Sky & Telescope October 1986 [antikvár]

In Search of Star ColorsDavid Malin, Anglo-Australian ObservatoryMeasuring the colors of stars is fundamental to our understanding of stellar evolution, and many astronomers have devoted their lives to this tedious but essential task. The massive body of data they have gathered forms a cornerstone of modern astrophysics. When combined with stellar luminosities, and enriched by extensive theoretical work, star colors help to give us a satisfying, though still incomplete, picture of how stars are born, live, and die.The early 19th-century French astronomer and statesman Francois Arago had surprising insight regarding star colors. In the 1855 English translation of his Popular Astronomy Arago noted:It will be reserved for time and precise observations to inform us if green or blue stars are not suns already in course of decay; if the different shades of those stars do not indicate a process of combustion in different stages. . . . It is probable, then, that upon this question of the colour of the stars, the part of observers may reduce itself, for a long time to come, to that of mere collectors of facts. The pleasure of connecting them with physical laws appears to be reserved for posterity.Arago was correct about the underlying importance of stellar colors, but he was referring specifically to double and multiple stars. As for single stars, he found that blue ones were conspicuously absent. He stated:The existence of so great a number of blue and green stars in binary systems of stars, is a fact so much more worthy of attention . . . that among the sixty or eighty thousand isolated stars . . . found in the catalogues of astronomers, there are none, I think, inscribed with any other indications, in regard to colour, than white, red, and yellow. The physical conditions which determine the emission of blue and green light appear then to exist only in multiple stars.Today we know that most of the bright stars are actually much bluer than the Sun. Unfortunately the eye rarely sees this color unless it is accentuated by a contrast effect like that found in double star systems such as Beta (ff) Cygni.With the invention of photography came a detector that, unlike the eye, was particularly sensitive to blue Ught. Although J. A. Whipple and W. C. Bond obtained the first daguerreotype of a star in 1850, another quarter century passed326 Sky & Telescope, OcWfter,/Sibefore any photographic plates became sensitive enough to make stellar photography practical.These early astropho-tographs provided the first completely objective way of measuring the stars' positions and brightnesses. But it was soon noticed that the magnitudes determined from photographs did not match those obtained visually. By 1891 David Gill and Jacobus Kapteyn began a photographic survey of the southern heavens. They found that a band of bright naked-eye stars inclined slightly to the plane of the Milky Way (these stars are now known as Gould's Belt) appeared systematically brighter on photographs than to the eye. Furthermore, Gill and Kapteyn correctly surmised that these stars are bluer or whiter.Astronomers realized that the difference between photographic (blue light) and visual (primarily yellow-green light) magnitudes is a measure of star color. That realization provided the basis of today's B - V (blue minus visual) color index, the most commonly used indicator of a star's surface temperature.Color measurements took on new importance when, around the turn of the century, E. Hertzsprung and H. N. Russell introduced their graphical comparison of star colors and luminosities. The Hertz-sprung-Russell diagram is central to astrophysics and links a star's color and brightness to its evolutionary state and its mass. From such relationships we know that the bright stars in Gould's Belt range in temperature from at least 40,000° Kelvin for the young, massive O stars to perhaps 3,000° K or less for a few red giants scattered among them. The hottest are as deeply blue as the daytime sky, while the coolest are a distinct yellow-orange.But the eye does not readily see these colors in individual stars. And, at firstBRIGHT STARS IN THE ORION REGIONVisualColorSpectralDistanceNamemagnitudeindextype(light-years)1a Ori0.501.85M2 la3102X Ori3.39-0.18083Ori4.41-0.16BO IV186040= Ori4.090.95KO 11119557 Ori1.64-0.22B2 111360656 Ori4.781.38K2 II780751 Ori4.911.17K1 III2858e Ori4.461.19K3 III2809W Ori6.173.45C6 II10tt' Ori4.471.40K2 II62011_4.531.22K2 III26012f Ori1.77-0.2109.5 lb111013f Ori1.70-0.19BO la121014J Ori2.23-0.220.95 11151) Ori3.36-0.17HI V75016groupof O and Bstars and the Orionnebula17|3 Eri2.790.13A3 III9118T Ori3.60-0.11B5 III4251929 Ori4.140.9608 11119020* Ori2.06-0.17B0.5 la6821S Ori0.12-0.03B8 la915glance, color photographs do not seem to show the pallid hues any better. Bright stars photograph as white, sometimes with the merest hint of color, and faint images that show color better are often too small to be seen without a magnifier.This diagram identifies stars in and around Orion listed in the table above.