March 3, 2000

Pressure Vs. Gravity round III

Stars: Midlife Crisis & Maturity

 
The earth was the woman, Sedi, and the sky was Melo, the man. And when they married it was feared that they would come together and crush what lay between them.
The beginning of a creation myth from the Mynyong, a tribe in northeastern India.
  Read Chapters 27, 28. Look up announcements for new extra credit possibilities. HW#4 solutions are now available.
Pressure Push vs. Gravity Pull - The first 10 billion years, and the ``midlife crisis'':

From the end of Friday's class: 

If Gravitation pull > pressure push the gas cloud / star shrinks. 

      If push > pull the star grows in size. 

      If push = pull the star is usually stationary. 
 

Let's examine the history of a sun-size star more carefully with this knowledge in mind.

Question 1: A star forms from an interstellar cloud of gas. As the cloud is contracting 
the gravitational pull on a cubic meter of gas is.... answer:  larger than the pressure

As the gas cloud contracts it warms up. The increased temp. creates pressure that stops the contraction, BUT THE TEMP IS DECREASED because energy is lost to radiation. That stage takes a few million years, until the temperature at the center of the forming star reaches ~15 million degrees Kelvin.

At that stage: Fusion starts.
Question #2: Who's larger at that stage: Pressure push or gravity pull? Answer: Their equal.

Also: Same energy produced by E=mc^2 as is radiated away for the next ~10 billion years. That's the stage our sun is in.

What happens then?

Question #3: As a star stops fusing hydrogen nuclei into helium in its core... Answer: the gravitational pull on a cubic meter of gas is larger than the pressure and the star contracts.
 

 

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