Astronomy, Kinds of

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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

Approximate Wavelengths (m) Wavelengths Other Units Photon Energies Greater Than Frequency Name for Spectral Brand Produced by Temperatures in Region of (K) Examples of Astrophysical Objects of Interest Examples of Present/Planned Telescopes to Use for Observations
10-13 80.6MeV Gamma-ray 108 Cosmic rays, Space only: CGRO (1991-
10-12 80.6MeV gamma-ray 2000), INTEGRAL (2002-),
10-11 0.8MeV bursters, nuclear GLAST (2005-)
10-10 1Å, 0.1nm 80.6keV Hard X-ray 107 Accretion disks in
10-9 10Å, 1nm 8.06keV binaries, black holes,
hot gas in galaxy
clusters, Seyfert Space only: ROSAT (1990-1999),
galaxies ASCA (1993-), Chandra (1999-),
XMM (2000-)
10-8 100Å, 10nm 0.806keV Soft X-ray 106 Supernovae remnants,
neutron stars, X-ray
stars, superbubbles
10-7 1000Å, 100nm 80eV XUV/EUV Far UV 105 White dwarfs, flare stars, Space only: EUVE (1992-),
O stars, plasmas FUSE (1999-)
2 x 10 -7 200nm Ultraviolet 105 Hot/young stars, Space only: HST (1990-), Astro-
Orion-like star nurseries, 1/2 (1990, 1995), SOHO (1996-)
interstellar gas, helium
from the big bang, solar
corona, Ly alpha forest
4 x 10 -7 400nm Violet 104 B stars, spiral galaxies, Ground: Keck, Gemini (1999-),
nebulae, Cepheids, Magellan (1999-), Subaru (1999-)
Visible QSOs VLT (1999-), MMT (2000-),
7 x 10 -7 700nm Red 104 K, M stars, globular Space: HST
clusters, galaxy mass
8-50 x 10-7 0.8-5μm Near-infrared Circumstellar dust shells Ground: CHFT, CTIO, IRTF, Keck
comets, asteroids, high Magellan, Subaru, UKIRT, VLT
z galaxies, brown Space: ISO (1995-98), SIRTF
dwarfs (2002-)
5-30 x 10-6 5-30μm Mid-infrared 103 Cool interstellar dust, Ground: IR optimized telescopes:
PAHs, organic molecules, IRTF, UKIRT, Gemini
planetary nebulae, Airborne: SOFIA (2005-)
molecular hydrogen Space: ISO (1995-1998), SIRTF
3-20 x 10-5 30-200μm Far-infrared Ultraluminous/starburst Airborne: SOFIA
galaxies, debris disks, Space: ISO, SIRTF
Kuiper Belt Objects
3.5-10 x 10-4 350mm-1mm Sub-millimeter High z galaxies/proto- Ground: HHT, JCMT, SMA (1999-)
galaxies; molecular Space: SWAS (1998-), FIRST
clouds; interstellar dust (2008-)
10-3 1 mm 300,000MHz, Millimeter Molecules in dark dense Gound: IRAM, ALMA
300GHz interstellar clouds (CO)
10-2 1cm 30,000MHz, 10 Cosmic microwave Space: COBE (1989-), MAP
30GHz background (2001-)
10-1 10cm 3000MHz, 1 Galaxy studies,
3GHz Hydrogen clouds
(21cm), masers
1 1m 300MHz Quasars, radio galaxies, Ground: Arecibo, VLA, VLBA,
hot gasses in nebulae MERLIN
Space: VSOP (1997-)
10 10 m 30MHz Radio <1 Synchroton radiation
(electronics spiraling in
102 100m 3MHz magnetic fields) from
supernovae remnants,
magnetic lobes of radio
103 1km Long wave No data yet. We could No missions planned, space
explore cosmic ray only due to opaqueness of
104 10km <30kHz Very long origins, pulsars, super- Earth's ionosphere. Lunar
and greater wave/very novae remnants, and telescope(s) perhaps.
low frequency look for coherent
SOURCE: Different "kinds" of astronomy separated by wavelength. Adapted and expanded from J. K. Davies, Astronomy from Space, 1997, Table 1.1, p.2.

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


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. <>.

The Solar and Heliospheric Observatory. European Space Agency/National Aeronautics and Space Administration. <>.


Astronomy, Kinds of