What is the universe for?
How did the universe come about?
In the night sky we can see many thousands of stars with the naked eye. With large telescopes we can look much further into space - over 100,000,000,000,000,000,000,000 kilometers, that's a one with 23 zeros! We see billions of galaxies, and within that, billions of stars. In short: the universe is unimaginably large and full of stars. But how did it come about?
Scientists are constantly watching the universe. They made an interesting observation: All galaxies in the universe fly away from each other - and the faster, the further away they are. So the universe is expanding and getting bigger and bigger.
Conversely, that means: In the past, space was smaller. If you trace the movement of the galaxies far back, they all meet at a point in time about 14 billion years ago. Back then, the entire universe and everything we see in it today was no bigger than the head of a pin - extremely tightly compressed and very hot. Everything flew apart in a huge explosion known as the “Big Bang”: the universe was born and has expanded in all directions ever since.
And what was before the big bang? We don't know, because the scientists cannot calculate any further back than the Big Bang.
What is the Milky Way?
If you look at the sky on a clear night, you can see a bright band that stretches across the sky. Because it looks so milky and light, it is called the Milky Way. The ancient Greeks thought that the gods spilled milk here and called it "galaxy" - after the Greek word for milk.
Of course, the Milky Way isn't actually made of milk. If you look at it with a telescope, you can see that it consists of many stars. With the naked eye, its light blurs into a bright band. But why do so many stars gather in this narrow strip of sky?
To solve this puzzle, astronomers had to look much further into space. There they discovered bright spots that they called "fog". Using a powerful telescope, they saw that these nebulae are a collection of many billions of stars - and that most of the nebulae were in the shape of large, flat discs. This made it clear: the sun itself is a star in such a disk. And because we live in the middle of this disk, it looks to us like a strip that stretches around us across the sky.
This disk, our galaxy, consists of several hundred billion stars, their planets and lots of dust and gas. This matter attracts each other with its gravitational pull, so the galaxy retains its shape: a flat disk with the stars arranged in spiral arms and revolving around the center of the galaxy.
Our sun is the center of the solar system, but itself only a small star in an outer spiral arm of the Milky Way. So it also moves and takes a little more than 200 million years to complete one lap.
Even our galaxy is by no means special, just a medium-sized galaxy among many billions in the universe. The closest galaxy is the Andromeda Nebula, about 2 million light years away from us. A simple look at the sky quickly leads to unimaginable expanses.
What is a star
When it is particularly dark at night and the sky is clear, we see thousands of stars as tiny points of light twinkling over our heads. But why do the stars shine? What are stars anyway?
Stars are simply spheres of gas. But inside it is unimaginably hot, many millions of degrees Celsius. Because of the intense heat, the gas glows and glows - like a light bulb, only a lot brighter. The light from the stars is so strong that we can see it from Earth, even though the stars are many trillions of kilometers away.
Stars appear to us like tiny points of light - but that's only because of the great distance: in reality, stars are huge. The smallest are about ten times the size of the earth, giant stars can be a hundred thousand times as big!
However, there is one star that is very close to us compared to any other: the sun. After all, it already appears to us as a bright disc in the sky. But even this impression is deceptive: the sun is about a hundred times the size of the earth. We see and feel its power every day, because it gives the earth light and warmth - like a large campfire, by which we sit in the cold universe.
However, a star does not burn wood. It consists mainly of hydrogen gas and draws its energy from the hydrogen atomic nuclei. So a star slowly burns itself, so to speak. When the fuel supplies are used up at some point, it becomes dark and collapses or explodes. Our sun will end like this one day too. But because stars are so big, the fuel lasts for a long time. Our sun, for example, will shine for another five billion years.
How are stars formed?
The stars shine in the sky - no doubt. But where do the stars actually come from?
To answer this question, scientists are interested in certain areas in space. In the telescope, these look hazy and blurred - and are therefore also called “fog”. One example is the Orion Nebula in our own galaxy. Stars were discovered in it that are still very young - just 30,000 years old. So there are still “baby stars”, because a star can be several billion years old. The nebula is, so to speak, the cradle of the stars.
And where do they come from? To this end, the nebula was examined more closely: it contains dense clouds of dust and, above all, of the gas hydrogen - the substance that stars are made of. So the gas clouds must be the origin of the stars.
But how does a gas cloud become a star? It is a process that will take many millions of years. The driving force is the gravity of the gas particles in the cloud. It causes the particles to attract each other. The cloud becomes smaller and denser over time - it becomes a massive ball of gas.
The gas particles inside the cloud are therefore under increasing pressure, they are literally squeezed together. As the pressure increases, so does the temperature. At some point it is so hot inside that the hydrogen atoms start to fuse together. This reaction releases a lot of energy so that the star begins to shine.
This “heating” from inside the star generates additional heat and counter pressure. The newly ignited star does not contract any further, but instead forms a stable, glowing gas ball.
In most cases, several stars emerge from one cloud at the same time. These attract each other and circle around each other. One then speaks of a double, triple or multiple star. But only one star emerged from “our” cloud - the sun.
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