| Object | Number | Diameter | Velocities miles per sec. | Distribution |
| 1. Solar System | ||||
| a. Planets | Eight | 3,000 to 88,000 mi. | 3 to 35 miles per sec. | Revolving in nearly circular orbits about the sun. |
| b. Sun | 864,000 mi. | 12½ mi. | Travelling through galactic systems of stars (Milky Way). | |
| 2. Stars | ||||
| a. Helium (bluish) | 8 mi. | All types of stars are more or less crowded toward plane of Milky Way in lens shaped formation. (Milky Way possibly a spiral nebula.) | ||
| b. Hydrogen (white) | Approx. 2,000,000,000 (Two thousand million) | Dwarfs | 14 mi. | |
| c. Solar (yellow) | 500,000 to 1,000,000 mi. | 18-19 mi. | ||
| d. Type M (red) | Including all types | Giants 10,000,000 to 400,000,000 mi. | 21 mi. | |
| 3. Nebulæ | ||||
| a. Diffuse or Gaseous | Numerous | Very extensive, many light years. | Very low. | In or close to Milky Way. |
| b. Spiral | Approx. 700,000 (seven hundred thousand) | Size and distance doubtful but very great. | Average 480 mi. | Far external to Milky Way and numerous near its poles. |
| c. Planetary | One hundred and fifty (150) | Several times that of the solar system on the average. | Average 48 mi. | In or close to Milky Way. |
| 4. Globular Star Clusters | About one hundred known | 22,000-220,000 light years. | Very high | External to Milky Way and spherically distributed about it. |
| 5. Magellanic Clouds | Two(Greater and Lesser) | Thousands of light-years. | Very high | Far beyond Milky Way. |
[XXVIII]
THE EVOLUTION OF THE STARS—FROM RED GIANTS TO RED DWARFS
The most casual of star-gazers is aware that the stars differ one from another in color and in brightness. There are red stars, yellow stars, white stars and bluish-white stars. There are the brilliant stars of first magnitude such as Vega, Capella and Antares, and there are, on the other hand, stars so faint that they can barely be glimpsed with the most powerful telescopes.
In general the most brilliant stars are the nearest and the faintest stars are the most distant, but there are many exceptions to the rule, since there are stars that appear faint even when comparatively near because they are small and shine with a feeble light. Such a star is the faint, sixth-magnitude star, 61 Cygni, one of the nearest of all the stars. Again, there are stars in far-distant clusters visible only in powerful telescopes that in actual brightness exceed our own sun several thousand times and in volume several million times. A star the size of the sun would be invisible in the most powerful telescope in existence if it were at the distance of many stars in the Milky Way or globular star clusters.
Stars differ in color because they differ in temperature. We are all aware of the fact that a piece of iron when heated first glows a deep red, then appears yellowish in color and finally attains to white heat. It is the same among the stars. The red stars are the coolest of all the stars and the bluish-white stars are the hottest of all the stars, while intermediate between them in temperature come the yellow and the white stars.
Now as the biologist and the geologist see in this world of ours signs of evolution, or gradual development and change from the simple to the more complex forms, and of growth and decay, so the astronomer sees among the stars signs of a continuous, progressive development from one type of star to another. Stars share in the general evolution that is the law of the universe, and are born, reach the height of their development, decline to old age and die.
Within the past few years important astronomical discoveries have been made that show the true order of this evolution of the stars. It was believed not so long ago that the blue-white helium stars—the type B stars the astronomers called them, or the Orion stars, since there are so many stars of this type in the constellation of Orion—were not only the hottest but also the youngest of the stars and that they represented the first stage in the development of a star from a primitive gaseous nebula such as the Great Orion Nebula. It is now known that these brilliant, hot helium stars represent the peak of development of the most massive of all the stars and not the first stages in the development.