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plate Facing page I The Great Nebula in Orion
The Great Nebula in Andromeda
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absolute magnitude absorbing clouds absorption curve absorption lines apparent distance approximately average Balmer brightness calculated centre clusters collisions colour excess colour indices continuous spectrum corresponding Cygnus dark nebulae density differential absorption co-efficient diffuse nebulae distribution elliptical emission lines emitted energy equation evaluate excitation forbidden lines frequency function Galactic latitude Galactic plane gives Greenstein H II regions Haute-Provence Observatory Hiltner Hubble Hulst hydrogen atoms increases interstellar gas interstellar lines ionized ions km./sec luminous intensity magn magn./kpc mass mean measurements Milky molecules neighbourhood nucleus observed obtained Oort opacity Orion parsecs particles photographic absorption photographic magnitudes photons planetary nebulae Plate polarized light privileged vibration probably proportion of polarized radial velocities radiation radius ratio reddened regions represents rotation scattering Schalen space spectral type spirals Spitzer Stebbins and Whitford stellar straight line suppose telescope theory tion ultra-violet visual magnitudes wave number wave-length