How big can a galaxy get?
Our home galaxy - the Milky Way
Diameter: 100,000 light years
If we look at the sky at night - far away from big cities that literally pollute the night sky with light - then we see a milky band there. The ancient Greeks called it the "Milky Way". But what is this object actually?
If you look closely with a telescope, you can see that the white band consists of countless stars. Scientists have calculated that it is between 100 and 200 billion. The exact numbers are not known because the Milky Way is huge. It has a diameter of about 100,000 light years. This means that it takes light 100,000 years to travel from one end of the Milky Way to the other at the speed of light.
But even the distance from star to star is unimaginable. If one star were as big as a soccer ball, then the next star would be as far away as New York is from us. With more than 100 billion stars in the Milky Way, their dimensions are huge.
Each of these many billions of stars is a sun, so to speak - although some of them may also have orbiting planets, as astronomers have suspected for a long time. But only the stars shine by themselves and are thus visible in the night sky.
Together these innumerable stars form a large disk. And we sit with our solar system in the outer third. What we can see in the night sky is the cross-section through this disk.
In addition to stars and planets, there are also interstellar gas clouds in the Milky Way. These consist largely of hydrogen and helium - just like our sun. After all, it emerged from such a gas cloud. In turn, there are also so-called "dark zones" in the gas clouds. These are clouds of dust from which planets like our earth are formed.
The Milky Way on a crash course
The near future of our home galaxy, the Milky Way, does not look very rosy - at least if you think in terms of cosmic dimensions. Because in a few million years the Milky Way will collide with our neighboring galaxy Andromeda.
Such an event is particularly exciting for astronomers, because there is a lot of space between the stars in the Andromeda galaxy and in the Milky Way. When the two galaxies collide, one planet does not meet the next, but the celestial bodies bring each other out of balance through their attraction.
However, the Milky Way has more mass and it also rotates faster than Andromeda. That is why the Milky Way is eating Andromeda, so to speak. And this creates a new galaxy in the end - when the many planets and stars have found a position of equilibrium.
The past of the Milky Way
However, our Milky Way itself was formed from gigantic gas clouds from the early days of the universe. However, the principle is similar. The main actor is always gravity. When the universe was formed around 14 billion years ago, there were no galaxies. The first of them emerged after about a billion years - including the Milky Way.
Similar to the collision of two galaxies, it was gas clouds that got out of balance. This causes the cloud to collapse due to its own gravity. At the same time it rotates faster and faster - similar to a figure skating pirouette: If the mass ("arms") is on the outside, it rotates slowly - if the mass is closer to the axis of rotation, it rotates faster. So did the collapsing gas cloud.
The faster rotation creates greater centrifugal forces and the cloud flattens out, just like a lump of clay on the turntable when making pottery. That is why the Milky Way is shaped like a flat disk.
How do you know all this?
Even the ancient Greeks observed and documented the Milky Way. However, they lacked the technical equipment to examine the Milky Way more closely.
It was only with the invention of the telescope by Hans Lipperhey around 1608, or its refinement by Galileo Galilei a year later, that the basis for the scientific description of the Milky Way was created.
State-of-the-art telescopes such as the Herschel space telescope are used today. It can resolve objects up to ten billion light years away. Using the movement patterns of celestial bodies, scientists can draw conclusions about what is happening in space.
It is of course important to know how far away an object is from us. The researchers use a lot of tricks to do this. For example parallax. In principle, the effect is very simple: pinch your left eye, stretch out your arm and use your thumb to aim at an object about three meters away. Now close your right eye and see where the thumb is now pointing. Slightly wrong.
This shift can be calculated and distances can be measured on a large scale. To do this, you aim for a distant star, also twice in a row - when the earth is at two different points in the orbit around the sun. The distance covered can be calculated and, together with the parallax shift, the distance can be determined. This gradually creates an ever more precise description of our galaxy.
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