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Basic Definitions


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Basic definition, page 3 Astronomy - Basic Definitions, 4 of 5 Basic definition, page 5

radio galaxy - Type of active galaxy that emits most of its energy in the form of long-wavelength radiation.

red giant - A giant star whose surface temperature is relatively low, so that it glows with a red color.

red shift - Change in the wavelength of light emitted from a source moving away from us. In the visible light range, red light is less energetic than blue light. The relative recessional motion causes the wave to have an observed wavelength longer (and hence redder) than it would if it were not moving. It is thought that the cosmological red shift is caused by the stretching of space as the universe expands.

resolving power - Also called 'resolution', refers to the ability of a telescope to distingush details.

right ascension - Celestial coordinate used to measure longitude on the celestial sphere. Measured in Hours, Minutes and Seconds and the zero point is the position of the Sun on the vernal equinox (or in Pisces where the ecliptic and the celestrial equator cross). It is usually used in conjunction with the declination to locate a celestial object.

shock waves - A wave front marked by an abrupt change in pressure caused by an object or material moving faster than the speed of sound. For example, an explosion or a sonic boom produced by an aircraft going faster than the speed of sound.

singularity - A point in the universe where the density of matter and the gravitational field are infinite, such as the center of a black hole.

solar wind - A flow of hot charged particles (mostly protons and electrons) leaving the Sun. The speed of a typical solar wind may reach 350 km per second.

spectral class - Star classification scheme which based on the stellar spectral lines. It indicates the color and the temperature of a star. When a substance (such as metal) is heated to a high enough temperature it begins to glow. This glowing heat is called blackbody radiation. The glow begins with a dark red then eventually turns brighter until becomes orange (such as melted iron in a blast furnace). At a temperature around 5500C the glow turns yellow. This is the surface temperature of the Sun (it is in fact a yellowish star). The glow eventually turns white at around 10 000C and finally turn bluish at round and above 15 000C. Table below shows spectral classification of stars based on the temperature.

Spectral class




40 000



27 000



10 000














Notice that each main spectral class (represented by the alphabets O, B, A, F, G, K and M) can be subdivided into ten smaller gradations which are indicated as numbers, in the order of increasing temperature, from 0 to 9. For example K0 is the hottest in the spectral class K and K5 is slightly cooler, at a temperature around 4000C. The Sun is classified as a G2 star, corresponds to a temperature of about 5500C.

Basic definition, page 3 Basic definition, page 5


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