A Black Forest and the Stars
Suppose some people are lost in the middle of a very large, old, dense forest that grew up uncultivated on a great plain. Since each generation of trees grew from seeds that older trees scattered, the trees are distributed randomly, not in ranks and files like an orchard. The people are looking for the edge of the forest where they hope to find landmarks that will lead them home.
When trees grow close together only their higher branches can get sunlight. Deprived of energy, the lower branches wither and die. Some trees are self-pruning, that is, their dead branches drop off. Let’s suppose that the black forest consists of this kind of tree. A real example is the kind of black forest one finds in southwestern Germany. There trees grow very straight and thin, putting all their energy into reaching the canopy of the older trees. There is very little undergrowth because strong sunlight does not reach the ground.
If the people stand close to any one tree, that tree blocks a considerable portion of their field of view. Other trees, farther away, obstruct only a small portion of the field of view. Yet if the forest is large enough, and the people are far in from the edge, trees close at hand or far away will stop all lines of sight in any horizontal direction. The trees prevent seeing any open space or any landmark on the edge of the forest.
Now replace the trees with stars. One star, close at hand, our Sun, supplies the Earth with heat and light. When the Earth turns away from the Sun, we can see other stars farther away, just as one can see many tree trunks in a forest. Using telescopes we can see farther. We see more and more stars, always as tiny regions of light in a dark background. Also we may see nebulae, that is, cloudy smudges of light or darkness. With still better telescopes we may resolve some of the bright nebulae into galaxies. We can see the nearer galaxies well enough to know that other galaxies contain stars and dark nebulae, just as our own galaxy.
In the 1950s telescopes like the 200-inch (5-meter) Hale telescope on Mount Palomar in California could see as far away as 2 000 million light years. The sky background was still dark, with the galaxies and stars making isolated spots of light. At that time we still wondered if a sufficiently powerful telescope would make every point of the background bright with stars. That would be the picture if every line of sight eventually ended on the surface of a star. But I think most of us rather hoped to see to the edge of the universe. We had no idea what the edge would look like, but we wanted to see something new and different. If there were a limit to the number of stars but no limit to the age of the universe, then the dark spaces between stars would not forever be filled in with new stars as the telescope power increased. We were like people hoping to see to the edge of a forest that is not too far away.
When trees grow close together only their higher branches can get sunlight. Deprived of energy, the lower branches wither and die. Some trees are self-pruning, that is, their dead branches drop off. Let’s suppose that the black forest consists of this kind of tree. A real example is the kind of black forest one finds in southwestern Germany. There trees grow very straight and thin, putting all their energy into reaching the canopy of the older trees. There is very little undergrowth because strong sunlight does not reach the ground.
If the people stand close to any one tree, that tree blocks a considerable portion of their field of view. Other trees, farther away, obstruct only a small portion of the field of view. Yet if the forest is large enough, and the people are far in from the edge, trees close at hand or far away will stop all lines of sight in any horizontal direction. The trees prevent seeing any open space or any landmark on the edge of the forest.
Now replace the trees with stars. One star, close at hand, our Sun, supplies the Earth with heat and light. When the Earth turns away from the Sun, we can see other stars farther away, just as one can see many tree trunks in a forest. Using telescopes we can see farther. We see more and more stars, always as tiny regions of light in a dark background. Also we may see nebulae, that is, cloudy smudges of light or darkness. With still better telescopes we may resolve some of the bright nebulae into galaxies. We can see the nearer galaxies well enough to know that other galaxies contain stars and dark nebulae, just as our own galaxy.
In the 1950s telescopes like the 200-inch (5-meter) Hale telescope on Mount Palomar in California could see as far away as 2 000 million light years. The sky background was still dark, with the galaxies and stars making isolated spots of light. At that time we still wondered if a sufficiently powerful telescope would make every point of the background bright with stars. That would be the picture if every line of sight eventually ended on the surface of a star. But I think most of us rather hoped to see to the edge of the universe. We had no idea what the edge would look like, but we wanted to see something new and different. If there were a limit to the number of stars but no limit to the age of the universe, then the dark spaces between stars would not forever be filled in with new stars as the telescope power increased. We were like people hoping to see to the edge of a forest that is not too far away.
