A light year, in general, refers to a year of journey. Its speed is faster than sound, and thus you see light first before hearing the thunderstorm's sound. It has a speed limit of 299792458 meters per second. Within the blink of an eye, the light has revolved the Earth seven times. This concept explains how fast the light is.
Now, all of you bring your cursor to the lower left of your computer's screen and start clicking on a "Start" button, followed by "All Program", browse through "Accessories" and search for "calculator", then start calculating. The year is 31.536 million seconds, and each second the light travels at 299792458m; the product of these two figures give you an answer of 9454254955488000 meters, which is equivalent to approximately 10 trillion kilometers.
Basically, scientists believe that from the birth of the universe until now, 150 million years have passed. Therefore, the broadest space's diameter in the universe that we can observe may have existed within the extent of 15 billion light years. The light originates from a place of 15-billion-light-years. What we see from our planet is actually the light that had passed through the universe for 150 million years. This is a consequence of the birth of the universe.
When we see the stars that have existed billions of light years away from the Earth, they are in fact the stars that have already existed in the universe billions of years ago. In other words, the stars we see emerging over the sky are those with a past appearance of a billion-light-years. What will the stars look like now? To see its present appearance, we have to wait for the passing of other billions of light years.
1 Billion Light-Years
The picture shows us the image of the universe observed in a time frame of billions of light years. Each dot represents a miraculous event that may occur in an unpredictable manner in our universe. The space seems as empty as this, which is a normal scene, and with a tenfold large view the space would show no new structure and thus the universe is almost uniform in such a dimension.
100 Million Light-Years
Now, we narrow the view of the universe ten times and examine by looking through the distant place in the Milky Way whether the clotted dusts that we observed in the previous picture are stars or not. At this stage, they may appear to us as stars but are they really stars in reality?
10 Million Light-Years
Again, we narrow the view of the universe ten times. We observe that each clotted dust is a single bright spot with each of them forming up a cosmic region. At this stage, each of them looks like a litter bright star. Also, its mutual gravity binds each of them into galaxies.
1 Million Light-Years
Now, we look at this image closer and closer. These "stars" appear so familiar to us that it is a galaxy that we learnt in the book of astronomy. The galaxy appears in its spiral structure. It is said to travel in space with two satellite galaxies, the irregular little Clouds of Magellan.
100 Thousand Light-Years
This image clearly shows the Milky Way. Within a time frame of 100 thousand light-years, we can see the entire galaxy clearly. The gravity encircle the central region is a mutually bounded hundred billions stars. The external galaxies skin scattered throughout space as far as our eyes could hardly catch. As they drift, they rotate gradually in a slow pace.
The diameter of the galaxy is 100 thousand light-years. If one has the ability to invent a spacecraft that has a speed as fast as the light, it will take him 100 thousand light-years to venture through this huge galaxy. However, this situation is impossible for a human to achieve as his life span can only attain as high as 200 years old.
10 Thousand Light-Years
While lying down on the grassland in Mongolia, one can see a clear galaxy particularly during the night of summer time. The galaxy appears in a dim light flashing through a pitch-dark night sky. From this point, we may estimate our position. If we assume the galaxy as huge as a flat plate, each of us is a small dot filling up the flat surface of the plate. If not, we will be hung above and below the galaxy and our sky may appear half-dark and half-bright but not the galaxy we are observing, which is relatively distributed in the space.
In this image, we can hardly see the existence of the sun. In fact, it is located in the centre of the image, near the border of one spiral arm. The slow-changing spiral patterns of the Galaxy disk are marked by clouds of stars, patches of darkening dust and glowing gas.
