The Night Sky Is Dark
The temperature of the Earth is an equilibrium value arising from the balance between heat received from the Sun and heat radiated into space. If the night sky were not dark less heat would flow to it, and the Earth temperature would rise. The night sky is dark because the universe is not eternal and uncreated, and because the universe is expanding. Let us examine this point in greater detail.
Here and there stars punctuate the darkness. Ancient peoples thought the number of stars was limited to those they could see. There are about 3 000 bright stars visible to the unaided eye. Modern telescopes reveal that our galaxy alone has 100 000 million stars. In the known universe there are about the same number of galaxies. With so many stars, why is the night sky not lit up from east to west and north to south?
Here and there stars punctuate the darkness. Ancient peoples thought the number of stars was limited to those they could see. There are about 3 000 bright stars visible to the unaided eye. Modern telescopes reveal that our galaxy alone has 100 000 million stars. In the known universe there are about the same number of galaxies. With so many stars, why is the night sky not lit up from east to west and north to south?
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.
Seeing to the Far Limit of the Universe
Let’s go back to the example of the trees. If we have not gone too far into the forest we can see to the edge of it, looking here and there between the tree trunks. But what does the edge look like? The forest grew up uncultivated, perhaps starting with one solitary tree in the middle of a great plain. Left to itself, every year the forest becomes larger, because the trees on the edge scatter seeds some distance out onto the plain. At the edge, last year’s trees are small saplings, and this year’s trees are seedlings. The closer we are to the edge, the younger the trees will be. Looking out from the depths of the forest, we will not be able to see the trunks of the youngest trees, because the seedlings are not tall enough. The saplings will have leafy branches all the way up, because on the edge of the forest all the outer branches are exposed to sunlight. In short, the edge of the forest must be younger and greener, different from the old, dense interior.
With telescopes we can see far away. When we see distant stars and galaxies, we see them not as they are now, but as they were when they were newer, because it takes thousands of millions of years for their light to reach us. The farther away the stars are, the newer they were when their light started on its journey toward Earth.
In the 1950s astronomers recorded their pictures on sensitive photographic plates. Now they record the pictures digitally using a new kind of detector called a charge-coupled device (CCD). These detectors are 100 times more sensitive than film. Light intensity falls off inversely as the square of distance. Therefore, an old telescope, newly equipped with a CCD instead of photographic plates, can see ten times farther. The Hale telescope in the 1950s could see out to 2 000 million light years. With a CCD instead of film it can now potentially see as far as 20 000 million light years, but beyond about 13 500 million light years there are no more stars or galaxies to see.
Does this mean that we can now see almost to the edge of the universe? Not really. We can see almost to the beginning of the universe. Stars in their earliest stages look different from fully formed stars. The first stars began as huge clouds of cold gas. Until these clouds contracted and heated up under the action of their own gravity, they emitted no light. That made them as invisible at a distance as the seedlings on the edge of the black forest.
The first stars became visible when their heat reached the temperature of ignition of hydrogen, their nuclear fuel. The first stars had no heavy elements to serve as catalysts for burning hydrogen into helium. They couldn’t start to burn until gravity brought their central temperature to millions of kelvins. When they burn their outer layers reach bluish-white heat. However, we see them as members of dark red galaxies too far away to resolve into individual stars. They look red, not bluish white, because expansion cools their light. The light from the farthest stars is the coldest. Though the most distant stars look red when we photograph them, we must remember that we are seeing the earliest stars. Now they look red, but when their light started out, it was so hot it was bluish white.
With telescopes we can see far away. When we see distant stars and galaxies, we see them not as they are now, but as they were when they were newer, because it takes thousands of millions of years for their light to reach us. The farther away the stars are, the newer they were when their light started on its journey toward Earth.
In the 1950s astronomers recorded their pictures on sensitive photographic plates. Now they record the pictures digitally using a new kind of detector called a charge-coupled device (CCD). These detectors are 100 times more sensitive than film. Light intensity falls off inversely as the square of distance. Therefore, an old telescope, newly equipped with a CCD instead of photographic plates, can see ten times farther. The Hale telescope in the 1950s could see out to 2 000 million light years. With a CCD instead of film it can now potentially see as far as 20 000 million light years, but beyond about 13 500 million light years there are no more stars or galaxies to see.
Does this mean that we can now see almost to the edge of the universe? Not really. We can see almost to the beginning of the universe. Stars in their earliest stages look different from fully formed stars. The first stars began as huge clouds of cold gas. Until these clouds contracted and heated up under the action of their own gravity, they emitted no light. That made them as invisible at a distance as the seedlings on the edge of the black forest.
The first stars became visible when their heat reached the temperature of ignition of hydrogen, their nuclear fuel. The first stars had no heavy elements to serve as catalysts for burning hydrogen into helium. They couldn’t start to burn until gravity brought their central temperature to millions of kelvins. When they burn their outer layers reach bluish-white heat. However, we see them as members of dark red galaxies too far away to resolve into individual stars. They look red, not bluish white, because expansion cools their light. The light from the farthest stars is the coldest. Though the most distant stars look red when we photograph them, we must remember that we are seeing the earliest stars. Now they look red, but when their light started out, it was so hot it was bluish white.