Stellar Formation
Star formation occurs in enormous gas clouds called nebulae.
A passing star or nearby nova explosion may cause the gas to clump/condense in a part of the nebula. Gravity takes over from there and the gas pocket continues to collapse inward.
The collapsing gas cloud can be anywhere from 1 LY across (resulting in the smallest red dwarf stars) to hundreds of light years (resulting in the largest blue hypergiant stars).
Let’s look at the formation of our home star, the Sun, as an example of stellar formation. We’ll keep a timeline at the top of the screen to see how events unfold.
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The formation of the Solar System began nearly 5 billion years ago with the gravitational collapse within part of a giant molecular cloud.
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The mass in the center of the collapsing cloud gets very dense and very hot. Though it glows intensely, it is not yet fusing H into He. This is know as a protostar.
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The protostar becomes so hot and dense that eventually the H nuclei smash with enough force to fuse into He.
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H
He
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The temperature increases, further accelerating the fusion reaction. Now it is a star.��For the Sun, this was ~4.9 billion years ago
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He
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The remaining H outside of the Sun is pushed away by the intense solar radiation. In the outer parts of the system the H will be part of Jupiter’s and Saturn’s formations.
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The heaviest debris in the cloud helps to form the inner rocky planets; Mercury, Venus, Earth, Mars, and the asteroids.
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The earliest part of the solar system is characterized by numerous massive collisions. At this time there may have been as many as 100 bodies we would classify as planets today.
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At just over 4.6 billion years ago the early solar system began to resemble what we see today, though there was still much debris and many collisions.
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Our moon was created in one such massive collision between 4.6 and 4.5 billion years ago.
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The last round of collisions, known as the Late Heavy Bombardment, scarred the inner solar system with craters that can still be seen today.
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On tectonically inactive bodies like the Moon and Mercury, craters from the Late Heavy Bombardment are clearly visible.
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Mercury
Lunar Surface
On tectonically active worlds like Venus and Earth it is hard to find craters from this time. But we do see evidence that impacts occasionally still happen.
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