Ultraviolet astronomy

Ultraviolet astronomy explores the universe by focusing on wavelengths of the electromagnetic spectrum that are shorter than those of visible light. This portion of the spectrum is particularly important to astronomy, as practically all stars and many of the most abundant elements in the universe emit energy in the ultraviolet range.

Overview

Any material with a temperature above absolute zero emits electromagnetic radiation, and that radiation carries with it information about the nature of the event that produced it. An object will emit radiation over a range of wavelengths, with a concentration at a single wavelength. Very hot objects produce shorter wavelengths, while cooler objects emit longer wavelengths. For example, as metal is heated, it first glows red (the longer wavelengths of visible light), then, as its temperature increases, it begins to glow in the shorter-wavelength yellow light. In space, objects that are very cold, perhaps only a few degrees above absolute zero, will emit radiation in the very long infrared and radio wavelengths. At the other extreme, very hot stars give off ultraviolet radiation, X-rays, and gamma rays.

Ultraviolet astronomy focuses on the area of the spectrum that is beyond violet light—the shortest wavelengths the eye can see. The ultraviolet portion of the spectrum begins at a Wavelength of 390 nanometers, ranges to the extreme ultraviolet at 90 nanometers, and merges into the X-ray portion of the spectrum at 10 nanometers.

While the visible portion of the spectrum can be observed from the surface of the Earth, observations at ultraviolet wavelengths must be done outside the Earth’s atmosphere. Ultraviolet radiation is readily absorbed by gases, both in space and in the Earth’s atmosphere. Only the longest wavelengths of ultraviolet light penetrate the atmosphere. It is this radiation that is responsible for the destructive tanning effects of the Sun on the skin. Screening effects of the atmosphere protect life on Earth from the more harmful, shorter-wavelength ultraviolet radiation. At the same time, the atmosphere prevents astronomers from easily collecting information about the universe from this important range of the spectrum.

Practically every object in the universe emits some radiation at ultraviolet wavelengths. Any material that has a temperature between 10,000 and 1 million kelvins thermally emits most of its energy in the ultraviolet. This range of the spectrum is important to the knowledge of celestial objects, since the atmospheres of most stars, the surfaces of very massive stars, white dwarf stars, and regions of hot interstellar gas all fall within this temperature range. Every element emits and absorbs energy according to a characteristic pattern. By analyzing the pattern, or spectrogram, astronomers can determine the chemical composition of very distant objects. A spectrum is governed by the temperature, density, and chemical composition of the object emitting the energy, as well as how the energy has been altered by intervening processes en route to the instruments. In addition, the elements that are most abundant in the universe, such as hydrogen, helium, carbon, nitrogen, oxygen, and silicon, all have spectral features that are prominent in the ultraviolet. For this reason, Ultraviolet astronomy can provide astronomers with important information about the universe.

Although instruments used in ultraviolet astronomy are designed to operate remotely, they can be very similar to optical instruments. Ordinary telescope mirrors will focus ultraviolet light. Electronic detectors record the image, or, in some cases, the image can be recorded on regular photographic film. A spectrograph can record such information by passing the radiation through a narrow slit and then through a prism, which separates the radiation into its component wavelengths. The result, a spectrogram, is then recorded on film.

The first ultraviolet telescopes were flown on high-altitude weather balloons. Later, they were launched on rockets, which raised them above the atmosphere for a few minutes at a time. The best way to gain access to ultraviolet information is to place an ultraviolet observatory in orbit. The first satellites to carry ultraviolet instruments were the Orbiting Astronomical Observatories (OAOs), a series of four identical satellites that carried different instrument packages to measure ultraviolet radiation from stars and interstellar gas. Only two of the four spacecraft proved functional. The first Satellite failed after two days in orbit, and another failed to reach orbit. The second OAO achieved a general ultraviolet survey of the sky, discovering ultraviolet sources within the galaxy and measuring ultraviolet light from bright nearby galaxies. The final OAO, Copernicus, was the first to target specific ultraviolet sources. Carrying a 0.8-meter telescope, it was launched on August 21, 1972, and was functional for nine years. Copernicustook the first detailed look at objects in a wide range of the ultraviolet spectrum.

Detailed ultraviolet pictures of the Sun were produced with the OAO mission and later using the Solar Maximum Mission (SMM) satellite. Spectacular ultraviolet solar studies resulted from the American crewed space station Skylab, which was launched in 1973. Three crews of three astronauts inhabited the space station for a total of five and one-half months. Throughout this time, they kept a continuous surveillance of the Sun with the Apollo Telescope Mount (ATM). The ATM carried eight telescopes, which observed the Sun in wavelengths ranging from visible light through the ultraviolet and into the X-ray range.

In 1978, two years before the OAO Copernicus ceased operation, the International Ultraviolet Explorer (IUE) was launched. The IUE was a joint venture by the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the British Science Research Council (SRC). Although the IUE was equipped with a telescope smaller than that of Copernicus (only 45 centimeters), it carried more modern ultraviolet detectors and was able to observe much fainter stars over a broader range of wavelengths. The IUE was run much like a traditional observatory and, as such, was designed to be used by visiting scientists rather than by a select group of researchers. The IUE remained stationed in an orbit that is geosynchronous (remaining constantly over one area of Earth), about 36,000 kilometers above the Atlantic Ocean, where it remained in continuous contact with at least one of two ground stations in the United States and Europe. This was an improvement over earlier satellites, which could not remain in continuous contact with ground stations. Their observations were also limited by low orbits, with a large percentage of the field of view blocked by the Earth. The IUE was decommissioned on September 30, 1996. Sadly it was merely shut off due to budget concerns, even though this workhorse observatory was functioning nearly at full capability.

Astro 1 was a Spacelab mission launched aboard the space shuttle in early 1990; it carried three ultraviolet telescopes. For the duration of the mission, project scientists conducted observations in the ultraviolet and X-ray regions. Other important ultraviolet satellites have been the European TD-1 and the Soviet Astron satellite. The TD-1, launched in 1972, measured the magnitudes of more than thirty thousand stars in four different spectral regions and gathered ultraviolet spectra from more than one thousand stars. The Soviet Astron satellite, launched in 1983, was similar in size and scope to the OAO Copernicus.

Ultraviolet astronomy advanced greatly with observatories such as the Hubble Space Telescope (HST), launched from the space shuttle Discovery on mission STS-31 on April 25, 1990; the Extreme Ultraviolet Explorer (EUVE), launched on June 7, 1992; and the Far Ultraviolet Spectroscopic Explorer (FUSE), launched on June 24, 1999. The HST has proved to be a powerful ultraviolet instrument, making exciting discoveries in the far ultraviolet, despite preliminary optical difficulties, which were repaired in 1993 during space shuttle mission STS-61. Thereafter the HST returned amazing images of distant galaxies, interstellar dust, and other objects from deeper in space than had ever before been observed. The EUVE was designed to survey the cosmos for objects emitting very energetic short-wavelength radiation in order to discover many new objects, including perhaps ten times as many white dwarf stars as previously known. FUSE was designed to carry a 2-meter ultraviolet telescope that would investigate objects in the far ultraviolet and extreme ultraviolet. NASA’s Wind mission was conceived to use multiple instruments, including ultraviolet instruments, to study the Solar wind and sample plasma waves, energetic particles, and electric and magnetic fields. The European Space Agency’s Lyman Observatory was designed to examine the dynamics of the Milky Way’s halo as well as comets.

FUSE was designed for only a three-year primary mission but was still in reasonable operational condition in 2002, despite having difficulties with its reaction wheels (used for pointing the observatory at selected celestial targets). Budget issues almost resulted in the cessation of FUSE observations, but the use of the orbital telescope was extended until July 12, 2007, when controllers lost the final reaction wheel. Despite efforts to restore the observatory, on September 7, 2007, FUSE investigations ceased. This relatively inexpensive Explorer-class satellite (also known as Explorer 77) led to more than four hundred published scientific papers and advanced the careers of many young astronomers specializing in ultraviolet astrophysics.

Applications

Each energy region in the electromagnetic spectrum allows astronomers to “see” objects in a unique way. The more information that can be discovered about the nature of a celestial object in each of these energy areas, the more completely the object can be understood.

All objects known to exist in the universe—from comets and planets to stars, galaxies, and quasars—can be effectively studied in the ultraviolet range. Ultraviolet telescopes see the hottest stars of all; as a result, they tend to pick out the youngest star groups in the sky. Ultraviolet astronomy can thus focus on the youthful clusters of stars that lie close to regions of star birth. With this particular window, ultraviolet astronomy has been very useful for mapping regions of star formation, both in the Milky Way and in distant galaxies. Other galactic studies have shown that the Milky Way, as well as other galaxies, is surrounded by a hot halo of gas.

There are excellent images of the Sun in the ultraviolet. Views of the ultraviolet Sun reveal different layers of its chromosphere, transition region, and lower corona. Bright, scintillating points of ultraviolet light in the Sun’s atmosphere provide a measure of magnetic activity within the Sun, with perhaps even more accuracy than the sunspots that are seen on the visible photosphere. By observing other stars in the ultraviolet, astronomers have gained valuable knowledge about the nature of stars, which correlates with what is known about the Sun. It has been shown that many stars have hot outer atmospheres similar to the Sun. A new class of stars was discovered that had distinguishing characteristics visible only in ultraviolet light.

Ultraviolet astronomy has been very useful in studying binary star systems. A binary system is a pair of stars orbiting around their common center of mass. In a binary system, one star can be much brighter in optical wavelengths, and only a single spectrum can be observed. If the companion star is much hotter, however, it will dominate the spectrum of the system at ultraviolet wavelengths. A binary system gives astronomers a tool with which they can study the nature of these dimmer hot stars. Previously unobserved hot companions have been discovered in stars not suspected earlier of being binaries.

The supernova is another area in which ultraviolet astronomy can contribute significantly. A Supernova is a stellar explosion in which all or most of the star’s mass is expelled. Astronomers have studied the remains of supernova explosions as well as observed them in the beginning stages of development. Ultraviolet observations can determine the chemical composition of the layers of the star expelled by the explosion. Astronomers discovered that a Nova explosion in Cygnus in 1978 produced much nitrogen, while the supernova that created the well-known Crab Nebula threw out relatively small amounts of carbon. These facts are important clues to learning how new elements are formed, as well as to understand the mechanisms behind supernovae.

The IUE observed Comet Kohoutek in 1976, finding a very bright image in ultraviolet wavelengths. Comet IRAS-Iraki-Alcock was observed in 1983 and Comet Halley was observed in 1986. In combination with observations at other wavelengths, it was found that the comets are similar in composition, suggesting that they have a similar origin.

Among IUE’s legacy of ultraviolet investigations were 104,000 individual observations, the discovery of aurorae at Jupiter, the first determination of the water-loss rate in a comet, the production of the first orbital Radial velocity curve for Wolf-Rayet stars, determination of the progenitor of Supernova 1987A, the first imaging of galactic halos, and the production of 44,000 stellar spectra per year, to name but a few. In a very real sense, IUE, more than anything before it, expanded ultraviolet astronomy from an interesting concept to a highly active and fruitful area of astrophysical observations.

Ultraviolet astronomy has confirmed some long-standing theories that previously lacked sufficient evidence. The theory of a “gravitational lens” had been predicted by the relativity theories of Albert Einstein but had never been supported by solid evidence. The gravitational lens is a process in which the gravitational field of a very massive object acts as a lens, bending the radiation from a more distant object behind it, distorting its image and often creating a double or multiple image. Observations by the IUE helped to indicate that such gravitational lenses exist. The first such lens system was discovered in 1979. Hubble routinely discovered new gravitational lensing objects.

A full list of scientific achievements of EUVE would be too long to include in this article. Among some of those discoveries were the production of an all-sky catalog containing a total of 801 ultraviolet-emitting objects (in wavelengths from 7 to 76 nanometers), participation in coordinated observations of numerous objects at different wavelengths across the electromagnetic spectrum, some of the first ultraviolet detections of extragalactic objects, measurements of quasi-periodic oscillations in dwarf novae, analyses of extreme ultraviolet spectral White dwarf star companions to main sequence stars, the detection of helium in a hot white dwarf, and the first extreme ultraviolet observations of the Coma Cluster.

Ultraviolet observations with the Hubble Space Telescope involved use of the observatory’s Goddard High ResolutionSpectrograph (GHRS), Faint Object Camera (FOC), and Faint Object Spectrograph (FOS). GHRS and FOS were replaced during the second Hubble servicing mission (STS-82 in February 1997). In their place were inserted the Space Telescope Imaging Spectrograph (STIS) and Near Infrared Camera and Multi-Object Spectrometer (NICMOS). During shuttle servicing mission 3B (STS-109 in March 2002) the FOC was replaced by the Advanced Camera for Surveys (ACS). STIS suffered a malfunction in August 2004, and portions of the electronics for ACS failed in 2006 and 2007. STIS and ACS were scheduled to be repaired during one final shuttle servicing mission, during which repairs would restore Hubble’s capability to conduct ultraviolet astronomy. That shuttle mission was also intended to insert a new Wide Field Camera 3 and the Cosmic Origins Spectrograph to expand the observatory’s capability to make ultraviolet measurements of objects at great distances (and hence tremendously early times in the cosmic past).

The July 2008, edition of Physics Today presented research using ultraviolet data from both FUSE and HST that indicated that as much as 40 percent of the anticipated baryonic matter is missing from the portion of the universe along the line of sight to energetic objects such as quasars. The study involved collecting ultraviolet spectral signatures of quasars. What was anticipated was a typical Quasar spectrum, which also incorporated into it absorption lines for the absorbing gas between the quasar source and the telescope in Earth orbit detecting that radiation. By analyzing the depth of absorption lines cutting into the quasar’s spectral emissions, astronomers were able to calculate the density of the baryonic matter (originally formed within just a few minutes of the Big Bang) along the line of sight. It turned out to be too low. Obviously, more investigations of the Intergalactic medium were necessary, or an adjustment of cosmological models would be in order.

FUSE provided insight into the abundance of deuterium in stars and studied a wide range of astrophysical objects, such as the intergalactic medium, cool stars, and galactic structures. By recording absorption and emission lines in the far-ultraviolet portion of the electromagnetic spectrum, FUSE increased our understanding of galactic, intergalactic, and extragalactic chemical processes.

On December 25, 2021, NASA launched its James Webb Space Telescope. In the years that followed, this telescope has provided images and data that have revolutionized scientific understanding in many different areas. The Hubble Space Telescope, nonetheless, remains the more capable instrument in terms of collecting data in the realm of ultraviolet light. The Webb telescope is more specialized toward the gathering of near and mid-infrared light.

Context

For thousands of years, the human eye was the only astronomical instrument. The eye evolved to be most sensitive to the range of visible light, the most abundant source of radiation at the surface of the Earth. Any celestial objects that were dim or emitted radiation at mostly non-optical wavelengths remained invisible to the eye, which limited the range of information about the universe scientists could study.

The invention of the telescope radically altered astronomy, not only because of the fainter objects it allowed astronomers to see but also because it opened up the possibility that there was more to the universe than what the human eye was able to image. Astronomy improved dramatically over the next few centuries but remained optical. The first sign that there was another way to look at the universe with anything other than optical wavelengths came in 1800, when Infrared radiation was discovered by Sir William Herschel, who placed a thermometer just outside the red range of the visible light separated by a prism.

The opening up of the wavelengths of electromagnetic energy got underway with the rapid growth of radio astronomy in the 1950s and 1960s and with the birth of the space program during the same period. The space program allowed ultraviolet astronomy to become an important new area of study. The potential value of ultraviolet observation from space was proposed to the U.S. Air Force in 1946 by the American astrophysicist Lyman Spitzer, Jr. With the establishment of NASA in 1958, the concept of placing orbiting observatories in Earth’s orbit became a reality, and a series of orbiting observatories were launched over the next twenty years.

The result of more than forty years of observing in all ranges of the spectrum is that astronomers now have a more complete understanding of the processes occurring in the universe. Today’s astronomers have taken images of the stars and galaxies that were unimaginable to the ancients, or even to the astronomers of a few decades past. Ultraviolet astronomy is now at a point where future missions will lose the “frontier” feel of the early missions, with increasingly complex and specialized missions. Even so, exciting new discoveries will continue to be made. In addition to new observations, research using information from the years of observations made by the satellites since the 1960s will allow astronomers to gain new insights into virtually every area of the universe.

Bibliography:

Arny, Thomas T. Explorations: An Introduction to Astronomy. 3d ed. New York: McGraw-Hill, 2003.

Barstow, Martin A., and Jay B. Holberg. Extreme Ultraviolet Astrophysics. Cambridge, England: Cambridge University Press, 2007.

Henbest, Nigel. Mysteries of the Universe. New York: Van Nostrand Reinhold, 1981.

"Hubble Launches Large Ultraviolet-Light Survey of Nearby Stars." NASA.gov, 5 Nov. 2020, www.nasa.gov/feature/goddard/2020/hubble-launches-large-ultraviolet-light-survey-of-nearby-stars. Accessed 28 Sept. 2023.

Karttunen, H. P., et al., eds. Fundamental Astronomy. 5th ed. New York: Springer, 2007.

Marten, Michael, and John Chesterman. The Radiant Universe. New York: Macmillan, 1980.

"Observing Ultraviolet Light." NASA Hubblesite 30 Sept. 2022, hubblesite.org/contents/articles/observing-ultraviolet-light. Accessed 28 Sept. 2023.

Time-Life Books. The New Astronomy. Alexandria, Va.: Author, 1989.