Astrophysicists use many branches of physics: nuclear physics to study power-generation in stars; atomic physics to understand the spectra of stars and gaseous nebulae; and gas laws and magnetic theory to probe starspots and flares on star surfaces.

Astrophysics is the study of the physical makeup and behaviour of celestial objects. Astrophysics and astronomy are not sharply distinguished. If one wishes to emphasize the application of physical laws or the physical understanding of astronomical objects, then the term astrophysics is used. The discipline of astrophysics involves the observation and analysis of planets and stars (including our sun), binary stars, clusters of stars, interstellar material, galaxies, quasars or anything else that is part of the universe. From a combination of theory and observation, astrophysicists deduce physical characteristics such as chemical composition, age, rotation rate, magnetic fields, space motions and so on. This information is then applied toward understanding how celestial objects came into being, how they change with time, how they will end up, and what forces and physical processes are involved.

Astrophysicists use many branches of physics: nuclear physics to study power-generation in stars; atomic physics to understand the spectra of stars and gaseous nebulae; and gas laws and magnetic theory to probe starspots and flares on star surfaces. Gravitational physics is fundamental to understanding planetary and binary-star orbits, the rotation of our galaxy and the expansion of the universe. Other sciences also make important contributions; for example, chemistry is needed to study atomic and molecular reactions in interstellar space; geology, geophysics and meteorology contribute to studies of planets. Technical skills in optics, electronics and computers are needed to collect astrophysical data and compare it to theory. Several fields of engineering contribute to the construction of astronomical telescopes (like the Canada-France-Hawaii Telescope-see Observatory), light detectors and spacecraft.

Information about astronomical objects arrives in the form of electromagnetic radiation, ie, light waves or photons. The most energetic form is gamma radiation, followed by X-rays, ultraviolet radiation, visible light (violet through red), infrared light, millimetre waves and radio waves, which have the least energy. Optical, X-ray, radio and other telescopes are required to measure the full range of radiation; only optical and radio radiation can be measured from the surface of the earth, because the atmosphere is opaque to the others. Photometry, polarimetry and spectroscopy are techniques for analysing light.

Photometry measures brightness. An object's brightness as we see it depends on how far away the object is, how bright the object actually is and how much obscuring material lies along the line of sight. Hotter stars emit more blue than red light, compared to cooler stars. Starlight collected with a telescope is measured by a light detector; the relative amount of light in different colours is established by placing colour filters in front of the detector. The DDO (David Dunlap Observatory) colour system, designed by R.D. McClure and S. Van den Bergh, has been widely used. X-ray photometry can be used to study the outermost parts of a star, the chromospheres and coronae. Stellar eclipses and pulsation are also studied by photometry.

Polarimetry measures the orientation of light waves. In natural-source radiation, waves are seen in all possible orientations and the light is unpolarized. If a magnetic field exists in the material emitting the light, the orientation becomes ordered and the light is polarized. Polarization also occurs when light is reflected or scattered, as starlight is by interstellar material. The polarization of visible light is detected by placing a Polaroid or Nichol prism in the light beam. In many cases, astronomical sources show only partial polarization (under 10%) if any. Magnetic fields are known to be strong in some types of galaxies because of the strongly polarized light and radio emissions.

Spectroscopy measures the detailed features in the electromagnetic spectrum. Most of these features are very narrow colour bands called spectral lines. They normally have less light than adjacent spectral regions, and they arise from the energy levels of the atoms in the celestial object. In the spectrum of a typical star, for instance, some of the more commonly seen spectral lines are produced by the elements hydrogen, sodium, magnesium, calcium, titanium, iron and nickel. Spectroscopic analysis of the strengths and shapes of spectral lines gives us information about the chemical composition, temperature, pressure, magnetic fields, convective motions, rotation rates and velocities of approach or recession of the object under study. Canadian researchers have done pioneering work in studies of stars, galaxies and quasars using spectroscopy.

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