Radio astronomy studies radiation with wavelengths greater than approximately one millimeter. Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.Though some radio waves are produced by astronomical objects in the form of thermal emission, most of the radio emission that is observed from Earth is seen in the form of synchrotron radiation. Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths. A wide variety of objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.

The instruments used to catch the radiowaves, are radiotelescopes (large-sized antennas), or an array of them. In some cases the separations between the antennas can be of hundreds of kilometers, allowing to attain a high angular resolution. Such technique is called Very Long Baseline Interferometry (VLBI).

The most representative instrument for radio astronomy is the Very Large Array (VLA), which consist on 27 antennas in a Y-shaped configuration, of 25 m diameter each. A new facility, ALMA (Atacama Large Millimeter/submillimeter Array) is under construction. It will be comprised of a giant array of 12-m submillimetre quality antennas, with baselines of several kilometres.

The Very Large Array

Optical astronomy. Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy. Optical images were originally drawn by hand. In the late nineteenth century and most of the twentieth century, images were made using photographic equipment. Modern images are made using digital detectors, particularly detectors using charge-coupled devices (CCDs). Although visible light itself extends from approximately 400 nm to 700 nm, the same equipment used at these wavelengths is also used to observe some near-ultraviolet and near-infrared radiation.

Among the largest and advanced telescopes on Earth there are the twin 10 m Keck Telescopes, the four 8-m telescopes Very Large Telescope (VLT) and the 10.4 m Gran Telescopio de Canarias (GTC). There are also telescopes in space, the most important being the Hubble Space Telescope. Future missions such as GAIA will create the largest and most precise three dimensional chart of our Galaxy.

The Gran Telescopio Canarias or GTC

Ultraviolet astronomy is generally used to refer to observations at ultraviolet wavelengths between approximately 10 to 320 nm. Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei. However, ultraviolet light is easily absorbed by interstellar dust, and measurements of the ultraviolet light from objects need to be corrected for extinction.

International Ultraviolet Explorer
Source: ESA

X-ray astronomy is the study of astronomical objects at X-ray wavelengths. Typically, objects emit X-ray radiation as synchrotron emission (produced by electrons oscillating around magnetic field lines), thermal emission from thin gases (called bremsstrahlung radiation) that is above 10⁷ K, and thermal emission from thick gases (called blackbody radiation) that are above 10⁷ K. Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be done from high-altitude balloons, rockets, or spacecraft. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.

XMM-Newton
Source: ESA

Gamma ray astronomy is the study of astronomical objects at the shortest wavelengths of the electromagnetic spectrum. High energy gamma rays may be observed directly by satellites such as the Gamma-ray Large Area Space Telescope (GLAST) or AGILE. To observe very high energy gamma rays (VHE) specialized telescopes called atmospheric Cherenkov telescopes, such as MAGIC or HESS, are required. The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.

The MAGIC Telescope
Source: MAGIC

In neutrino astronomy, astronomers use special underground facilities such as GALLEX, Kamioka, SuperKamioka or the Sudbury Neutrino Observatory for detecting neutrinos. These neutrinos originate primarily from the Sun but also from supernovae.

Super Kamiokande
Source: Super Kamiokande

Cosmic ray radiation consists on very high energy particles, coming from outside the solar system that can be observed hitting the Earth's atmosphere.

Gravitational waves are extremely difficult to detect. A few gravitational wave observatories have been constructed or are under construction.