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Galaxy Formation

The Big Bang
Early Universe
Life and Death of Stars
Galaxy Formation
The Solar System
Exotic objects
End of the Universe
Who created the universe?
What is Time?
Life beyond Earth
NASA Missions
Particle Map
Glossary
Sources
The Universe today
Thanks to Edwin Hubble, we know that the universe is continuously expanding at an ever-increasing rate. This means that the more distant galaxies are moving away from us and everything else proportionately faster than the closer ones. We know this by observing the doppler shift from blue to red, indicating shorter wave lengths (frequencies). A galaxy that is 3 times farther than a closer one is moving 3 times faster. As we look into the universe, we are looking back in time. The farther we look, the earlier in time we are seeing. Because the light from distant objects takes billions of years to reach our telescopes, our distant views are a snapshot of the universe as it was billions of years ago. We can literally see the universe as it was 12 billion years ago or 1 billion years after the Big Bang. We can not see further back than the end of the Dark Age of the Universe because there was no light to illuminate the universe at that time, so we see nothing. We have access to this view of the early universe thanks to the Hubble Deep Field images. To further illustrate the concept of seeing back in time, consider the fact that our Sun, which is 93 million miles from us (1 astronomical unit), is 8 light minutes away. This means that it takes light 8 minutes to travel from the Sun to our location in space. So, when we view the Sun, we are seeing 8 minutes back in time.



Formation of The Earliest Galaxies
To explain the formation of the earliest galaxies, which may have occurred some 1 billion years after the Big Bang, we make three assumptions about the conditions in the early universe: (1) the early universe was composed of clouds of dust and gas, primarily hydrogen and a small amount of helium, and a predominance of dark matter; (2) there was an asymmetry of density (some areas were slightly denser than others); (3) the universe was uniformly expanding everywhere at once. As the universe was expanding, the density was decreasing, but in regions where the density was a bit higher than average, the expansion was slightly slower than elsewhere and the density decreased more slowly. Gravity slowed the expansion of high-density regions more rapidly than that of low-density regions, so the contrast between high and low density regions steadily increased. By the time the Universe was a few hundred million years old, the densest regions had ceased expanding and begun to collapse due to the effect of increasing gravity. These regions of higher density began clumping together due to their stronger gravity and as the clumps of matter grew, they heated up and the strength of their gravity increased even further. Dark matter played a large role in the process of clumping of ordinary matter as the gravitational force of dark matter, which was also clumped together, pulled more ordinary matter together through its own gravitational pull. Eventually, the clouds of gas in these areas of higher density collapsed and formed protogalactic clouds. As gravity continued to collapse the clouds, they formed rotating disks. The rotating disks attracted more gas and dust with gravity and formed galactic disks. Inside the galactic disk, new stars formed. What remained on the outskirts of the original cloud were globular clusters and a halo composed of gas, dust and dark matter. Conservation of angular momentum caused remaining gas to rotate faster and flatten into a rotating spiral disk with a bulge in the center, just as happens to a figure skater who, while spinning, gradually brings their arms closer in to their body and spins faster.

A typical spiral galaxy. M101 is 170,000 light-years across or nearly twice the size of the Milky Way galaxy. It contains at least one trillion stars and as many as 100 billion of them may be similar to the Sun.