Astronomy 104 - Chapter 17 - Key Terms

A measure of the apparent brightness of a star as seen from the earth. The scale is based on the faintest stars visible to the unaided eye in a clear dark sky. These stars are given a magnitude value of 6.0. A star 100 time brighter will have a magnitude value 5 less or 1.0. A star 100 times fainter will have a value 5 greater or 11.0. See the Pogson Scale.

A system which consists of two stars in orbit about their common center of mass, held together by their mutual gravitational attraction. Most stars are found in binary-star systems. Term can also be applied to star systems containing more than two stars.

The characteristic way in which the intensity of radiation emitted by a hot object depends of frequency. The frequency at which the emitted intensity is highest is an indication of the temperature of the radiating object. Also referred to as the Planck curve.

The continuous spectrum emitted by a blackbody. The flux at each wavelength is given by a formula known as Planck's Law.

Large, hot, bright star at the upper left end of the main sequence on the H-R diagram. Its name comes from its color and size.

The very largest of the large, hot, bright stars at the uppermost left end of the main sequence on the H-R diagram.

A convenient method of quantifying a star's color by comparing its apparent brightness as measured through different filters. If the star's radiation is well described by a black-body spectrum, the ratio of its blue intensity (B) to its visual intensity (V) is a measure of the object's surface temperature.

A way of plotting stellar properties, in which either apparent magnitude or absolute magnitude is plotted against color index. It is a functional equivalent to the H-R diagram.

Binary system in which spectral lines of both stars can be distinguished and seen to shift back and forth as the stars orbit one another.

System containing two stars in orbit around one another. See binary star.

Any star with radius comparable to, or smaller than, that of the Sun (including the Sun itself).

Rare binary-star system that is aligned in such a way that from Earth we observe one star pass in front of the other, eclipsing each other. Eclipsing binaries are a special case of spectroscopic binaries.

A plot of luminosity (or absolute magnitude) against temperature (or spectral class) for a group of stars.

The variation in brightness of a star with time.

One of the basic properties used to characterize stars, luminosity is defined as the total energy radiated by star each second, at all wavelengths.

A classification scheme which groups stars according to the width of their spectral lines. For a group of stars with the same temperature, luminosity class differentiates between supergiants, giants, main sequence stars and subdwarfs.

A system of ranking stars by apparent brightness, developed by the Greek astronomer Hipparchus. Originally, the brightest stars in the sky were categorized as being of first magnitude, while the faintest stars visible to the naked eye were classified as sixth magnitude. The magnitude scale is logarithmic where a difference of one magnitude equals a factor (muiltiplier) of approximately 2.512... which is the fifth root of 100 (1001/5). This scale is used for both apparent magnitude and absolute magnitude. The scheme has since been extended to cover stars and galaxies too faint to be seen by the unaided eye. Increasing magnitude value means fainter stars, and a difference of five magnitudes corresponds to a factor of 100 in apparent brightness.

A well-defined band on an H-R diagram, on which most stars tend to be found, running from the top left of the diagram to the bottom right.

Relation between the component masses of a single-line spectroscopic binary.

A measure of distance based on the parallax of an object. One parsec is the distance to an object that exhibits a heliocentric parallax of one arc second. One parsec equals approximately 3.26 light-years or 206,265 Astronomical Units.

Branch of observational astronomy in which intensity measurements are made through each of a set of standard filters.

The English astronomer Pogson standardized the approximate magnitude scale created by Hipparchus. His scale pegged the faintest stars visible to the unaided eye on a clear dark night at magnitude 6.00. Since the Hipparchus Scale that preceded it had brighter objects with smaller numbers, the magnitude scale decreases as objects get brighter. A difference of one magnitude is about 2.512.. making a difference of 5 magnitudes a factor of 100. This allowed Hipparchus' Scale to expanded to telescopic objects (apparent magnitudes greater than 6) and objects brighter than the typical brightest star (magnitudes less than 1 and negative).

The angular movement of a star across the sky, as viewed from Sun, measured in seconds of arc per year. This movement is a result of the star's actual motion through space. When the apparent motion due to heliocentric parallax is subtracted from the apparent motion as seen from the Earth, the remaining motion is called the "proper motion".

Motion along a particular line of sight (an increase or decrease in the distance), which induces apparent changes in the wavelength (or frequency) of radiation received. See Doppler Effect.

Component of a star's velocity along the line of sight.

A mathematical proportionality, arising from Stefan's Law, which allows astronomers to indirectly determine the radius of a star once its luminosity and temperature are known.

Small, cool faint stars at the lower-right end of the main sequence on the H-R diagram, whose color and size give them their name.

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

The section of the evolutionary track of a star that corresponds to continued heating from rapid hydrogen shell burning, which drives a steady expansion and cooling of the outer envelope of the star. As the star gets larger in radius and its surface temperature cools, it becomes a red giant.

The upper right hand corner of the H-R diagram, where red giant stars are plotted.

An extremely luminous red star. Often found on the asymptotic giant branch of the H-R diagram.

Binary system in which one star is too faint for its spectrum to be distinguished, so only the spectrum of the brighter star can be seen to shift back and forth as the stars orbit one another.

Classification scheme, based on the strength of stellar spectral lines, which is an indication of the temperature of a star.

A binary-star system which from Earth appears as a single star, but whose spectral lines show back-and-forth Doppler shifts as two stars orbit one another.

Method of determining the distance to a star by measuring its temperature and then determining its absolute brightness by comparing with a standard H-R diagram. The absolute and apparent brightness of the star give the star's distance from Earth.

A star with a radius between 100 and 1000 times that of the sun.

Motion perpendicular to a particular line of sight, which does not result in Doppler shift in radiation received.

Component of star's velocity perpendicular to the line of sight.

True motion of a star, taking into account both its transverse and radial motion according to the Pythagorean theorem. Sometimes referred to as Space Velocity.

A binary-star system in which both members are resolvable from Earth.

A subdwarf star with a surface temperature that is hot, so that the object glows white. White dwarfs are typically about the size of the earth with a mass comparable to the sun.

The bottom left-hand corner of the H-R diagram, where white dwarf stars are plotted.