Observational astronomy

Telescopes, adaptive optics, interferometry, and the Event Horizon Telescope

What is observational astronomy?

Observational astronomy is the practice of actually gathering data about the universe — using telescopes and instruments to collect light and other signals from space, which is essentially the only way astronomers can ever study distant objects directly (since we cannot travel to them or bring samples back, except in a very few cases within our own solar system).

Telescopes: gathering more light than the eye can

A telescope's main job is to collect far more light than the human eye can gather on its own, using a large mirror or lens, and focus it into a sharp image or a measurable signal. Larger telescopes collect more light and can detect fainter, more distant objects, which is why the history of astronomy has been closely tied to building ever-larger telescopes.

Beyond visible light

Visible light is only a small part of the full range of light that exists (the electromagnetic spectrum, covered in the Electromagnetism article). Modern astronomy also studies the universe using radio waves, infrared, ultraviolet, X-rays, and gamma rays, each revealing different objects and processes that emit primarily in that particular range — for example, very hot gas emits strongly in X-rays, while cool dust clouds emit mainly in infrared.

Space-based telescopes

Earth's atmosphere blocks or distorts much of the light coming from space, particularly certain wavelengths like X-rays and much of the infrared and ultraviolet range. Telescopes placed in space, above the atmosphere, avoid this problem entirely and can also observe with far greater clarity, since they aren't affected by atmospheric turbulence blurring the image, the way ground-based telescopes are.

Why observational astronomy matters

Observational astronomy is the foundation all of astronomy is ultimately built on — every theory about stars, galaxies, planets, and the universe as a whole is developed and tested against data actually gathered by telescopes and instruments, making the careful, precise collection of that data essential to everything else in this branch.