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Our Modern view of the universe

 What is our place in the universe?

Earth is a planet orbiting the Sun. Our Sun is one of more than 100 billion stars in the Milky Way Galaxy. Our galaxy is one of about 40 galaxies in the Local Group. The Local Group is one small part of the Local Supercluster, which is one small part of the universe

Billions of other galaxies are scattered throughout space. Some galaxies are fairly isolated, but many others are found in groups. 

Our Milky Way, for example, is one of the two largest among about 40 galaxies in the Local Group. Groups of galaxies with more than a few dozen members are often called galaxy clusters. 

On a very large scale, observations show that galaxies and galaxy clusters appear to be arranged in giant chains and sheets with huge voids between them. 

The regions in which galaxies and galaxy clusters are most tightly packed are called superclusters, which are essentially clusters of galaxy clusters. 

Our Local Group is located in the outskirts of the Local Supercluster.

Together, all these structures make up our universe. In other words, the universe is the sum total of all matter and energy, encompassing the superclusters and voids and everything within them. 

How did we come to be?

The universe began in the Big Bang and has been expanding ever since, except in localized regions where gravity has caused matter to collapse into galaxies and stars. The Big Bang essentially produced only two chemical elements: hydrogen and helium. The rest have been produced by stars, which is why we are “star stuff.” 

The Big Bang and the Expanding Universe

Telescopic observations of distant galaxies show that the entire universe is expanding, meaning that the average distances between galaxies are increasing with time. This fact implies that galaxies must have been closer together in the past, and if we go back far enough, we must reach the point at which the expansion began. We call this beginning the Big Bang, and from the observed rate of expansion we estimate that it occurred about 14 billion years ago. 

The universe as a whole has continued to expand ever since the Big Bang, but on smaller scales the force of gravity has drawn matter together. Structures such as galaxies and galaxy clusters occupy regions where gravity has won out against the overall expansion. That is, while the universe as a whole continues to expand, individual galaxies and galaxy clusters do not expand. 

Stellar Lives and Galactic Recycling

Within galaxies like the Milky Way, gravity drives the collapse of clouds of gas and dust to form stars and planets. Stars are not living organisms, but they nonetheless go through “life cycles.” A star is born when gravity compresses the material in a cloud to the point where the center becomes dense and hot enough to generate energy by nuclear fusion, the process in which lightweight atomic nuclei smash together and stick (or fuse) to make heavier nuclei.

The star “lives” as long as it can generate energy from fusion and “dies” when it exhausts its usable fuel. 

In its final death throes, a star blows much of its content back out into space. In particular, massive stars die in titanic explosions called supernovae. The returned matter mixes with other matter floating between the stars in the galaxy, eventually becoming part of new clouds of gas and dust from which future generations of stars can be born. Galaxies therefore function as cosmic recycling plants, recycling material expelled from dying stars into new generations of stars and planets. Our own solar system is a product of many generations of such recycling. 

BOOK: The Essential Cosmic Perspective with MasteringAstronomy (Sixth Edition) 

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