Editing Astrophysics (section)

==Observational astrophysics==
[[Image:N 63A- Chandra and Hubble - Heic0507f.tif|thumb|right|200px|Supernova remnant LMC N 63A imaged in x-ray (blue), optical (green) and radio (red) wavelengths. The X-ray glow is from material heated to about ten million degrees Celsius by a shock wave generated by the supernova explosion.]]
[[Observational astronomy]] is a division of the astronomical science that is concerned with recording and interpreting data, in contrast with [[theoretical astrophysics]], which is mainly concerned with finding out the measurable implications of physical [[model (abstract)|models]]. It is the practice of observing [[celestial object]]s by using [[telescope]]s and other astronomical apparatus.

Most astrophysical observations are made using the [[electromagnetic spectrum]].
* [[Radio astronomy]] studies radiation with a [[wavelength]] greater than a few millimeters. Example areas of study are [[radio waves]], usually emitted by cold objects such as [[interstellar gas]] and dust clouds;  the cosmic microwave background radiation which is the [[redshift]]ed light from the [[Big Bang]]; [[pulsar]]s, which were first detected at [[microwave]] frequencies. The study of these waves requires very large [[radio telescope]]s.
* [[Infrared astronomy]] studies radiation with a wavelength that is too long to be visible to the naked eye but is shorter than radio waves. Infrared observations are usually made with telescopes similar to the familiar [[optical]] telescopes. Objects colder than stars (such as planets) are normally studied at infrared frequencies.
* [[Optical astronomy]] was the earliest kind of astronomy. Telescopes paired with a [[charge-coupled device]] or [[spectroscope]]s are the most common instruments used. The Earth's [[atmosphere]] interferes somewhat with optical observations, so [[adaptive optics]] and [[space telescope]]s are used to obtain the highest possible image quality. In this wavelength range, stars are highly visible, and many chemical spectra can be observed to study the chemical composition of stars, galaxies, and [[nebula]]e.
* [[Ultraviolet]], [[X-ray astronomy|X-ray]] and [[gamma ray astronomy]] study very energetic processes such as [[binary pulsar]]s, [[black hole]]s, [[magnetar]]s, and many others. These kinds of radiation do not penetrate the Earth's atmosphere well. There are two methods in use to observe this part of the electromagnetic spectrum—[[space-based telescope]]s and ground-based [[imaging air Cherenkov telescope]]s (IACT). Examples of [[Observatory|Observatories]] of the first type are [[RXTE]], the [[Chandra X-ray Observatory]] and the [[Compton Gamma Ray Observatory]]. Examples of IACTs are the [[High Energy Stereoscopic System]] (H.E.S.S.) and the [[MAGIC (telescope)|MAGIC]] telescope.
Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances. A few [[gravitational wave]] observatories have been constructed, but gravitational waves are extremely difficult to detect. [[Neutrino]] observatories have also been built, primarily to study the Sun. Cosmic rays consisting of very high-energy particles can be observed hitting the Earth's atmosphere.

Observations can also vary in their time scale. Most optical observations take minutes to hours, so phenomena that change faster than this cannot readily be observed. However, historical data on some objects is available, spanning [[century|centuries]] or [[millennia]]. On the other hand, radio observations may look at events on a millisecond timescale ([[millisecond pulsar]]s) or combine years of data ([[Rotation-powered pulsar|pulsar deceleration]] studies). The information obtained from these different timescales is very different.

The study of the Sun has a special place in observational astrophysics. Due to the tremendous distance of all other stars, the Sun can be observed in a kind of detail unparalleled by any other star. Understanding the Sun serves as a guide to understanding of other stars.

The topic of how stars change, or stellar evolution, is often modeled by placing the varieties of star types in their respective positions on the [[Hertzsprung–Russell diagram]], which can be viewed as representing the state of a stellar object, from birth to destruction.