The universe holds so many mysteries for us human beings. Its enormous size invites people to keep learning and finding out new information about it. Today, we will learn about three basic structures in the universe. Let’s start with number one: galaxies. A galaxy is a gravitationally bound system consisting of stars, stellar remnants, interstellar gas and dust, and dark matter. There are approximately 170 billion galaxies in the observable universe. Can you imagine how huge that is?
数以亿计的星系总称为银河系，其中就包括了我们的太阳系。银河系的英语是Milky Way ，因为它看上去就像打翻的牛奶(Milk）。银河系中有数以亿计的星球，比地球大的就至少有170亿。
Of those billions of galaxies, one is called the Milky Way. It is the galaxy that contains our solar system, where Earth is located. It is called the Milky Way because the disk of the galaxy runs across the sky as a band of glowing light, like spilled milk. Astronomers estimate that there are between 100 and 400 billion stars in the Milky Way, and think that each star has at least one planet. So there are likely hundreds of billions of planets in the Milky Way, and at least 17 billion of those are the size and mass of Earth.
The Sun is located in the Orion Arm, a region of space in between the two major arms of the Milky Way. The Sun and all the objects that orbit it, either directly or indirectly, form the solar system. It takes the Sun about 240 million years to orbit the Milky Way once.
So imagine this: the Sun orbits around the Milky Way, and Earth orbits around the Sun. Everything in the universe orbits around something, or is orbited around by something else … or both! It paints a huge, complex, beautiful picture, doesn’t it?