What is Astrophysics?

•  Branch of astronomy that uses the laws of physics to understand astronomical objects and the processes occurring within them. The terms ‘astronomy’ and ‘astrophysics’ are however often used as synonyms. Astrophysics effectively dates from the first applications of the SPECTROSCOPE to study astronomical objects. Sir William HUGGINS’ identification of some of the chemical elements present in the Sun and stars in the 1860s was the first major result of astrophysics. Today, the use of computers for data processing, analysis and for modelling objects and processes is an integral part of almost all aspects of astrophysics. 

• SPECTROSCOPY is the most powerful technique available in astrophysics. Individual atoms, ions or molecules emit or absorb light at characteristic frequencies (or ‘lines’, from their appearance in a spectroscope) and can thus be identified and their quantity assessed. Information on the physical and chemical state of an object can be obtained from an analysis of the way its SPECTRUM is modified by the collective environmental conditions within the object. Line-of-sight velocities of the whole object or regions within it are indicated by the DOPPLER SHIFT. The most intensively studied spectral region is the ground-accessible ‘optical’ region (the NEAR-ULTRAVIOLET to the NEARINFRARED), but much information now comes from observations in the RADIO, MICROWAVE, FAR-ULTRAVIOLET, X-RAY and GAMMA-RAY regions. 

• Stellar spectra characteristically show a continuous bright background that by its colour distribution approximately indicates the ‘surface’ temperature (but is made up of radiation contributed in some measure from throughout the whole of the stellar atmosphere). Several different processes produce these continua. In cooler stars (like the Sun) the interaction chiefly involves electrons and hydrogen atoms; in hotter stars, electrons and protons; and in the hottest stars, electrons alone. The dark spectral lines superimposed on the continuum, which are due to atoms, ions and molecules, strictly do not arise from absorption by a cooler outer layer as a simple visualization suggests, but in a complex depth-dependent manner in which radiation dominantly escapes from outer and therefore cooler (darker) regions of the atmosphere near the centre of an absorption line, or deeper, hotter (brighter) regions forthe continuum. The fact that the lines appear dark indicates that the outer layers are cooler, and indeed the temperature generally is expected to decrease outwards from the centre of any star. For the Sun and certain other stars some bright lines are also seen, particularly in the farultraviolet region, indicating that in these the temperature is rising again in extended, extremely tenuous layers of the outer atmosphere. The strengths, widths and polarization of spectral lines give information on the chemical composition, ionization state, temperature structure, density structure, surface gravity, magnetic field strength, amount of turbulent motion within the atmosphere of the star and stellar rotation.