Jongjin Park
There is a famous nursery rhyme called Half Moon that begins with ‘Blue Sky Milky Way’. The Milky Way, which appears in the first line of the song, is the name of the galaxy we belong to. In addition to the Milky Way, there are scientifically estimated 2 trillion galaxies in this universe. In each such galaxy, hundreds of billions of stars twinkle. It’s a number that doesn’t come into my head no matter how much I stretch my imagination. A number that is so enormously large is called an astronomical number.
Among the total of 2 trillion galaxies mentioned earlier, the Milky Way galaxy to which we belong and the nearest neighbor, the Andromeda galaxy, have about 1 trillion stars. It takes 2.5 million years to travel from the Milky Way to Andromeda at the speed of light. So what we see now is what the Andromeda Galaxy looked like 2.5 million years ago. The two galaxies are said to be attracted by each other’s gravity and will merge completely in about 4.5 billion years. Impatient people have named the two merged galaxies ‘Milcomeda’. You don’t have to worry because it’s a distant future that you can’t even imagine.
According to recent research results, it is generally said that a supermassive black hole exists at the center of a galaxy, but our galaxy also has a black hole in the middle. The mass of this supermassive black hole is said to be about 4.5 million times that of the sun. When viewed from directly above, the Milky Way looks like a disk with two spiral arms, but when viewed from the side, it looks like a dish with a convex center and a tapering edge. It is said to be about 15,000 light-years thick at the center, and about 100,000 light-years in diameter from one end to the other.
The moon takes one month to orbit the earth, and the earth takes one year to orbit the sun. It doesn’t end there, but our sun also takes 230 million years to orbit once around the center of the galaxy. Just as we have determined that one orbit around the sun is one year, the period during which the sun orbits the center of the galaxy is called a galactic year. For reference, the age of the Milky Way is about 13.2 billion years, almost the same as the age of the universe.
The Milky Way is said to look like spilled milk, so it is called Milkyway in English and Mirinae in pure Korean. There are an estimated 400 billion stars in the Milky Way galaxy, the best known of which is our Sun. The closest neighbor to our star, the Sun, is Alpha Centauri, and it is said that it will take more than 40,000 years for Voyager, which has just left the solar system after flying for 40 years, to reach Alpha Centauri. So no more guesses or imaginations.
The sun lies on the relatively inactive outskirts of the Milky Way and has been stable for the long ages necessary for life to emerge and evolve to the point of civilization. That’s how our human race was born and evolved to this extent to achieve civilization. Judging by the size of the galaxy, there would be an enormous number of civilizations in the Milky Way galaxy alone, but they would be so far apart that any kind of communication would be impossible until the end of the galaxy’s lifetime.
In the above description of our galaxy, many astronomical numbers have been cited as examples. It’s not a number that we normally use, so we won’t get a feel for it. Just think of it as a huge, distant, or large number. (author)
Jongjin Park
