Pressure Vs. Gravity round IV

2. Pressure Push vs. Gravity Pull - To understand what happens next, refer to question by student regarding my last in-class question: The star collapses. But we know that it forms a red-giant. How come?

Answer: We have to look at different parts of the star: Core, outer core, outer layer. 

This will explain:
Core Collapse -  part I
Deep layers persistent fusion
Outer layers bubbling up
Red Giant stage (AGB)
Helium flash & planetary nebula 

The core is collapsing after hydrogen is exhausted. That means: Particles speeding up towards one another &
temp goes up. 
Hot things glow brighter -->  energy lives the core even faster than during fusion!  

Question #1: When the core of a 1 solar mass star stops fusing hydrogen into helium the layers just outside the core... Answer:  warm up.

A result: Temp in the outer core exceeds ~15 million degrees, Fusion starts in the outer core! The whole core gives off many Joules / second.

Question #2: As the outer core of a 1 solar mass star keeps on fusing hydrogen into helium and the inner core keeps contracting the outer layers expand because of... Answer: The increased temperature and pressure from the inner layers.

That pressure results in the outer layers "bubbling up" like milk that boils. Radius of the star from ~10^8 m to ~ 10^11 m or more. 

Star becomes red: Less intensity/flux from each square meter of surface - but overall MORE POWER (Intensity x surface area!). 

These are the red-giant stars, or the AGB (Asymoptotic Giant Branch). Stars of the size of the sun are red giants for a few million years.

In the meanwhile, the core is contracting and heating up.

Question #3: As the inner core reaches temperatures of 100,000,000 degrees triplets of helium nuclei are fused into carbon. The “Helium Flash” may result in... Answer:  More Push on the outer layers.

During the Helium Flash, or soon thereafter, the great push on the outer layers throws them away from the core.

This separation is the onset of a planetary nebula. 

Slide show: 

Some planetary nebulae. (Name because of a French astronomer looking for planets in the late 18th century).

Note: In a FEW THOUSANDS OF YEARS outer layers move ~thousand A.U. outwards.

Planetary nebulae shine because the core - now a separate star - illuminates the gas that than shines like the 
discharge arcs in the lab. 

Late last year it has been suggested that the assymetrical shapes of planetary nebulae may be the result of planets inside some of them. A Jupiter-sized planets "churns - up" the star, and spins up its outer layers, creating a donut of gas that makes it aspherical. (There are other explanations, though).