EARTH AND NATURAL HISTORY ORIENTATION 



I 



mosphere probably has been much the same for over 

 two billion years, and perhaps only moderate changes 

 in the percentages oi existing gases have occurred. 



STAR DEATH 



The life expectancy of a star is partly related to its 

 size. The smaller the star, the longer its life expect- 

 ancy! Giant stars may last only ten million years, but 

 some small stars may last for fifty billion years. 



According to many astronomers the life cycle of a 

 star assumes the following general pattern, which is 

 directly related to the burning of hydrogen; 



For the first half of its life, a star burns steadily 

 until 15 per cent of its hydrogen is consumed. The 

 star then starts to change. In addition to cooling, 

 the star in time grows in size, its diameter increasing 

 50 to 100 times that of the original, to form a red giant 

 or supergiant star. (Our average size star, the sun, 

 will probably not reach its half-life for another ten 

 billion years, and will reach the red giant stage five 

 billion years beyond that.) When approximately 60 

 per cent of the hydrogen has been burned, internal 

 pressure decreases and the giant starts a cycle of 

 contraction. Such stars are unstable and take one 

 of two possible paths of development. Some alter- 

 nately expand and contract, the pulsating stars. Others, 

 the novas, undergo a series of explosions, while in still 

 others, the supernovas, the explosion may be a single 

 gigantic blast (or very few). No matter which of these 

 alternatives occurs, if sufficient matter remains, the 

 end product is a feebly glowing, white dwarf star. 



THE SOLAR SYSTEM 



Data about the solar system are summarized in 

 Table 1.1. Included in the table are distances of each 

 planet from the sun, plus the diameter, mass, escape 

 velocity, surface gravity, inclination of orbit and 

 planet axis, revolution period, maximum surface 

 temperature, atmospheric composition, and number 

 of satellites, where applicable, for solar system bodies. 

 In general, these are approximate figures. 



Some of the terms and/or data need clarification. 

 None of the planets maintains a constant distance from 

 the sun. The closest distance of a plant in its orbit, or 

 path around the sun is called perihelion and the great- 

 est, aphelion. The escape velocity is that velocity at 

 which a moving body can escape from its gravita- 



tional field. The ability of a celestial body to retain 

 an atmosphere around it depends on the escape veloc- 

 ity from its surface. Surface gravity is the intensity of 

 the force of gravity at the surface of a planet, and is a 

 function of mass, radius, and rotation speed. The 

 scale for surface gravity is given in relation to the 

 earth; and (as you can see from Table 1.1) contrary 

 to what you might have heard, a human might suffer 

 much discomfort but probably could "walk" upon the 

 surfaces of all of the planets. Inclination is the angle 

 formed between the orbital plane of a planet and the 

 ecliptic. A.n orbital plane is that imaginary flat surface 

 defined by the orbit of a given planet. The ecliptic is 

 the plane containing the center of the sun and the 

 orbit of the earth. In short, the ecliptic is the orbital 

 plane of the earth. Thus, the ecliptic serves as a ref- 

 erence point in the calculation of the inclination of orbit 

 and the inclination of equator of the planets in the solar 

 system. Revolution period is the time necessary for a 

 planet to complete its orbit around the sun (i.e., a 

 planetary year); it is given in earth years. Rotation 

 period is the time required for a planet to complete a 

 turn upon its central axis (i.e., a planetary day). In 

 the cases of Jupiter and Saturn, each planetary day is 

 for the equator. This is because different latitudes of 

 these two planets rotate at different speeds! 



LIFE ON OTHER PLANETS 



The data in Table 1.1 must be used in any specu- 

 lation about "advanced" life on other planets. In our 

 conjectures we can, on the basis of our present knowl- 

 edge, eliminate planets beyond Mars. These outer 

 planets not only have methane-ammonia atmos- 

 pheres (some on Mars?), but also low surface tem- 

 peratures; they could not support life as we know it. 

 Mercury has extreme temperature changes and no 

 atmosphere; the asteroids are too cold and have 

 little, if any, atmosphere. Venus and Mars are the 

 only planets deserving serious consideration as sup- 

 porters of life. Venus apparently cannot support our 

 "complex" life forms; high carbon dioxide and other 

 contents cause the atmosphere to be poisonous and 

 temperatures (because the atmosphere traps heat 

 that otherwise would escape from the surface) too 

 severe. There is evidence of simple plant life, with 

 what appear to be seasonal expansions and contrac- 

 tions of vegetation, on Mars. However, the thin at- 

 mosphere on that planet would prevent complex 

 living forms from existing for long. Man might be 



