SP-419 SETI: The Search for Extraterrestrial Intelligence
[ii] 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 0, 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
- Back -