Problems arise when only one colour index is observed. If, for instance, a star is found to have, say, a B − V colour index of 1.0 (i.e., a reddish colour), it is impossible without further information to decide whether the star is red because it is cool or whether it is really a hot star whose colour has been reddened by the passage of light through interstellar dust. Astronomers have overcome these difficulties by measuring the magnitudes of the same stars through three or more filters, often U (ultraviolet), B, and V (see UBV system).
Observations of stellar infrared light also have assumed considerable importance. In addition, photometric observations of individual stars from spacecraft and rockets have made possible the measurement of stellar colours over a large range of wavelengths. These data are important for hot stars and for assessing the effects of interstellar attenuation.
Embryonic-stars-in-the-Eagle-Nebula-This-detail-of-aEmbryonic stars in the Eagle Nebula (M16, NCG 6611)[Credits : Photo AURA/STScI/NASA/JPL (NASA photo # STScI-PRC95-44b)]
Light-curve-of-Algol-an-eclipsing-variable-or-eclipsing-binaryLight curve of Algol (Beta Persei), an eclipsing variable, or eclipsing binary, star system. The …[Credits : Encyclopædia Britannica, Inc.]
Schematic-spectrum-luminosity-correlation-of-spiral-arm-stars-in-theSchematic spectrum–luminosity correlation (Hertzsprung–Russell diagram) of spiral-arm …[Credits : From Astrophysical Journal, reproduced by permission of the American Astronomical Society]
Colour-magnitude-diagram-for-an-old-globular-cluster-made-upColour-magnitude (Hertzsprung-Russell) diagram for an old globular cluster made up of Population II …[Credits : From Astrophysical Journal, reproduced by permission of the American Astronomical Society]
Eta-CarinaeEta Carinae.[Credits : Photo AURA/STScI/NASA/JPL (NASA photo # STScI-PRC96-23a)]
N81-and-new-stars-observed-by-the-Hubble-Space-TelescopeN81 and new stars, observed by the Hubble Space Telescope.[Credits : Photo AURA/STScI/NASA/JPL (NASA photo # STScI-PR98-25)]
Centre-of-the-Orion-Nebula-Astronomers-have-identified-some-700Centre of the Orion Nebula (M42).[Credits : Photo AURA/STScI/NASA/JPL (NASA photo # STScI-PRC95-45a)]
Stellar-parallaxFigure 3: Stellar parallax[Credits : Encyclopædia Britannica, Inc.]
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