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.
A star, it is now known, comes into existence as a giant, reddish star of enormous size and of a density only about one-thousandth that of the earth's atmosphere at sea-level. It is inconceivably tenuous or rare, and its temperature is comparatively low, about 3,000° Centigrade or less. It is not evolved from the luminous, gaseous nebulæ because red stars are never found associated with the gaseous nebulæ, as are the blue-white stars. The origin of these red giant stars is uncertain, but it is possible that they may be gradually evolved in some manner from the dark clouds of obscuring matter or dark nebulæ that exists so abundantly in the heavens.
In the next stage of its development the deep-red giant star increases in temperature as it contracts under the action of gravitation and its color gradually changes from red to yellow. Its density increases slightly and its volume decreases. It is now a yellow giant star. As the evolution progresses in the course of ages the star continues to contract, its temperature increases greatly as does also its density and it continues to decrease in volume. It is now a brilliant white star, a hydrogen star, so called because its spectrum is chiefly characterized by the lines of hydrogen.
As the star contracts under the gravitation of its parts and increases in temperature and density there comes more and more into play an important factor that has a great effect upon its future development. This is light-pressure or radiation pressure which acts in opposition to gravity and exerts a strong outward pressure upon matter within the depths of the star, tending to push it outward from the center where the temperature is greatest and the light is most intense. It is a most interesting fact that if the mass of a star, that is the quantity of matter that it contains, exceeds a certain value the pressure of light or radiation within it overbalances the gravitational attraction that draws matter towards its center and the star disintegrates or ceases to exist as a star. This accounts for the fact that the stars differ very little among themselves in the quantity of matter that they contain, that is, in their masses, though they may differ enormously in size. Stars that exceed a certain mass will become unstable and this may account for the association of luminous nebulæ with the hottest of all stars, the nebulæ possibly being puffed off from the surfaces of these stars under the action of radiation pressure.
After a star has reached the height of its development as a bluish-white helium star with a temperature of something like 10,000° Centigrade and a density about one-tenth that of the sun, it begins to lose heat and to cool gradually though it continues to contract and increase in density.
It is now on the descending scale of evolution and is to be counted among the dwarfs instead of the giants. A brilliant blue-white helium or Orion star is about one hundred times more luminous than the sun, and its diameter is about ten times that of the sun.
Our own sun, we find, is on the descending scale of stellar evolution. It is a yellow dwarf star of temperature about 6,000° Centigrade and density one and one-fourth that of water, which is probably about as great a density as is attained by any star since even the non-luminous planets Jupiter and Saturn have lower densities than the sun.
The last stage in the development of a star is represented by the dwarf red star of high density and low temperature. The diameter of the dwarf red star probably averages about five hundred thousand miles and its temperature is 3,000° Centigrade or less. After this we have the extinct stars, similar possibly to our planet Jupiter, though considerably larger, with a dense gaseous atmosphere and a certain degree of internal heat.
We have traced the evolution of a star from a red giant to a red dwarf through the intermediate stages from yellow giant to a giant helium star with increasing temperature and thence to yellow dwarf and red dwarf as the temperature decreases. Only the most massive stars pass through this entire chain of evolution. Stars of small mass never attain to the splendor of brilliant blue-white helium stars, but begin to decrease in temperature and brightness before this stage is reached.
The time required for the evolution of a star from red giant to red dwarf is not known, but it must be very great. The age of the earth, which is probably equal to that of the solar system, is estimated as something like one thousand million years. It is probable that the average life of a star far exceeds this limit.