for navigation. Most early civilizations were so awed by the universe that religions, mythologies, and astrology were invoked to explain it.

Gradually the superstitions were dislodged, and man began to understand the universe for what it is: a physical entity, full of secrets, and a marvelous challenge for him to decipher. Piece by piece the foundations of the science were established. In 600 B.C., the Greek Thales suggested that Earth was round. About A.D. 140, the Greco-Egyptian astronomer Ptolemy defined a universe that revolved around a stationary Earth. Not until the mid-15th century did the Polish astronomer Nicolaus Copernicus properly place the Sun in the center of the solar system. In the following century, the German astronomer Johannes Kepler used data collected by his former teacher Tycho Brahe to define the elliptical orbits of the planets. The development of the telescope, about 1609, provided a closer view of the planets and gave strong evidence to support the Copernican system. Newton expanded upon Kepler's laws of motion to develop his universal law of gravitation, which marked a significant milestone in that it provided the means for the mathematical prediction of planetary orbits. In the 20th century, Albert Einstein's theory of relativity expanded the conceptual universe by relating energy to mass as well as establishing that measurement of motion and time is dependent upon their frame of reference. Furthermore, technological advances in astronomical instruments in the 20th century permitted Hubble and others to view far beyond the Milky Way. Today, with the knowledge of atomic structure, spectral analysis, and remote observational satellites, opportunities to acquire new information seem limitless.

Stellar observations have historically been performed in the visible region of the electromagnetic spectrum, but scientists have learned that a vast amount of information can be obtained from the other spectral regions. While other spectral regions cannot be seen with the human eye, they can be sensed by instruments and transformed either mechanically, electronically, or photographically into a meaningful form. A frustrating problem for astronomers is that energy from many of these spectral regions is absorbed by Earth's atmosphere before it can reach the surface. This is particularly true of certain X-ray and ultraviolet (UV) radiations. One of the specific objectives of Skylab's observation program was to take advantage of the space station's position above the atmosphere to enhance X-ray and UV astronomy.

A research technique exploited extensively by Skylab in its astronomy observations was that of [96] spectroscopy. Spectroscopy is the interpretation of electromagnetic radiation to determine the chemical composition and temperature of a radiating source.

Spectroscopy is based upon the fact that excited atoms of individual elements radiate a characteristic pattern of wavelengths. By separating the light from a star into individual wavelengths, we can detect these patterns and determine what elements are contained in the star.

Four student experiments were concerned with stellar astronomy. Two used Skylab's UV stellar astronomy cameras to obtain photographs of emissions from pulsars and quasars. Two others used the X-ray spectrographic telescope of its solar observatory to photograph stars and the planet Jupiter in six X-ray wavelength bands.

 

UV From Quasars

Quasars are some of the most perplexing phenomena ever observed. Astronomers do not understand the mechanism involved and are not even certain just what or where quasars are.

Quasar is a shortened version of the term "quasi-stellar" radio source. Visually a quasar appears to be a rather ordinary, relatively faint star. But in the early 1 960's these stars were identified as sources of electromagnetic emission in the radio wavelengths. Galaxies and nebulae had previously been recognized as radio sources, but never had such strong radio emission been associated with stars. Quasars then became a new classification of celestial bodies, objects that look like stars, but whose radio emissions do not fit the normal stellar pattern.

In 1963, astronomers at Mount Palomar in California succeeded in recording the line spectra from two quasars. To their amazement the wavelengths of their spectral lines did not coincide with any known elements. They appeared to be entirely composed of exotic substances never before observed. Physicists were certain that this was impossible, but they had no explanation.

Maarten Schmidt of the Hale Observatories offered the key to the quasar puzzle. He noticed that the emission pattern of the Balmer spectrum of hydrogen seemed to be present in their spectra, but it was displaced by a constant factor toward longer wavelengths toward the red end of the visible spectrum. In the case of the brightest known quasar, 3C 273 (number 273 in the Third Cambridge Catalogue of Radio Sources), the wavelengths are 1.158 times as long as the emissions of identical elements on Earth. This "red shift" is most often explained as a Doppler effect, meaning that the emission source is speeding away from Earth so fast that the wavelengths of its emitted energy appear to be stretched.

Hubble's law states that the distance to an object is proportional to its observed "red shift." This association is valid when considering distant galaxies and implies that some quasars are receding at 90 percent of the speed of light at a distance from Earth of some 10 billion light-years. Thus, these quasars would be the most distant objects of the known universe.

Another characteristic of quasars is their relatively large and rapid variation in brightness. Astronomers have known that the brightness of a celestial object does not significantly change faster than the time required for light to travel across it. Some quasars have been known to double their brightness in 1 week, in contrast to the 100 000 years required for light to traverse our galaxy. This fact suggests that quasars, for such small objects, are extremely powerful energy sources to be detectable at these enormous distances. Some quasars would have to produce hundreds of times more energy than the Milky Way. Yet their size is thousands of times smaller than the galaxy. No such powerful means of producing energy is known.

The alternative description of quasars is to accept them as stars within the Milky Way and attribute their "red shift" to something other than the Doppler effect. This resolves the energy dilemma; however, no accepted explanation for the "red shift" in this situation has yet been provided. A more thorough description of quasars is required before this controversy can be settled.

John C. Hamilton of Aiea High School in Aiea, Hawaii, suggested an experiment to photograph various quasars with the Skylab UV spectrometer. He hoped to identify further elements contained in quasars whose emission lines would shift from the far-UV or X-ray regions into the UV area.

Karl Henize, a NASA astronaut and astronomer at the University of Texas, was the principal scientist for one of Skylab's UV stellar astronomy experiments and had an instrument required for Hamilton's experiment. As a result of working with....

 


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John C. Hamilton worked with Dr. Henize on an experiment to detect quasars using Skylab instruments.

John C. Hamilton worked with Dr. Henize on an experiment to detect quasars using Skylab instruments.

 

[98] ...Henize, Hamilton decided to attend the University of Texas and major in astronomy and physics.

The list of prospective celestial objects for his experiment was expanded by Hamilton to include several Seyfert galaxies (small, intense galaxies that exhibit similar characteristics to quasars). He did so after a consultation with Henize revealed that the sensitivity of Henize's spectrometer was marginal for detecting distant quasars.

Photographic plates were exposed on June 17, 1973, in an attempt to observe several quasars and Seyfert galaxies. However, only quasar 3C 273 was identified. In order to maximize the spectrometer's sensitivity, its spectral separating prism was removed from Hamilton's experiment. Therefore, he changed his experiment to determine the apparent brightness of 3C 273 in the spectral region of wavelengths between 1250 and 5000 Angstroms (an Angstrom is 10-10 meter). Preliminary results indicate that the apparent ultraviolet magnitude of 3C 273 is 12.6 ±0.5 on the brightness scale used to compare stars. This value is very near the sensitivity limit of Skylab's UV instruments but corresponds favorably with apparent magnitudes of 3C 273 previously measured. The internal energy required to produce this magnitude is dependent upon the quasar's distance from Earth. Eventually, enough of these small additions to knowledge of quasars will lead to an overall understanding of their nature.

 

UV From Pulsars

In 1967, Jocelyn Bell, an astronomy student at Cambridge University was investigating fluctuations in the strength of radio waves emitted by distant galaxies. Unexpectedly, she discovered several celestial areas emitting short, rapid signals at short intervals. None of them lasted longer than one-hundredth of a second. The intervals between the signals were extremely constant, as precise as a clock that would vary only 1 second or less in a year. The discovery was so surprising that at first the results were not announced.

The first thought of some astronomers (and many science fiction fans) was that intelligent beings from other galaxies were beaming messages to Earth. But soon too many such sources were discovered for this idea to be realistic; the signals simply covered too broad a band of frequencies for efficient transmission. The radio pulses had to be natural phenomenon, and their sources became known as pulsars, a contraction for pulsating stars.

Another confusing thing about pulsars was that at first no visual objects were found to correspond with the sources of the radio emission. A clue to the solution of this mystery lay in the sharpness of the pulses. When a burst of radio energy is emitted from a large source, the radio waves from different portions of the object arrive at different times, blurring the signal. The smaller the object, the shorter and more precise the pulse. Knowing this, astronomers calculated that pulsars could be no more than 10 miles in radius. Until this discovery, the smallest, most dense stars in the universe were thought to be about 10 000 miles in radius.

Consideration also had to be given to a prediction made several decades previously. Theoretical astronomers had described the forces involved in the collapse of a large, dying star to be so great that individual atomic nuclei at the star's center touch each other and electrons are forced to combine with protons to form a core of solid neutrons. A supernova explosion blasts off the star's bright shell, leaving behind the core of neutrons. This hypothetical ball of neutrons became known as a neutron star, the most compact, most dense form of matter thought possible.

In 1968, a pulsar was discovered in the center of the Crab Nebula, at precisely the location where scientists had searched for a neutron star, the remnant of the supernova explosion that created the Crab. Indeed, pulsars seem to be neutron stars. Neutron stars would rotate rapidly enough to produce the precise radio bursts of a pulsar. The Crab pulsar "beeps" at a rate of 30 times per second.

Several theories have been proposed as the mechanism for production of these radio bursts. The most popular explanations are: a surface storm on the pulsar that emits a beacon of radio waves throughout its revolution, but is only seen when pointed toward the Earth; the poles of a strong magnetic field of the pulsar are set from its axis of rotation and are focusing ejected particles and their radiation into emission beams; or a plasma held in position by a strong magnetic field surrounds the pulsar and rotates with it. At a certain radius, the rotational speed of the plasma will approach the speed of light, causing a portion of it to break away from the magnetic field and release a pulse of electromagnetic energy.

 


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Three theories have been advanced to explain how pulsars produce their radio-wavelength emissions. [Link to a larger picture]

Three theories have been advanced to explain how pulsars produce their radio-wavelength emissions.

 

Neal W. Shannon, a high school student working at the Fernbank Science Center, Atlanta, Gal, proposed the observation of several pulsars with Skylab's UV spectrometer to determine their intensities in that portion of their spectra. A more detailed description of a pulsar's electromagnetic emission profile would be expected to further define means by which its energy is released.

Unfortunately, upon examination of the photographic plates containing the data from Shannon's experiment, it was found that an alinement error of the spectrometer had prevented the detection of any of the pulsars. Thus, the mystery of pulsars remains.

 

X-Ray Stellar Classes

X-ray astronomy is one of the younger branches of an ancient science, only recently made possible with the advent of space technology. With the development of reliable spacecraft, new regions of the electromagnetic spectrum became accessible to astronomers for investigation.

By 1962, X-ray emission from the Sun had been detected and mapped in some detail. But if X-rays were produced by other stars in strengths comparable to those released by the Sun, such sources would be too weak to be detectable at their great distances from Earth. It was thus a major surprise when strong sources of X-rays were observed by instruments in sounding rockets throughout the heavens, many of which appear to have no visible counterpart. Uhuru, the first X-ray satellite, was launched in 1970 from a platform off the coast of Kenya. It scanned the sky for 3 years and detected additional new X-ray sources, some point sources, some large regions of radiation, and a diffuse, uniform, background glow.

This intriguing new science inspired Joe W. Reihs of Tara High School, Baton Rouge, La., to propose X-ray observation from Skylab of several stars in each spectral classification to determine if there might be a direct relationship between the magnitude of a star's X-ray emission and its age, or stage of stellar evolution.

Reihs' experiment was associated with Skylab's solar observatory X-ray spectrographic telescope, which could be pointed at specific star fields and record X-rays. However, it was determined that the X-ray telescope designed for solar studies did not have sufficient sensitivity to detect any stellar....

 


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picture of Neal W. Shannon

Neal W. Shannon, shown above right with science adviser John Humphreys

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Neal W. Shannon, shown above right with science adviser John Humphreys, proposed an experiment for Skylab that would investigate the ultraviolet properties of pulsars.

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The Uhuru satellite, launched in 1970, gathered data for this X-ray map of the Milky Way. Note that the sources are concentrated along the Equator. [Link to a larger picture]

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The Uhuru satellite, launched in 1970, gathered data for this X-ray map of the Milky Way. Note that the sources are concentrated along the Equator.

 

[101]....X-ray sources and was thus unable to acquire data for Reihs.

However, he did benefit from his experiment in that it provided him the opportunity to work for American Science Engineering, Inc., both in Boston and Houston, for most of 1973, as the company reviewed the performance of its X-ray telescope.

 

X-Rays From Jupiter

The ancient Greeks first identified Jupiter as a planet or a "wanderer" different from the fixed....

 


Joe W. Reihs suggested an experiment for observing X-ray emissions of certain stars as a possible index of their age of stage in evolution. Below right, he is greeted by Astronauts Russell Schweickart and Owen Garriott; Leland Belew, Skylab Program Manager; and David Newby, Marshall Space Flight Center Director of Administration and Technical Services.

picture of Joe W. Reihs

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picture of Joe W. Reihs

Joe W. Reihs is greeted by Astronauts Russell Schweickart


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Skylab's X-ray spectrographic telescope took this picture of the Sun on August 19, 1973.

Skylab's X-ray spectrographic telescope took this picture of the Sun on August 19, 1973. The large bright areas are X-ray emissions that represent gases in the corona at temperatures greater than 3 600 000°F. The great number of small, bright points shown had not been suspected prior to Skylab.

 

....stars. In 1610, Galileo identified four of the moons (nine more have since been observed). It was, in fact, observations of Jupiter's "miniature solar system" that encouraged acceptance of the Copernican model of planets orbiting the Sun.

Jupiter is quite unlike Earth. Its diameter is approximately 11 times as large, and its mass is 317.8 times greater. More significantly, Jupiter has no surface such as Earth's. There are no oceans and continents. It has no distinct boundary between its surface and its atmosphere. The greater part of its innermost regions is solid hydrogen. The huge planet may have a very small metallic core, which is covered by a sea of liquid hydrogen that blends...

 


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The Great Red Spot on Jupiter has been seen from Earth for some 300 years.

The Great Red Spot on Jupiter has been seen from Earth for some 300 years. This picture, made from the space probe Pioneer 10, indicates that the spot may be a huge storm within the planet's atmosphere.

 

....into a mixture of gases such as hydrogen and helium. Above it are turbulent clouds of ammonia ice crystals, the tops of which can be seen from Earth with the larger telescopes. Beneath this topmost layer probably lie regions of ammonium hydrosulfate crystals, and perhaps, still lower, water ice crystals.

Some astronomers feel that X-ray emissions from Jupiter may be produced by a nonthermal process within Jupiter's atmosphere or the bremsstrahlung method as electrons carried by the solar wind interact with Jupiter's magnetosphere following an increased period of solar activity. Bremsstrahlung (German for "braking radiation") is the....

 


Jupiter consists almost entirely of hydrogen in some state; however, it may have a small, rocklike core. [Link to a larger picture]

Jupiter consists almost entirely of hydrogen in some state; however, it may have a small, rocklike core.


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picture of  Jeanne L. Leventhal

Jeanne L. Leventhal proposed examining Jupiter with Skylab's solar X-ray telescope.

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Jeanne L. Leventhal proposed examining Jupiter with Skylab's solar X-ray telescope.

 

....form of energy release caused by a rapid deceleration of charged particles such as electrons. X-ray emission has never been detected from Jupiter, but the effect has been detected in the upper limits of Earth's magnetosphere. Photographs of X-ray emission from Jupiter would reveal new information about the conformation and mechanism of Jupiter's magnetosphere and magnetospheres in general.

Jeanne L. Leventhal of Berkeley High School, Berkeley, Calif., proposed using the Skylab solar observatory's X-ray telescope to observe Jupiter. The experiment was scheduled for the second Skylab mission, but the space vehicle experienced problems with its power supply during the most probable time for observing Jupiter. The normal orientation for the solar cell arrays, Skylab's power generating system, was toward the Sun. Reorientation of the solar observatory in order to view Jupiter would have required a relatively complex maneuver which could not be accommodated in the minimal power situation. Thus, it became obvious to Ms. Leventhal, a working member of [105] the American Science Engineering, Inc., X-ray telescope support team at the JSC Mission Control Center at the time, that the Jupiter observation would not be performed. She therefore proposed an alternative target, the Cygnus Loop or Veil Nebula, which required a smaller spacecraft maneuver angle for observation.

The Veil Nebula had been identified as a source of X-ray emission in 1968. Improved instruments launched in 1971 showed that the X-rays are primarily produced in the outer edges of the filamentary structure of the nebula. In 1973, an X-ray "hot spot" was observed near the center of the Veil Nebula that apparently possessed the size, luminosity, and temperature characteristics of a neutron star. Confirmation of this "point" source of X-rays would add support to the theory of supernovas.

Ms. Leventhal then proposed using Skylab's X-ray telescope to further define the X-ray source near the center of the Veil Nebula. The observation was planned for the third Skylab mission. Concurrent with preparation for the Veil Nebula observation, data from the second Skylab visit were being analyzed. Close examination of the returned film from the solar observatory revealed that the sensitivity of the solar X-ray telescope, in combination with Skylab pointing uncertainty, had prevented the detection of an X-ray target within the Scorpius constellation (the brightest stellar X-ray source in the sky). This failure suggested that attempts to detect the Veil Nebula would be similarly unsuccessful, so the Skylab experiment was terminated in order to utilize the solar X-ray telescope for the higher priority observations of the Sun.


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