SP-404 Skylab's Astronomy and Space Sciences

 

2. Stellar and Galactic Astronomy

 


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Figure 2-1. Large Magellanic Clouds. (A) In red light from the ground; (B) in far-ultraviolet from the Moon (Apollo 16 mission); (C) in ultraviolet by the Skylab stellar astronomy experiment; (D) in ultraviolet taken by the ultraviolet panorama camera on Skylab. Link to a larger picture.

Figure 2-1. Large Magellanic Clouds. (A) In red light from the ground; (B) in far-ultraviolet from the Moon (Apollo 16 mission); (C) in ultraviolet by the Skylab stellar astronomy experiment; (D) in ultraviolet taken by the ultraviolet panorama camera on Skylab.

 

[7] Astronomy was limited to observations in the narrow band of visible wavelengths until a few decades ago. Investigations in other parts of the electromagnetic spectrum began in the 1930s with radio astronomy and have since been extended to cover almost the whole spectrum.

The greatest single obstacle to ground-based observations is the atmosphere. The ozone, oxygen, and nitrogen molecules of the Earth's upper atmosphere almost completely absorb ultraviolet radiation and prevent any Earth-based astronomy at wavelengths shorter than about 3000 Å. Various vehicles have been used to transport instruments beyond the atmosphere's interference. Balloons, aircraft, sounding rockets, and unmanned satellites have provided large amounts of new information. These observations, however, were incomplete in the far-ultraviolet to soft-X-ray region and were confined to bright objects. The Skylab experiments were designed for additional observations at these wavelengths and for the observation of faint objects.

 

SKYLAB'S ULTRAVIOLET EXPERIMENTS

 

Figure 2-1 illustrates the different information that can be obtained from photographs in different wavelength bands. It shows the Large Magellanic Clouds in red light from Earth, in the far ultraviolet from the Moon's surface, and in the ultraviolet from Skylab.

 

Importance of the Ultraviolet Region

 

The ultraviolet region of the spectrum is of special importance to astrophysics. The strongest emissions of the....

 


Figure 2-2.&emdash;Astronaut Alan Bean operating the ultraviolet stellar astronomy experiment (S019) in the orbital workshop.

Figure 2-2. Astronaut Alan Bean operating the ultraviolet stellar astronomy experiment (S019) in the orbital workshop.

 

...two most abundant elements in the universe, hydrogen and helium, lie in the ultraviolet, as do the strongest emissions of other important elements, such as carbon, nitrogen, and oxygen and their ions. (An ion is an atom that has lost one or more electrons. A neutral carbon atom is designated by C I; a carbon atom with one electron missing is C II, etc.)

Stars are classified by spectral class according to a system of letters (table 2-1), each letter class being further subdivided from zero to 10. The youngest, hottest, and most massive stars, of the spectral classes O and B, emit most of their radiation in the ultraviolet; for example, a B star emits 80 percent of its total radiation at wavelengths shorter than 3000 Å. Although their visible radiation gives some information on the nature of the hot stars and was used for the conventional classification of stars, ultraviolet data are essential for calculating their total rate of energy loss and their lifespan.

 

[8] Table 2-1. Spectral Classification of Stars

Class

Temperature range, K

Spectral lines in visible light

.

O

30 000-50 000

Ionized helium

B

10 000-30 000

Neutral helium

.

.

Strong hydrogen

A

7500-10 000

Very strong hydrogen

F

6500-7500

Strong hydrogen

.

.

Ionized calcium

.

.

Weak metal lines

G

5000-6000

Weak hydrogen

.

.

Many metal lines

K

3500-5000

Many metal lines

M

2000-3500

Molecular spectra, especially TiO,

.

.

Many metal lines

 

Observations in the ultraviolet have recently revealed that many of these stars are blowing away their outer layers. This reaction not only affects their evolution but also replenishes and modifies the interstellar gas and dust from which new stars will be formed.

 

Ultraviolet Stellar Astronomy Experiment (S019)

 

The S019 ultraviolet stellar astronomy experiment was designed by astronaut Karl S. Henize while an astronomer at Northwestern University to survey the ultraviolet spectra of a much larger number of stars than had been possible in earlier similar programs. It was able to reach somewhat dimmer stars than could the Orbiting Astronomical Observatories launched in the 1960s and early 1970s, and therefore could contribute a broader statistical base for the interpretation of these spectra.

The experiment was operated during all three Skylab missions and photographed spectra in a total of 188 star fields. These fields cover an area in the sky of approximately 3660 square degrees and include roughly 24 percent of a 30°-wide band centered on Gould's belt, a band around the sky in which the brighter stars concentrate. Nearly 1600 stars photographed by Skylab have measurable brightnesses at wavelengths of 2000 Å or less. Of these, 400 show useful data at 1500 Å or less, and about 170 show easily measurable absorption lines. Before Skylab, only about 30 spectra below 1500 Å had been obtained, with sounding rockets.

 

Instrumentation

 

The S019 ultraviolet stellar astronomy experiment used a 15-cm-aperture telescope converted into an objective-prism spectrograph by placing a prism of calcium fluoride in front of it. Such a system has the advantage of being able to obtain the spectra of several stars (all the bright stars in a 4° by 5° field of view) in a single exposure, whereas a conventional slit spectrograph photographs only one star at a time.

Since the objective-prism spectrograph operated from the antisolar scientific airlock, an articulated-mirror system was extended through the airlock and then rotated and tilted in order to view any desired part of the sky. This mirror system was also used in other astronomical experiments operated through the airlock. Figure 2-2 shows the equipment being operated by astronaut Alan Bean. The operation was entirely manual. The astronauts pointed the mirror, verified (during the first exposures of each observing session) the mirror pointing by observing stars in a finding telescope, advanced the film, opened and closed the shutter, and timed the exposures.

 

Operating Modes

 

The calcium fluoride prism disperses starlight into a spectrum. Since the fine spectral lines in such narrow....

 


Figure 2-3.-Ultraviolet spectra in the 1500-Å region taken with the objective-prism spectrograph in the widened-spectrum mode.

Figure 2-3. Ultraviolet spectra in the 1500-Å region taken with the objective-prism spectrograph in the widened-spectrum mode.

 

[9] ....spectra tend to be obscured by the graininess of the photographic film, most of the spectra were also widened to 0.6 mm in the direction perpendicular to the dispersion by slowly tilting the rear end of the mirror canister (to which the spectrograph was attached) during each exposure. This widened-spectrum mode (fig. 2-3) showed improved detail in the spectra and was able to produce spectra in the 1500-Å region of BO stars as faint as visual magnitude 6.5 (barely visible to the naked eye).

It was possible to obtain spectra from even fainter stars in the same exposure time if the spectra were not widened. Even though the unwidened-spectrum mode yielded less spectral detail (fig. 2-4), it was used in a large fraction of the fields to record fainter stars.

A no-prism mode was also used. The prism was removed from the spectrograph, and lateral chromatic aberration, which would normally be considered a defect in the optical system, was used to produce very low resolution spectra. The ultraviolet image of each star was closer to the center of the plate than was its visible-light image, thus producing images elongated radially outward from the center of the plate (fig. 2-5). Although the no-prism mode produced only very crude spectra, it had the advantage of showing the spectral distribution of energy in much fainter stars than could the other two modes. This mode was used mainly to observe the distribution of very hot faint stars in galaxies relatively close to the Milky Way.

Figures 2-3, 2-4, and 2-5 show spectra taken in each mode in the region of M8, a cluster of hot stars enveloped in a glowing cloud of gas and dust from which the stars may have been formed.

 

Character of the Widened Spectra

 

Figure 2-6 is a negative print of a typical star field in the southern constellation Carina and illustrates the information available in the widened spectra. A wavelength scale in angstrom units is shown above the star Alpha Carinae on the right. Several other stars are marked with their catalog number, visual magnitude, and spectral type.

In Alpha Carinae, the absorption lines of ionized carbon (C II) at 1335 Å and ionized silicon (Si IV) at 1394 and 1403 Å are visible; in Iota Carinae (upper right), a line caused by magnesium ions (Mg II) at 2800 Å and a cut-off caused by the blending together of several ionized iron (Fe II) lines at 2416 Å are seen.

These two stars illustrate how the relative amount of ultraviolet radiation depends on the surface temperature of the star. Although Iota Carinae (surface temperature....

 


Figure 2-4.-Ultraviolet spectra of faint stars taken with the objective-prism spectrograph in the unwidened-spectrum mode.

Figure 2-5.-Ultraviolet spectra obtained without a prism.

Figure 2-4. Ultraviolet spectra of faint stars taken with the objective-prism spectrograph in the unwidened-spectrum mode.

Figure 2-5. Ultraviolet spectra obtained without a prism.


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Figure 2-6. Negative print of the widened ultraviolet spectra of a typical star field in the southern constellation Carina. Link to a larger picture.

Figure 2-6. Negative print of the widened ultraviolet spectra of a typical star field in the southern constellation Carina.

 

....7000 K) is twice as bright as Alpha Carinae in visible light, its brightness diminishes rapidly between 3000 and 2000 Å, whereas Alpha Carinae (surface temperature 22000 K), is still bright at 2000 Å and still measurable at 1300 Å. It is thus possible to gain a better knowledge of both the temperature and the absolute brightness of these stars by measuring the brightness distribution of their ultraviolet and visible spectra.

 

Peculiar Stars: The Ultraviolet Spectra of Wolf-Rayet Stars

 

Wolf-Rayet stars are stars whose outer layers have been stripped away by evolutionary processes and in which a very hot and disturbed core remains. Their spectra are very strong in the ultraviolet and show broad emission lines of ionized carbon, nitrogen, oxygen, and helium. Wolf-Rayet stars fall into one of two groups, WC and WN. For reasons not clearly understood, the WC stars show strong emissions of carbon and oxygen in their visible spectra but no nitrogen, whereas the WN stars show a strong emission of nitrogen and almost no carbon and oxygen.

Only two of these stars are bright enough in the ultraviolet to have been analyzed by the Orbiting Astronomical Observatory satellites. Skylab experiment S019 was able to obtain the ultraviolet spectra of 12 Wolf-Rayet stars. Figure 2-7 shows the spectra of six stars with strong carbon and oxygen emissions. The spectra of HD 156385, and possibly HD 165763 (the HD designation refers to the Henry Draper Catalog), show two lines (1718 and 1805 Å) that are best identified with nitrogen ions and may be the first unambiguous examples of the appearance of nitrogen in WC Wolf-Rayet stars.

The spectra of Gamma Velae and Theta Muscae are more like those of typical supergiant stars than like the emission-line-rich spectra of Wolf-Rayet stars. Both stars are known to have companions whose spectra are barely detectable at visible wavelengths. The ultraviolet spectrum of the companion star is stronger than that of the primary. The companion stars are thus even hotter than their primaries. Similar data for the WN stars show that three of the six have previously unknown companion stars. These discoveries give strong support to the concept that the observed characteristics of all WolfRayet stars are partly attributable to effects of the companion stars.

 

Attenuation of Ultraviolet Light by Interstellar Dust

 

The plane of the Milky Way contains clouds of very minute particles known as interstellar dust. Starlight passing through these clouds is attenuated, with blue wavelengths being affected more than red. Thus starlight is "reddened" by the interstellar dust. The attenuation is greatest at about 2200 Å in the ultraviolet and then decreases somewhat at still shorter wavelengths. The effects of interstellar dust are shown in figure 2-8, in which star BS 6187 exhibits a pronounced dip m brightness at 2200 Å but slowly brightens again at wavelengths of 2000 to 1800 Å. BS 6188 is somewhat nearer to the Earth and shows very little of these absorption effects. (BS denotes Bright Star Catalog numbers.)

Since the effects of interstellar dust are greatest in the ultraviolet, ultraviolet spectra such as these should be useful in clarifying the nature of interstellar dust and its distribution in space.

 

Laboratory Rectification of Widened Spectra

 

Some of the recorded spectra were distorted by movement of Skylab during exposures. To compensate for this, the spectra were scanned with a computer-controlled microdensitometer in a series of very narrow strips. These measurements were then reassembled into a smoothed, geometrically rectified spectrum.

[11] The rectification process is illustrated in figure 2-9. A print of the original spectrum is shown at the bottom of the picture; only two absorption lines are clearly evident. Trace 1 shows the rather noisy densitometer data from one 30-µm-wide strip. Trace 2 shows 20 such traces combined in their original positions along the wavelength scale. The noise is obviously reduced, but the wavelength resolution is degraded. Finally, trace 3 shows the result after adjustments ("wavelength shifts") have been made in each strip to obtain the best fit. The top portion of figure 2-9 is a "playback" of the rectified spectrum and shows a third line of moderate strength and several weaker features not seen in the original spectrum. However, care must be taken in interpreting such weaker lines since they may be remnants of photographic grain noise.

 

Broad Overview of the Ultraviolet Spectra of Hot Stars

 

An array of stars arranged vertically by spectral class (i.e., temperature) and horizontally by luminosity is shown in figure 2-10. The spectra have been rectified by the computer-controlled microdensitometer process described above.

 


Figure 2-7. Far-ultraviolet spectra of WC Wolf-Rayet stars. Link to a larger picture.

Figure 2-7. Far-ultraviolet spectra of WC Wolf-Rayet stars.



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Figure 2-8. Attenuation of ultraviolet light by interstellar dust, manifested in the spectrum of star BS 6187. Link to a larger picture.

Figure 2-8. Attenuation of ultraviolet light by interstellar dust, manifested in the spectrum of star BS 6187.

 

A B5 star has a surface temperature of approximately 16 000 K, while 07 stars have surface temperatures ranging to approximately 40000 K. Luminosity class V....

 


Figure 2-9.-Rectification of spectral distortions caused by Skylab movement during exposures. Trace 1: <<noisy >> densitometer trace of one 30- & #181 ;m-wide strip. 2 : combined from 20 strips without wavelength shifts. 3 after appropriate shifts in each

Figure 2-9. Rectification of spectral distortions caused by Skylab movement during exposures. Trace 1: "noisy" densitometer trace of one 30-µm-wide strip. Trace 2: combined trace from 20 strips without wavelength shifts. Trace 3: combined trace after appropriate wavelength shifts in each strip.

 

....stars are those, like the Sun, that lie on the main sequence of the color luminosity diagram. Class I stars are large-diameter supergiants with brightnesses 50 to 100 times greater (mainly because of their larger size). The class II and III giants lie between.

Some interesting trends are evident. In all luminosity classes, the C IV line grows gradually stronger as the stellar temperature increases. The Si IV lines behave differently in class V and I stars, peaking very sharply at B1 in class V stars but remaining strong from B1 to 06 in class I stars. At spectral type B1, the strength of the C IV line increases dramatically with luminosity. Furthermore, the C IV and Si IV lines are wider in the more luminous stars, and emission appears on the red edge. The emission and broad absorption indicate that stars of higher luminosity are ejecting matter at such a rapid rate that it must affect their evolutionary cycle.

The Skylab data from experiment SOI9 and others covered a wide range of stars and showed for the first time that the rate of gas ejection depends mainly on the intrinsic brightness of the star summed over all wavelengths (the so-called bolometric magnitude) and that all stars above a certain value of total radiated energy seem to eject gas. The supergiants, whose luminosity is due chiefly to their enormous sizes, can eject matter at lower surface temperatures. (The surface layers of these giants are far from their centers, and hence the force of gravity is relatively weak.) On the other hand, a smaller, class V star must be much hotter before mass ejection begins.

 

[13] The S201 Far-Ultraviolet Electrographic Camera

 

The S201 far-ultraviolet electrographic camera was sent aloft on the last Skylab manned mission, primarily to observe Comet Kohoutek. This camera was designed to take pictures in the far ultraviolet, including the region of the very strong Lyman-alpha line of hydrogen at 1216 Å. It was used during the Apollo 16 mission for both stellar and extragalactic photography from the Moon as well as for photographing the ultraviolet emissions of the Earth's upper atmosphere. The principal investigator for the Skylab experiment was Thornton Page, of the U.S. Naval Research Laboratory in Washington, D.C.

The electrographic camera was designed by George Carruthers, of the U.S. Naval Research Laboratory, to produce extremely sensitive photographs in ultraviolet wavelengths. It combines the light-gathering power of a telescope, the sensitivity of a photomultiplier tube, and the cumulative recording feature of film. It is therefore able to obtain wide-field pictures containing farultraviolet images of dim stellar sources.

Figure 2-11 is a schematic diagram of the camera. The incoming ultraviolet radiation passes through a corrector plate, which also serves as a filter. Either of two different wavelength regions can be photographed, depending on which corrector plate is used. A lithium fluoride plate permits radiation of wavelength longer than 1050 Å to pass; a calcium fluoride plate narrows the wavelength region to wavelengths longer than 1230 Å, thus excluding the Lyman-alpha radiation of hydrogen. The light is then reflected from a concave....

 


Figure 2-10. Ultraviolet spectra of hot stars arranged by spectral type and luminosity class. Link to a larger picture.

Figure 2-10. Ultraviolet spectra of hot stars arranged by spectral type and luminosity class.


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Figure 2-11. Schematic diagram of the S201 far-ultraviolet electro-graphic camera. Link to a larger picture.

Figure 2-11. Schematic diagram of the S201 far-ultraviolet electro-graphic camera.

 

.....mirror to form an optical image on a photocathode, a thin layer of potassium bromide that emits electrons from points at which star images are focused. Potassium bromide responds only to light of wavelengths shorter than 1600 Å. The net result is operation at one of two wavelength bands, 1050 to 1600 Å or 1230 to 1600 Å.

These electrons are accelerated away from the photocathode by a 25 000-V negative potential on it and are focused onto an electron-sensitive film by a 300-gauss magnetic field. This film thus records an electron image that is proportional in brightness to the photon image focused on the photocathode. A very thin aluminum barrier membrane keeps visible and ultraviolet light from reaching the film but is easily penetrated by the electrons. In summary, the camera has two unique advantages: densities in the developed film may be accurately related to the far-ultraviolet brightness of objects photographed, and it is "blind" to ordinary visible light and ultraviolet light of wavelengths longer than 1600 Å. The camera does have two operating constraints: the photocathode cannot be exposed to humid air, and exposures can be made only in a hard vacuum. Skylab's airlock enabled these conditions to be fulfilled.

 

Electrographic Data

 

One of the pictures returned from the electrographic camera aboard Skylab was of the Gum Nebula, a supernova remnant. It is shown in a 107-sec exposure (fig. 2-12) that illustrates the far-ultraviolet radiation of the hot blue stars in that region. A hazy glow fills the entire frame. Accurate measurements can reveal what portion of this background glow comes from gases in the Gum Nebula by comparison of this photograph with those of adjacent regions.

The star images on all the photographs were measured for far-ultraviolet brightness and "color" (difference in brightness between photographs taken with the two filters). Star positions allowed identification with those in catalogs of stars, nebulae, and galaxies photographed by Earth-based telescopes. Preliminary results showed that about 90 percent of the stars photographed were hot, blue stars in the Smithsonian Catalog. A few.....

 


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Figure 2-12.-Gum Nebula photographed by the S201 electrographic camera (107-sec exposure, 1250- to 1600- Å band).

Figure 2-12. Gum Nebula photographed by the S201 electrographic camera (107-sec exposure, 1250- to 1600- Å band).

 

Others were nebulae and galaxies, but 5 or 10 percent were unknown objects-possibly faint stars of extremely high temperature or small gas clouds with strong farultraviolet emission lines.

 

Analysis Techniques

 

Quantitative variations in brightness can be emphasized by drawing isophote contours, lines that connect points of equal brightness. For example, figure 2-13 shows the Small Magellanic Cloud, a galaxy first observed by Magellan in his voyage in 1519. Four or five clouds of stars and gas in this galaxy can be compared with visible-wavelength data from ground-based photographs, allowing an estimate of the density distribution of interstellar gas in various regions of this small galaxy, which is quite near the Milky Way.

To make the isophote plot (fig. 2-14), a negative of the print (fig. 2-13) was scanned with a microdensitometer that measured film density at points only 2.5 µm apart. These millions of measurements were recorded on magnetic tape and then fed to a computer for connecting points of equal density or brightness. This chart was then compared with a similar one of the Large Magellanic Cloud, photographed during the Apollo 16 mission. Similarities and differences between these two nearest galaxies (about 100000 to 150000 light-years away) could thus be established.

 

Pleiades

 

Pictures of the Pleiades, better known as the Seven Sisters, show evidence of dust nebulosity around the brighter stars. The glow so produced is also evident in ground-based photographs. Figure 2-15 is an isophote plot made from a 15-sec exposure of the Pleiades taken.....

 


Figure 2-13.-Far-ultraviolet photograph (107-sec exposure) of the Small Magellanic Cloud.

Figure 2-13. Far-ultraviolet photograph (107-sec exposure) of the Small Magellanic Cloud.


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Figure 2-14. Isophote plot of the Small Magellanic Cloud, constructed from the photograph shown in figure 2-13.

Figure 2-15. Isophote plot of the Pleiades, constructed from a photograph taken by the electrographic camera with a lithium fluoride filter (15-sec exposure, 1050- to 1600-Å band).

Figure 2-14. Isophote plot of the Small Magellanic Cloud, constructed from the photograph shown in figure 2-13.

Figure 2-15. Isophote plot of the Pleiades, constructed from a photograph taken by the electrographic camera with a lithium fluoride filter (15-sec exposure, 1050- to 1600-Å band).

.

Figure 2-16. Isophote plot of the Pleiades, constructed from a photograph taken with the electrographic camera with a calcium fluoride filter (30-sec exposure, 1230- to 1600-Å band).

Figure 2-16. Isophote plot of the Pleiades, constructed from a photograph taken with the electrographic camera with a calcium fluoride filter (30-sec exposure, 1230- to 1600-Å band).

 

....with the electrographic camera, through the lithium fluoride filter. With that filter, the camera recorded wavelengths from 1050 to 1600 Å. Figure 2-16 shows the isophote plot of a 30-sec exposure through the calcium fluoride filter, which limited sensitivity to wavelengths from 1230 to 1600 Å. Differences between these two illustrations result from radiation in the interval 1050 to 1230 Å.

 

The S183 Ultraviolet Panorama Experiment

 

The S183 ultraviolet panorama experiment was conceived by Georges Courtes and his colleagues at the Laboratoire d'Astronomie Spatiale in Marseilles, France. It was based on previous experiments with sounding rockets. The objective was to obtain ultraviolet intensities, at three wavelengths, of hot stars, clusters of stars, large stellar clouds in the Milky Way, and nuclei of other galaxies. Data in two wavelength regions centered on 1878 and 2970 Å were recorded with high accuracy on glass plates by a photographic photometer. Data for the band centered on 2574 Å were recorded by direct, wide-field imaging with a 16-mm camera also used in Skylab for other experiments. The camera viewed the same area of the celestial sphere (with a slight offset) as the photometer. The two instruments [17] were mounted in the same housing, shown in figure 2-17 in place at the airlock.

 

Photometer Design

 

The two-wavelength photometer employed a sophisticated optical design shown schematically in figure 2-18. The articulated mirror system common to several experiments and shown previously in figure 1-7 directs light from the selected 7° by 9° sky area into the photometer. This beam is focused onto a grating to achieve spectral dispersion, and the two spectral ranges (centered on 1878 and 2970 Å) are isolated by appropriately placed diaphragms. Immediately in front of the photographic plate, an array of cylindrical lenses made of magnesium fluoride form the light from a star into an.....

 


Figure 2-17. Instrumentation for the S183 ultraviolet panorama experiment.

Figure 2-17. Instrumentation for the S183 ultraviolet panorama experiment.


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Figure 2-18. Optical train of the S183 ultraviolet panorama experiment. Link to a larger picture.

Figure 2-18. Optical train of the S183 ultraviolet panorama experiment.

 

....elongated rectangular image for each wavelength. The light from a calibration source is formed into a similar rectangular image on the photographic plate.

These spread-out, rectangular images are useful for accurate comparisons of the relative brightness of stars or other objects in the specified ultraviolet spectral ranges. Such images do not suffer from problems of photographic overexposure, such as may occur in the center of a tightly focused image of a star.

 

Schmidt-Cassegrain Camera System

 

The instrument for sky photographs in the 2574-Å region used a small Schmidt-Cassegrain telescope and a standard 16-mm data acquisition camera to record the....

 


Figure 2-19. Ultraviolet spectra recorded by the two-wavelength photometer in the S183 ultraviolet panorama experiment and corresponding microdensitometer traces. The spectral types and visual magnitudes of the stars are indicated above the tracings. Link to a larger picture.

Figure 2-19. Ultraviolet spectra recorded by the two-wavelength photometer in the S183 ultraviolet panorama experiment and corresponding microdensitometer traces. The spectral types and visual magnitudes of the stars are indicated above the tracings.

 

[19] ....sources in a 5° by 7° field of view. The desired wavelength selectivity was obtained by multilayer interference coatings deposited on the two mirror surfaces of the telescope. The ultraviolet region used here lies between the wavelengths recorded by the two-wavelength photometer. Detailed structure of extended sources in the ultraviolet could be recorded by this camera system but this was sacrificed to gain accuracy in the photometer. The two instruments in the S183 experiment were thus complementary in the data they generated.

 

Photometer Results

 

An exposure (fig. 2-19) made during the first Skylab mission shows nearly rectangular images for brighter stars. For dimmer stars, the images are less well formed. Also shown in this figure are the corresponding photographic density traces. For each star, the bands centered around 1878 and 2970 Å are on the right and left, respectively. The spectral types and visual magnitudes of the stars are given above the tracings. The far-left example is the unresolved superposition of a pair of stars. The expected decrease in ultraviolet intensity for stars of increasing visual magnitude is apparent (in going from a first magnitude to a sixth magnitude star the intensity decreases by a factor of 100, or approximately 2.512 per step). The intensity plots show the relative weakening of the far-ultraviolet (1878-Å) band from the hottest star on the left to the coolest star on the right.

 

Camera Results

 

The Schmidt-Cassegrain camera system obtained ultraviolet pictures of 36 star fields. A typical observation target, the Pleiades (fig. 2-20), was photographed in a 21-min exposure. It is an example of a group of some 300 young, hot stars (not all seen here) formed about 60 million years ago. The stars are embedded in nebulous matter, which produces the haze seen around the bright stars. This haze is seen in the ground-based,.....

 


Figure 2-20. Ultraviolet photograph of the Pleiades taken with the Schmidt-Cassegrain camera system (21-min exposure).

Figure 2-21. Star field and nebula around Eta Carinae (21-min exposure).

Figure 2-20. Ultraviolet photograph of the Pleiades taken with the Schmidt-Cassegrain camera system (21-min exposure).

Figure 2-21. Star field and nebula around Eta Carinae (21-min exposure).


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Figure 2-22. Color isophote plot of the star field and nebula around Eta Carinae, constructed from the ultraviolet photograph shown in figure 2-21.

Figure 2-22. Color isophote plot of the star field and nebula around Eta Carinae, constructed from the ultraviolet photograph shown in figure 2-21.

 

[21] ....blue-light photographs as well as in the 2574-Å image obtained with the Schmidt-Cassegrain camera system. The fainter stars appear doughnut-shaped because of a slight misalignment of the optics in the 16-mm camera.

 

Ultraviolet Intensity Variations Emphasized by Color Isophotes

 

A useful and aesthetically pleasing series of photographs resulted from the S185 experiment. They were processed at the Marshall Space Flight Center's Image Data Processing Facility. The derived intensity differences are reproduced in several colors to emphasize temperature variations. The hotter and brighter stars and regions appear white, whereas the decreasing intensity of ultraviolet radiation is shown by the other colors.

An example is shown in figures 2-21 and 2-22. Figure 2-21 is a 21-min ultraviolet exposure of the stars and the nebula in the neighborhood of the star Eta Carinae; figure 2-22 is the isophote plot derived from it. The isophote plots dramatically emphasize the decreasing intensity of reflected ultraviolet light in the nebula outward from the hot stars shown in white.

These isophote plots are very useful in observing features and trends, which can then be studied more quantitatively by microdensitometry of the original negatives.

 

A Galaxy as Viewed by All Three Ultraviolet Instruments

 

The millions of galaxies known in the cosmos are classified into four basic forms: elliptical, spiral, barred spiral, and irregular. The Large Magellanic Cloud, one of a pair of galaxies that are relatively close companions of the Milky Way, is an irregular galaxy with some structural features suggestive of a barred spiral. It is evident that star formation is still taking place in this galaxy, for it is studded with numerous knots of hot, young stars. "Hot" stars are always equated with "young" stars because their energy is radiated so rapidly that it is exhausted within a few tens of millions of years. On the other hand, the cooler stars, like the Sun, radiate their energy slowly and may exist for billions of years.

Figures 2-23 through 2-26 show the value of ultraviolet observations in locating regions where stars are being formed, defining the extent of these regions, and showing their relationship to the overall structure of the galaxy. The first photograph (fig. 2-23) is a red-light picture taken from the Earth at the University of Michigan observatory by Karl G. Henize. The cool, red stars are emphasized more than the hot, blue, or ultraviolet stars. The dominant feature is a long central "bar" that consists mainly of faint yellow or red stars. Also conspicuous are several bright nebulosities that are clouds of hydrogen caused to glow in the red by ultraviolet light from hot stars embedded within them. Thus the nebulosities give some indication of the location of the hot stars. On comparing this photograph with figure 2-24, which is a far-ultraviolet photograph taken from the surface of the Moon with the electrographic camera (Apollo 16 mission), it is interesting to note that every nebulous patch in figure 2-23 has a corresponding farultraviolet patch in figure 2-24. However, not every farultraviolet patch in figure 2-24 is conspicuous in figure 2-23. For example, region A is the most prominent feature of figure 2-24 but is barely visible in figure 2-23. This indicates a lack of gas in region A and suggests that the gas and dust have been depleted and that star formation may be ended within this region. On the other hand, the 30 Doradus nebulosity, the most prominent feature in figure 2-23 other than the "bar," is rather weak in the far ultraviolet, which indicates the presence of a great mass of gas and dust from which only few stars have been formed so far. This region, therefore, is probably the youngest and most active of those in which stars are formed.

Figure 2-25 is an ultraviolet photograph taken in the S019 ultraviolet stellar astronomy experiment (no-prism mode, effective wavelength of about 2500 A). The ultraviolet photograph in figure 2-26 was taken with the 16-mm camera of the S183 ultraviolet panorama experiment (effective wavelength 2574 Å); it shows an interesting transition between figure 2-23 (effective wavelength 6500 Å) and figure 2-24 (effective wavelength 1400 Å). For example, in figure 2-24, region B is slightly brighter than the complex around 30 Doradus, whereas it is slightly fainter in figures 2-25 and 2-26. This indicates that Region B has more hot, new-born stars than the complex around 30 Doradus, an observation that is not at all evident in photographs taken from the Earth.

Another interesting aspect of figure 2-24 is that the overall structure of the Large Magellanic Cloud differs considerably from that shown in figure 2-23. The ground-based photograph shows a 'bar" from which rudimentary spiral arms may trail. The far-ultraviolet picture suggests that the dynamic center of the galaxy lies in region A and that one of its major arms spirals outward in a clockwise direction and splits into two arms near region C. Thus the ultraviolet morphology presents a totally new concept of the basic structure of the galaxy.

 


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Figure 2-23. (top left) Large Magellanic Cloud: red-light photograph (effective wavelength 6500Å) taken from the Earth.
Figure 2-24. (bottom, left) Large Magellanic Cloud: far-ultraviolet photograph (effective wavelength 1400Å) taken from the surface of the Moon with the electrographic camera (Apollo 16 mission.)
Figure 2-25. (top, right) Large Magellanic Cloud:  ultraviolet photograph taken during Skylab's S019 stellar astronomy experiment (no-prism mode, effective wavelength ~2500Å)
Figure 2-26. (bottom, right) Large Magellanic Cloud: ultraviolet photograph (effective wavelength 2574Å) taken by the 16mm Schmidt-Cassegrain camera system during Skylab's S183 ultraviolet panorama experiment. Link to a larger picture.

Figure 2-23. (top left) Large Magellanic Cloud: red-light photograph (effective wavelength 6500Å) taken from the Earth.

Figure 2-24. (bottom, left) Large Magellanic Cloud: far-ultraviolet photograph (effective wavelength 1400Å) taken from the surface of the Moon with the electrographic camera (Apollo 16 mission.)

Figure 2-25. (top, right) Large Magellanic Cloud: ultraviolet photograph taken during Skylab's S019 stellar astronomy experiment (no-prism mode, effective wavelength ~2500Å)

Figure 2-26. (bottom, right) Large Magellanic Cloud: ultraviolet photograph (effective wavelength 2574Å) taken by the 16mm Schmidt-Cassegrain camera system during Skylab's S183 ultraviolet panorama experiment.


 

[23] GALACTIC X-RAY EMISSIONS

 

X-Rays from Beyond the Sun

 

Nothing was known about the appearance of the sky in the soft-X-ray portion of the electromagnetic spectrum, at wavelengths of 1 to 100 Å, before NASA began experiments with equipment mounted in sounding rockets. These wavelengths are absorbed in the uppermost regions of the Earth's atmosphere, which even the most advanced research balloons cannot reach.

By 1962, X-rays from the Sun, which are produced in the solar corona and in flares, had been discovered and mapped in some detail. Astronomers concluded that, if other stars emitted X-rays at the same rate as the Sun, there was little hope of discovering any other sources of X-rays in the galaxy, the nearest star being over 250000 times more distant than the Sun. It was therefore a major astronomical surprise when strong sources of X-rays were found, many at locations in the sky where only a distant, barely visible star, or no star at all, was known to exist.

Well over a hundred celestial objects emitting X-rays had been identified by 1973. One of the surprising results was the diverse nature of the discoveries. Sources of X-rays have been found both in and outside the Milky Way. Some of them are compact, almost point sources; others extend over large regions of the sky. More intriguing is the finding that some of them vary irregularly; some are pulsars and emit short pulses of X-rays on a regular schedule.

Moreover, a perplexing background glow of X-rays was found, coming almost uniformly from all directions. At the longest X-ray wavelengths observed (40 to 100 A), this background glow, when integrated over the whole sky, amounts to more power reaching the Earth than from the brightest point source, Scorpius X- 1. Even though sounding-rocket experiments continued until after Skylab, this diffuse glow has refused to admit resolution into individual starlike sources.

 

Skylab's Galactic X-Ray Mapping Experiment

 

The S150 galactic soft X-ray experiment was designed by William Kraushaar, Alan gunner, and their colleagues at the University of Wisconsin for the difficult 40- to 100-Å band. An important objective was to extend the search for the origin of galactic X-rays beyond the sensitivity possible with short flights of small research rockets, by placing a large-area soft-X-ray detector in orbit to collect data for a much longer time.

Unlike the "hard" X-rays used by hospitals and industry to penetrate deep into materials, the "soft" X-rays are completely absorbed by even a very thin layer of the Earth's atmosphere. Even the tenuous neutral gas in interstellar space absorbs these soft X-rays. The S150 experiment had a plastic film entrance window 2 µm thick, yet still tough enough to hold back the gas pressure of the argon-methane mixture in the proportional counter. A sophisticated pressure regulator kept this gas mixture at constant density, in spite of a continual, slow leakage of gas through the thin window until eventually an inevitable direct exposure to the burning rays of the Sun melted the membrane.

 

The S150 X-Ray Instrument

 

The S150 instrument, shown in figure 2-27, was a single large proportional counter 1500 cm2 in collecting area, electrically divided by fine wire ground planes into separate signal-collecting areas, and looking between collimating vanes (these vanes are the bronze-colored...

 


Figure 2-27. The S150 X-ray instrument.

Figure 2-27. The S150 X-ray instrument.


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Figure 2-28. The S150 X-ray instrument deployed in position.

Figure 2-28. The S150 X-ray instrument deployed in position.

 

.....objects in figure 2-27). These collimators, directed to different portions of the sky, defined three intersecting fields of view that allowed pin-point location of an X-ray star within 30 minutes of arc. The two star sensors on the side of the experiment feed pointing information to a computer.

The S150 instrument was not in Skylab but in the instrument unit of the second stage of the Skylab 3 Saturn IB rocket, which briefly orbited behind and below, Skylab. The S150 experiment, therefore, could not be attended by the astronauts. It was activated only after the Command and Service Module carrying the crew toward linkup with Skylab had pulled away, exposing the experiment to space. The instrument then unfolded as shown in figure 2-28, into its operating position and began automatically recording the X-rays on a tape recorder for later playback to ground tracking stations. The entire 130-ton second stage of Saturn IB was rolled and pitched by attitude-control thrusters to permit the X-ray instrument to scan selected areas of the Milky Way.

Figure 2-29 shows the Milky Way as seen from the solar system; the shaded regions are those scanned by the S150 experiment. The galactic center is approximately at the center of the map.

Because the experiment was designed to establish whether the seemingly diffuse X-ray emissions came....

 


Figure 2-19. The Milkay Way as seen from the solar system. The three overlapping rectangles illustrate the fields of view of the three collimators at one instant of time. Link to a larger picture.

Figure 2-19. The Milkay Way as seen from the solar system. The three overlapping rectangles illustrate the fields of view of the three collimators at one instant of time.



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Figure 2-30. X-ray emission from the Crab Nebula, for two collimator fields of view. Several energy channels were seen successively by two counters, each in three energy channels. Link to a larger picture.

Figure 2-30. X-ray emission from the Crab Nebula, for two collimator fields of view. Several energy channels were seen successively by two counters, each in three energy channels.

 

....from many unresolved weak X-ray sources, the response of the instrument to localized sources was important. The strong source associated with the Crab Nebula provided a calibration of the instrument characteristics. Figure 2-30 shows a record from the Crab Nebula for two of the fields of view and several energy channels. The field of view of counter 3 swept across the target first; the field of view of counter 2 swept across it about 30 sec later.

Analysis of the data has provided strong evidence that the soft-X-ray background glow cannot be explained as the cumulative effect of thousands of unresolved X-ray stars. Thus it remains an enigma despite Skylab's efforts. Nearby representatives of every category of star known to exist in the Milky Way (in numbers sufficient to provide a possible class of closely spaced X-ray sources) passed slowly through the experiment's field of view, while the long-wavelength X-ray flux failed to reveal an X-ray star among even the closest candidates. Together with clues collected in other investigations, the Skylab experiment suggests that the soft-X-ray glow originates, not from stars of any kind, but from previously unsuspected hot plasmas in the vast reaches between the stars.


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