B-V colour
Encyclopedia
B-V colour is one of the two colour indices
in the UBV photometric system
.
Astronomers measure the amount of light a star
gives off in different colours. Hot stars give off more blue light than red; cool stars give off more red light than blue. Coloured filters are used to measure different wavelengths of light from stars. The magnitude
of the star is measured first through a standardized B-band ("blue") filter. Then the star's magnitude is measured through a V-band ("visible", peaking in green) filter. The value of V is subtracted from B to get the B-V colour index.
As a star
gets cooler and therefore more red, the B-V colour index increases, since smaller magnitudes correspond to brighter light. Hot stars have a small B-V and cool stars have a large B-V. Hotter stars therefore appear to the left on the Hertzsprung-Russell diagram and cooler stars appear on the right.
Color index
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...
in the UBV photometric system
UBV photometric system
UBV photometric system, also called the Johnson system , is a wide band photometric system for classifying stars according to their colors. It is the first known standardized photoelectric photometric system. The letters U, B, and V stand for ultraviolet, blue, and visual magnitudes, which are...
.
Astronomers measure the amount of light a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
gives off in different colours. Hot stars give off more blue light than red; cool stars give off more red light than blue. Coloured filters are used to measure different wavelengths of light from stars. The magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
of the star is measured first through a standardized B-band ("blue") filter. Then the star's magnitude is measured through a V-band ("visible", peaking in green) filter. The value of V is subtracted from B to get the B-V colour index.
As a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
gets cooler and therefore more red, the B-V colour index increases, since smaller magnitudes correspond to brighter light. Hot stars have a small B-V and cool stars have a large B-V. Hotter stars therefore appear to the left on the Hertzsprung-Russell diagram and cooler stars appear on the right.