Chapter 10 Flashcards
Parsec
The distance at which a star must lie in order for its measured parallax to be exactly 1 arc second; 1 parasec equals 206,000 AU.
Distance (in parasecs) = 1 / (parallax [in arc seconds])
Proper motion
The angular movement of a star across the sky, as seen from Earth, measured in seconds of arc per year. This movement is a result of the star’s actual motion through space.
Apparent brightness
The brightness that a star appears to have, as measured by an observer on Earth
Notes|
Apparent brightness o< luminosity / distance^2
Magnitude scale
A system of ranking stars by apparent brightness, developed by the Greek astronomy 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 scheme has since been extended to cover stars and galaxies too faint to be seen by the unaided eye. Increasing magnitude means fainter stars, and a difference of five magnitudes corresponds to a factor of 100 in apparent brightness.
Apparent magnitude
The apparent brightness of a star, expressed using the magnitude scale.
Absolute magnitude
The apparent magnitude a star would have if it were placed at a standard distance of 10 parasecs from Earth.
Spectral class
Classification scheme, based on the strength of stellar spectral lines, which is an indication of the temperature of a star.
Letter order O, B, A, F, G, K, M. (“Oh, Be A Fine Guy/Girl, Kiss Me”)
Radius-
luminosity-
temperature-
relationship
A mathematical proportionality, arising from Stefan’s law, that allows astronomers to indirectly determine the radius of a star once its luminosity and temperature are known.
Luminosity 0< [(radius^2)/(temperature^4)]
Giants
a star with a radius between 10 and 100 times that of the Sun.
Red giant
A giant star whose surface temperature is relatively low so that it glows red
Supergiant
A star with a radius between 100 and 1000 times that of the Sun.
Red supergiant
An extremely luminous red star. Often found on the asymptotic giant branch of the Hertzsprung-Russel diagram
White dwarf
A dwarf star with sufficiently high surface temperature that it glows white.
H-R (Hertzsprung-Russel) diagram
A plot of luminosity against temperature (or spectral class) for a group of stars
Main sequence
Well-defined band on the Hertzsprung-Russel diagram on which most stars are found, running from the top left of the diagram to the bottom right.
Blue giant
Large, hot, bright star at the upper-left end of the main sequence on the Hertzsprung-Russel diagram. Its name comes from its color and size.
Blue supergiants
The very largest of the large, hot, bright stars at the uppermost-left end of the main sequence on the Hertzsprung-Russel diagram.
Red dwarfs
Small, cool, faint star at the lower-right end of the main sequence on the Hertzsprung-Russel diagram.
White-dwarf region
The bottom-left corner of the Hertzsprung-Russel diagram, where white dwarf stars are found.
Red giant region
The upper-right corner of the Hertzsprung-Russel diagram, where red giant stars are found.
Spectroscopic parallax
Method of determining the distance to a star by measuring its temperature and then determining its absolute brightness by comparing with a standard Hertzsprung-Russel diagram. The absolute and apparent brightness of the star give the star’s distance from Earth.
Luminosity class
A classification scheme that 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.
Binary-star system
A system that 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.
Visual binaries
A binary star system in which both members are resolvable from Earth.
Spectroscopic binaries
A binary-star system that appears as a single star from Earth, but whose spectral lines show back-and-forth Doppoer shifts as two stars orbit one another.
Eclipsing binary
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 the other star.
Light curve
The variation in brightness of a star with time.
