chapter 17 astro1010-lee.com twlee2016@gmail.com uvu survey of astronomy chapter 17 formation of...

Chapter 17A

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Chapter 17Formation of Stars

Chapter 17A

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y Star formation is an ongoing process in the Universe. Star-forming regions are seen in our galaxy as well as others

Chapter 17A

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y Star formation happens when part of a dust cloud begins to contract under its own gravitational force; as it collapses, the center becomes hotter and hotter until nuclear fusion begins in the core.

When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion.

Even a massive cloud of gas and dust will remain just a cloud until some shock wave or pressure wave arrives to initiate its gravitational collapse

Chapter 17A

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y The collapse process from nebula to star is similar for all stars and can be followed by observing the temperature produced by the compression

Star FormationThe collapse process from nebula to star is similar for all stars and can be followed by observing the temperature produced by the compression

Chapter 17A

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Chapter 17A

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y Stage 1: An interstellar cloud starts to contract, probably triggered by a shock or pressure wave. As it contracts, the cloud fragments into smaller, irregular size pieces.

Chapter 17A

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Stage 2:Individual cloud fragments begin to collapse. Once the density is high enough, there is no further fragmentation.

Stage 3:

The interior of the fragment has begun heating, and is about 10,000 K.

Chapter 17A

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Stage 4:

The core of the cloud is now a protostar, and the surface temperature is high enough to make its first appearance on the H–R diagram

Chapter 17A

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By stage 5 Planetary formation around the star has likely begun, but the protostar itself is still not in equilibrium – all heating that effects the system comes from the gravitational collapse.

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The last stages can be followed on the H–R diagram.

The protostar’s luminosity decreases even as its temperature rises because it is becoming more compact.

Chapter 17Star Formation

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At stage 6, the core reaches 10 million K, and nuclear fusion begins. The protostar has become a star but it not yet on the main sequence. This stage is often called the T Tauri stage and it is a period of adjustment.

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y T Tauri stars are mostly:•between 105 and 108 years in age; •of low mass (0.5 to 3.0 M¤); • surrounded by hot, dense

envelopes; and • losing mass via stellar winds with

typical v¥= ~100 km/s.

Chapter 17Star Formation

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The star continues to contract and increase in tem-perature, until it is in equilibrium. This is stage 7: the star has reached the main sequence and will remain there as long as it has hydrogen to fuse.

This H–R diagram shows the evolution of stars somewhat more and somewhat less massive than the Sun.

The shape of the paths is similar, but they wind up in different places on the main sequence.

Chapter 17A

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Chapter 17A

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The main sequence is a band, rather than a line, because stars of the same mass can have different compositions.

Most important: Stars do not move along the main sequence! Once they reach it, they are in equilibrium, and do not move until their fuel begins to run out.

Chapter 17A

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In-Class Quiz

The T Tauri stage in a star’s development is a period of ______________.

Chapter 17A

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End of Chapter 17