Astronomy, Kinds of

Astronomers study light, and almost everything we know about the universe has been figured out through the study of light gathered by telescopes on Earth, in Earth's atmosphere, and in space. This light comes in many different wavelengths (including visible colors), the sum of which comprises what is known as the electromagnetic spectrum. Unfortunately, Earth's atmosphere blocks almost all wavelengths in the electromagnetic spectrum. Only the visible and radio "windows" are accessible from the ground, and they thus have the longest observational "history." These early restrictions on the observational astronomer also gave rise to classifying "kinds" of astronomy based on their respective electromagnetic portion, such as the term "radio astronomy."

Over the past few decades, parts of the infrared and submillimeter have become accessible to astronomers from the ground, but the telescopes needed for such studies have to be placed in high-altitude locations (greater than 3,050 meters [10,000 feet]) or at the South Pole where water absorption is minimal. Other options have included balloon experiments, airborne telescopes, and short-lived rocket experiments.

Presently, the field of astronomy is enriched immensely by the accessibility of several high-caliber airborne telescopes (e.g., Kuiper Airborne Observatory [KAO], Stratospheric Observatory For Infrared Astronomy [SOFIA]) and space telescopes, all of which are opening up other, previously blocked windows of the electromagnetic spectrum (such as gamma ray,X ray , ultraviolet , far infrared, millimeter, and microwave). Additionally, modern astronomers often need to piece together information from different parts of the electromagnetic spectrum to build up a picture of the physics/chemistry of their object(s) of interest. The table on page 12 summarizes some of the links between wavelength, objects/physics of interest, and current/planned observing platforms. It provides a flavor of how the field of astronomy today varies across wavelength, and hence, by the energy of the object sampled.

The field of astronomy is also quite vast in terms of the physical nature, location, and frequency of object types to study. The field can be broken down into four categories:

1. Solar and extrasolar planets and planet formation, star formation, and the interstellar medium;
2. Stars (including the Sun) and stellar evolution;
3. Galaxies (including the Milky Way) and stellar systems (clusters, superclusters, large scale structure, dark matter ); and
4. Cosmology and fundamental physics.

The Study of Planets, Star Formation, and the Interstellar Medium

One of the most important developments in the first category over the past few years has been the detection of several planets orbiting other stars along

with the detections, through deep infrared sky surveys, of substellar objects (brown dwarfs ), whose spectral characteristics have been found to be similar to that of giant planets. Additionally, through superb Hubble Space Telescope (HST) imaging with its infrared camera and through infrared instruments on large ground-based telescopes, astronomers have started to directly observe the protostellar disks out of which planets are forming.

Astronomers have learned that the formation of stars and protostellar disks start in the interstellar medium, the vast "vacuum" of gas and dust between the stars, but astronomers are only just learning what the structure of the interstellar medium really is and how it affects and is affected by stellar birth (dust-enshrouded stars) and death (planetary nebulae and supernovae ). Another step forward is to understand star formation in other galaxies, for astronomers readily see active star formation in the arms of spiral galaxies and in the collisions of galaxies.

The Study of Stars and Stellar Evolution

The study of stars and their evolution is perhaps one of the oldest subfields of astronomy, and has benefited greatly from observational evidence dating back over hundreds of years. This is the core of astronomy because stars are truly the fundamental blocks of the universe, creating and destroying chemical elements, acting as light posts in galaxies, and giving insights into understanding mysterious phenomena, such as black holes and gamma-ray bursts. Understanding such exotic and high-energy events is critical to the advancement of astronomy and fundamental physics, where such "events" occur in conditions impossible to create on Earth. Astronomers are even continuing to learn new things about the nearest star, the Sun, through, for example, recent amazing images (e.g., solar storm activity) from the Solar and Heliospheric Observatory (SOHO) satellite.

The Study of Galaxies and Stellar Systems

Just as stars are the building blocks of galaxies, galaxies are the building blocks of the universe. The study of their types, sizes, distribution, and interactions with neighbors is essential to understanding the nature and future of the universe. The study of the earliest galaxies (galaxy "seeds") is the main motivating factor behind building larger ground-based telescopes and more sensitive infrared space telescopes, such as the Space InfraRed Telescope Facility (SIRTF) and the Next Generation Space Telescope (NGST). Astronomers know from the deepest HST images that the early universe was composed of many irregular, active, star-formation-rich galaxies. Astronomers do not know, however, how such a chaotic early universe evolved to what is seen in our local group, whose component galaxies are quite different.

Among the many mysteries in the universe is the dark matter in galaxies and clusters. We know little about its amount (speculated to be roughly 10 to 100 times greater than the observed mass), structure, location, and makeup, despite evidence from beautiful HST pictures of gravitational lenses , and observations of hot gases in galaxy clusters measured by sensitive X-ray telescopes (e.g., German Röntgensatellit (ROSAT), Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA), American Chandra).

Another very active field is the study of elusive quasars , observed out to a distance when the universe was less than 10 percent of its present age. Recent far infrared and X-ray data have revealed a large population of these objects, indicating that many of them might be heavily obscured by dust and therefore not seen by earlier visible light surveys. Astronomers know very little about the power mechanisms of these objects, and this field is a very active area for today's radio, X-ray, and gamma-ray astronomers.

The Study of Cosmology and Fundamental Physics

The area of cosmology and fundamental physics is perhaps the most elusive and yet also the most important field in astronomy because it encompasses the other three categories. Cosmology literally means "the study of the beginning of the universe." Cosmologists, however, strive to answer questions not only about the universe's origin but also about its evolution, contents, and future.

It is now widely believed that the universe started with a "big bang," with the most conclusive evidence being precise measurements of variations in the big bang signature 2.7K cosmic microwave background by the Cosmic Background Explorer satellite in 1997. Other recent advances in this subfield have come through all-sky infrared surveys, which have mapped out the distribution of galaxies across the sky; additional observational evidence that has led to more accurate estimates of the rate of expansion of the universe and its deceleration parameter; and increased computing power for numerical simulations that attempt to solve the ever-present many-bodied problem .

Astronomers can comprehend the universe only through what they can see (limited by the sensitivities of the instruments used), what they can infer from observational data and numerical simulations, and what is supported by theory. As time has progressed, so too has the toolkit of the astronomer, from easier access to satellites, large ground-based telescopes, arrays of telescopes around the world working as one, increased computing power, and more sensitive cameras and spectrometers . As long as there is a way to improve detection techniques and strategies, astronomers will never run out of new discoveries or rediscoveries among the many "kinds" of astronomy.

see also Hubble Space Telescope (volume 2); Observatories, Ground (volume 2); Observatories, Space-Based (volume 2).

Kimberly Ann Ennico

Bibliography

Davies, John K. Astronomy from Space: The Design and Operation of Orbiting Observatories. Chichester, UK: Praxis Publishing, 1997.

Henbest, Nigel, and Michael Marten. The New Astronomy, 2nd ed. Cambridge, UK:Cambridge University Press, 1996.

Maran, Stephen P., ed. The Astronomy and Astrophysics Encyclopedia. New York: Van Nostrand Reinhold, 1992.

Internet Resources

The Hubble Space Telescope. Space Telescope Science Institute. <http://www.stsci.edu/hst/>.

The Solar and Heliospheric Observatory. European Space Agency/National Aeronautics and Space Administration. <http://sohowww.nascom.nasa.gov/>.