Page:The Search for Extraterrestrial Intelligence, NASA, 1977.pdf/3



A portion of the star field taken from the same photographic plate as shown on the cover is reproduced on the right with the addition of designations of stellar type for some of the stars. The letter designation (e.g., A) indicates the spectral type of the star. The conventional spectral types are O, B, A, F, G, K, and M, with O-stars being the hottest (effective surface temperatures in excess of 30,000 K) and M-stars being the coolest (effective surface temperatures of 3,000 to 4,000 K; the effective surface temperature of the Sun is about 5,800 K). The prefix "g" indicates that a star is a giant star, a star that has moved away from the main sequence. Stars indicated only with the spectral designation are main sequence stars, deriving their energy primarily from the conversion of hydrogen into helium. The two important aspects of the figure are first that stars like the Sun, spectral type G, are very numerous in the Galaxy, and second, in any typical group of stars, most stars are of spectral types G, K, and M. These types of stars are long lived (10 billion years or greater). The figure shows that viewed from the perspective afforded by interstellar distances, the Sun would be a rather common and ubiquitous type of object. This suggests that the Sun's retinue of planetary companions, and perhaps the intelligent life forms existing on one of these planets, may also be common and ubiquitous phenomena.