The Interaction of Radiation and Matter Understanding how the light from celestial sources can provide information on the properties of those sources, requires knowledge of how radiation and matter interact, and how those interactions alter the pattern of emitted radiation. Radiation and matter interact in four ways: 1. Emission: Matter can release energy (which it previously absorbed, or generated internally) by emitting radiation. On the atomic level, electrons “drop” from higher to lower energy levels, emitting individual photons (“pieces” of energy). For example, when an object is heated, energy is transferred to it, that energy is subsequently released as the object attempts to reach equilibrium with its surroundings. 2. Absorption: Matter can absorb radiation; the absorbed energy causes the absorbing material to heat up for a time. Eventually the absorbed radiation will be released, mostly likely in some other part of the spectrum. 3. Transmission: Radiation may pass through materials, for example, light through glass. The atoms & molecules in glass do not absorb the passing radiation, permitting it to transit through the glass. 4. Reflection: Radiation may be “bounced” from a material object. In effect, the radiation is absorbed and emitted instantly, without changing its spectrum. However the direction of the radiation is changed. Light shining on a mirror is an example. The type of spectrum radiated by a celestial (or terrestrial) object depends critically on the temperature, density, and composition of that object. The principles which define the type of spectrum were outlined by the physicist Kirchhoff: Kirchhoff’s “Rules” for Spectra: (1) A solid or dense object will emit electromagnetic radiation at a range of wavelengths without gaps in the emitted spectrum. The amount of emission at each wavelength is defined by the object’s temperature according to the blackbody curve. This object will emit a continuous spectrum. Stars behave much like blackbodies. (2) A low-density, hot gas will emit emission lines: light at specific wavelengths, with no radiation emitted between the emission lines, producing an emission line spectrum. Gas clouds in interstellar space usually exhibit an emission line spectrum. (3) A cool gas seen against a background source of radiation will absorb light from that source, imprinting an absorption line spectrum onto the spectrum of the background source. If there is no background source, an observer will, of course, see nothing. Spectral lines are unique to the atoms and molecules which produce them – permitting the composition of stars, planets, comets, galaxies, etc, to be determined at a distance. Knowledge from Spectra What can we learn about objects in space by observing their spectra ? Remember that the spectrum of an object is defined as the emitted intensity of radiation at each wavelength (or frequency). Spectra are displayed with wavelength along the X-axis and intensity along the y-axis Temperature: The temperature of a celestial object can be found from the shape of its spectrum. By measuring the peak wavelength of emission, Wien’s law gives an object’s temperature, if that object radiates like a blackbody. The temperature of stars can be found using this approach. Energy production: Total energy produced by stars can be found using Stefan’s law, which relates the total emitted energy to an object’s temperature. Stefan’s law is most useful for estimating the energy output from stars. Composition: can be determined using both the pattern and strength of spectral lines. Atoms and molecules display unique spectral patterns. These patterns permit identification of specific substances in an object. The pattern of lines defines which substances are present, while the strength of the lines measures how much of a substance is present. Stronger lines imply more of a substance. Velocity: Motion towards or away from the observer can be determined from the Doppler effect, which is the shift in the wavelength of spectral lines caused by an object’s motion. Motion towards the observer produces a blueshift. Motion away produces a redshift. Rotation: The spin of a planet or star can also be determined via the Doppler effect. That part of a planet or star moving towards the observer causes a blueshift in the object’s spectrum; the other “half” of a rotating object produces a redshift in the object’s spectrum. Taken together, rotation will “fatten” a spectral line, making a sharp (“spiky”) line appear broader.