In astronomical terms, the Sun is a "main sequence" star of spectral type G-zero with a surface temperature of about 11,000° F.1 Although a medium-small star, it is over a thousand times as massive as Jupiter, and over 300,000 times as massive as the Earth. Its energy output, as light and heat, is extremely constant, probably varying no more than about 0.5 percent from the average value.2 However, it is much more variable in its Production of ultraviolet radiation, radio waves (solar static), and charged particles.

At infrequent intervals, extremely intense solar outbursts of charged particles (cosmic rays) have been observed. The most recent of these outbursts, which occurred on February 23, 1956, and lasted about 18 hours, resulted in a peak intensity of ionizing radiation above the atmosphere estimated at about 1,000 times the normal value.

All usable forms of energy on the Earth's surface, with the exception of atomic and thermonuclear energy, are directly or indirectly due to the storing or conversion of energy received from the Sun.

Although not a theater of operations in the first phases of the space age, it is worthwhile to mention the larger setting in which the solar system itself figures.

The nearest neighbor of our solar system is the star system Alpha Centauri, a bright object in the southern sky at a distance of about 4 light-years. (Pluto is 5½ light-hours from the Sun.) Alpha Centauri is a double star whose two main components orbit about one an other. A third star called "Proxima" is also associated with this system and is actually at the present time the star closest to our solar system. ("Proxima" itself may also be doubled.)

It is not known whether the Alpha Centauri system has any planets, but observations of some other nearby stars, e. g., 61 Cygni, indicate, from wobbles in their motion, the possible presence of orbiting dark bodies with masses comparable to Jupiter's. There is, then, what might be considered indirect evidence for the existence of other planetary systems. Within 20 light-years of the Sun there are known to be about 100 stars with possibly 2 or 3 planetary systems, if the interpretation of the "wobbles" are correct. Kuiper estimates on the basis of the ratio of the masses of components of double stars that not more than 12 percent of all stars may have planetary systems.¹² When we realize that there are some 2 x 10¹¹ stars in our galaxy, this would give 1 to 10 billion with planetary systems. It seems reasonable to speculate that out of this vast number there surely must be some systems with earth-like planets, and that on some of these planets life similar to our own may have evolved.^13-16