Observation in astronomy is almost completely confined to detecting the electromagnetic radiation from planets, stars, gas, and dust. Most objects appear through the visible light that they emit or reflect. With the help of photography, electronics, and rocketry, some objects are observed in other bands of the electromagnetic spectrum. Cool objects are observed in the infrared. The hottest stars are observed in the ultraviolet. Objects that have strong gravitational fields, such as white dwarfs, neutron stars, and black holes, or that are generate by explosions, such as supernovae remnants, are seen in by the x-rays and gamma-rays they emit. Hot gases in magnetic fields are seen by their radio emission.
At its birth, astronomy was an optical science. Our eyes are designed to see the Sun at its most brilliant at Earth's surface. The light emitted by the Sun is the emission from a 5000° body, which has a peak emission that falls into the middle of our eye's frequency range. Hotter stars produce light that is predominately blue. Cooler stars produce red light. Objects cooler than these red stars are invisible to us, although they emit infrared radiation. Inevitably the limitation of our eyes confine visual astronomical studies to the study of the Solar System planets and the stars.
Much of the electromagnetic radiation flowing through space is unable to reach us. Most infrared and ultraviolet radiation and all x-ray and gamma-rays from space are absorbed by the atmosphere. To see this radiation, we must place our instruments into space. Even optical astronomy is improved when we place our telescopes into space, because turbulence within the atmosphere limits the resolution of an optical telescope. The birth of modern rocketry was therefore the birth of modern astronomy.
Once above the Earth's atmosphere, we are only limited in what we can see by the gas and dust within our own Galaxy. Our galaxy is transparent to gamma-rays and to the more energetic x-rays, but it is opaque to a large portion of the ultraviolet spectrum. At infrared and optical frequencies we can see out of the Milky Way when we look perpendicular to the plane of the Galaxy, but our view in the plane of the galaxy is limited by clouds of gas and dust.
The atmosphere is transparent to bands of radio spectrum , so radio astronomy remains ground-based. This enables us to build radio telescopes that are many times the size of the largest optical ground telescope. Radio telescopes have an additional property: they can be linked together electronically to produce an even larger telescope. When the signals of widely separated telescopes are combined, the resolution of the image they produce improves. This is why arrays of radio telescopes, such as the Very Large Array (VLA) in New Mexico, are used in astronomy. The most precise images of the sky are from the combined observations of telescopes at opposite ends of the Earth.
We observe the most energetic gamma-rays by watching them interact with Earth's atmosphere. These gamma-rays can carry a million times the energy that is carried by the typical gamma-ray observed in space. Such gamma-rays pass through space-based detectors without interacting, but when one of these gamma-rays strikes the atmosphere, it creates a cascade of particle that can be seen through the optical light they emit, or through an interaction with a particle detector on Earth's surface.
Stars emit other types of radiation besides electromagnetic radiation. A large fraction of the thermonuclear energy released within a star escapes as neutrinos. These fundamental particles interact weakly with atoms, so they are difficult to detect. Experiments to detect them have observed them from only two identifiable sources: the Sun and a nearby supernova. Finally, very energetic atomic nuclei and fundamental particles, collectively called cosmic rays, continually bathe the Earth. Cosmic rays are observed both at Earth's surface and in space. They are particles trapped within the Galaxy's magnetic field; because they do not move in a straight line in these magnetic fields, they cannot be linked directly to specific sources.