Did you know that how really big is the Universe? it is hard to imagine how big it is. Astronomers (scientists who study the stars, planets, and other objects in the universe) estimate it to be about 10 billion (10,000 million) light years in diameter. A light-year is the distance traveled by light for a whole year at the speed of 300,000 kilometers per second.
It is estimated that millions of galaxies are distributed in this huge universe. Each galaxy is itself composed of billions of stars and is millions of light-years from the nearest galaxy. Our galaxy, the Milky Way, is about 100,000 light-years across and it belongs to what is known as the Local Group. Other galaxies are composed of countless star system such as our won solar system. To such a star system belongs our tiny planet Earth.
One of the more important recent astronomical discoveries points to strong evidence that the galaxies are moving away from one another, and some travel at a speed of more than 50,000 kilometers per second. This means that the universe is expanding. It is theorized that as the galaxies flee form each other, new ones are created. This is called the continuous-creation theory, which assumes that the universe is infinitely old and large.
The Magnitude of Stars
By mapping the night sky, astronomers were able to estimate that around 3,000 stars are easily visible to the naked eye. The rest, numbering many billions, can be seen by the use of telescopes; the more powerful the telescope is the more stars are seen through it.
Stars differ in brightness. Some are bright and others are faint. The apparent brightness of a star is called its magnitude. The stars of the first magnitude are the brightest. They are two-and-a-half times brighter than a star of the second magnitude. In turn a second magnitude star is two-and-a-half times brighter than a third-magnitude star, and so on. The faintest stars seen with the unaided eye are sixth-magnitude stars. Powerful telescopes have observed stars of the twenty-third magnitude. Antares and Polaris are of the first and second magnitudes, respectively. The magnitude of a star is determined by its size, distance, and temperature. The big stars shed more light than the small stars. A star close to us appears brighter than a distant star of the same size. Likewise, a hot star shines brighter than a cooler star.