Now we can explain what the universe looks like. Roughly speaking about the structure of the recent universe, the stars gather to form one group called a galaxy, and those groups gather to form a larger group called cluster of galaxies, and so on.

Newton
Credit: Rex Features Ltd

Stars are attracted to each other by the force of gravity. Perhaps you have heard of the phrase "universal gravitation." It is famous because of an episode of Isaac Newton when he saw an apple fall from a tree and discovered gravity. This is an idea that all matter with mass attracts toward other matter. This attractive force becomes larger if the mass of matter gets larger.

The stars gather because of their own attractive forces, their mass increases, and they attract another set of stars. And so the universe is formed.
To the simulation of Universal Gravitation made with Java.

Structure of Universe

Well, let's look at the structure of recent universe known from the observations up till now. To make it easier to understand, we will begin with our planet Earth. Bear in mind, as we have said before in the section of the Universe Expansion, that there is no center of the universe. Although we begin our explanation with Earth, it does not mean that the earth is the center of the universe.

Galaxy

The earth is a planet belonging to the solar system. In the universe, there is uncountable number of groups of stars with similar structure to our solar system, that is, we can think of many small stars circling around a fixed star with some probability. A fixed star is a star that glows by causing nuclear reaction inside itself. A good example in our solar system is the sun. About 100 million of those groups of stars and fixed stars gather to form one galaxy.

Milky Way Galaxy
Credit: AstroArts

When thinking about the universe in the terms of cosmology, the most basic unit is this galaxy, and can be separated into few kinds, which we'll explain later on. Our solar galaxy is usually called "the Galaxy" or "Our Galaxy" or "Milky Way (Galaxy)". It is disc-shaped, with a diameter of 100 thousand light years, and a thickness of 3000 light years. Converted into kilometers, this would be a diameter of 900 quadrillion km, a thickness of 30 quadrillion km. Although this is the smallest unit of the universe, it has such a massive size.

So, let's go on to some larger structures.

Hierarchical Structure of Galaxies

A small group of no more than 50 galaxies is called a group of galaxies, and if more than 50 galaxies are gathered within the diameter of 10 million light years, it is called a cluster of galaxies. Our Milky Way galaxy is part of a small group called a local group, together with 25 other galaxies within a diameter of 5 million light years.

Virgo cluster of galaxies
Credit: AstroArts

Among the cluster of galaxies, there is the giant cluster of galaxies, which consists of more than 1000 galaxies within the diameter of 10 million light years. Our nearest giant clusters of galaxy is the Virgo cluster of galaxies. In fact, this Virgo cluster of galaxies attract many other groups of galaxies by the strong gravity caused by its great mass. The Milky Way galaxy, which is 50 million light years from Virgo cluster of galaxies, is no exception. This large group of galaxies with the Virgo cluster at its center of that local supercluster. The Milky Way galaxy is on the edge.

Supercluster of galaxies
Credit: ST Scl W.Baum(U.WA),NASA

This has a far greater mass than normal galactic clusters, and has attracted many other small clusters around itself by the force of gravity. Other than the Virgo cluster of galaxies, Coma galactic clusters, Perseus galactic clusters and Hercules galactic clusters all are said to belong to the supercluster of galaxies.

This makes us wonder how big these clusters and superclusters can go. With structures as large as these, the distances between their galactic clusters will become far. Gravity can not transmit its force to objects at such a far distance. Therefore, if you look at this relatively, the force of attraction(gravity) becomes weaker than the force of universe expansion. The supercluster spreads outward with the expansion, and even if it pulls inward with gravity, it does not exist as a group. So in the sense of other gigantic galactic clusters, whether or not supergalactic clusters are formed, is not yet known.

Further Larger Structure of Universe

The structure of universe is expanding even further.

Distribution of galaxies
(black parts are spaces, and blue parts are the galaxies)
Credit: Maddox-Sutherland-Efstathiou-Loveday

The galaxies, groups of galaxies, clusters of galaxies, and superclusters of galaxies do not exist independently but have some connection with each other. It was found that stars are connected like bubbles stuck together. The galaxies aren't seen inside the bubbles but on the surface, and inside of a bubble is large hollow. For example when you insert a straw in to some liquid and blow air into it. This structure is called a bubble-like structure. Because the face is similar to bee nests, is it also called a bee's nest structure. A diameter of one bubble is a maximum of 100 million light years. This means that in one bubble, about 1000 of our galaxies can fit in a straight line. One bubble is called a void.

Large scale structure

The largest structure ever found is a gigantic dividing wall called the Great Wall. The Great Wall is a thin, wall like structure made from galaxies and several thousand galaxies are gathered there. And its length is said to be longer than 6 hundred million light years. There are many Great Walls and in fact , observations tell us that great walls exist periodically every 4 hundred million light years. Because of their size, we don't know how they are placed in universe.

Until recently, every time a wider area was observed, a bigger structure was found. The Great Wall is likely to form a further large structure in the vast area of several billion light years.
To the animation of Structure of Universe made with Shockwave

List of Related Links

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• Interactive universe - Both local galaxy and universe models have an expanding semi-transparent sphere surrounding the position of the Milky Way galaxy. (Change and Delete)
• Theories of the early universe - The great excitement of modern cosmology comes from the confrontation of theoretical ideas with a rapidly growing body of data. (Change and Delete)
• Large scale structure formation - The clustering of baryons and cold dark matter induced by a single moving string is being analyzed numerically. (Change and Delete)
• The origin of the large scale structure - There are strong reasons to believe that the fluctuations which seeded the large-scale structure of the universe must have been primordial in origin. (Change and Delete)

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