Why Are Giant Stars So Low in Density?

[triabva2003]

Why giant stars are so low in density?

 

[DaveC426913]

Read up on stellar evolution of red giants here.
http://en.wikipedia.org/wiki/Red_giant#Stellar_evolution
It explains the process.

 

[Radrook]

Why giant stars are so low in density?

By the time they become giants they have burnt up a lot of their fuel and their outer layers have bloated due to less gravity. It doesn't mean that their cores aren't very dense though. It just means their average density is lower than main sequence stars which are still fusing hydrogen and have enough gravity to keep their outer layers compact..

 

[Ken G]

Here's one way you can think about it. Main-sequence stars are considered dwarfs, so giant stars are giant in relation to main-sequence stars. Main-sequence stars have an internal structure that is sort of "all one thing"--they inject heat essentially at the center and the whole rest of the star lives under the conditions of needing to pass outward that heat. So they have a simple structure that can be simply described by the mass of the star, and the size is controlled by the average gravitational potential energy. But a giant star has fusion occurring in shells that are around the outside of the core, so the entire outside of the star is passing outward energy that the core is not. This means giant stars are basically not "all one thing", they are fundamentally made of a core plus an envelope, which are carrying different amounts of energy and behave differently. What happens is the need for the outer region (the "envelope") to carry more energy causes it to puff out to much lower densities than it would have otherwise, and this is what makes the star attain a "giant" radius.

 

[Radrook]

In harmony with what has already been said above.
Here is the sequence that leads a sun-like star to the Red Giant stage.


1. Star fuses Hydrogen to helium in its core

2, Hydrogen into Helium fusion in Core ceases

3. Star compresses

4.. Helium into oxygen and carbon fusion begins in core

5. A layer of Hydrogen that surrounds the core begins fusing Hydrogen to Helium

6. Both types, core and shell fusion combine and cause the star to grow enormous as their combined outward thrust overpowers gravity.

7. Helium fusion at the core stops-outward pressure decreases

8. Gravity causes red giant star to collapse and shrink

9. Insufficient mass to fuse oxygen and Carbon = inert fusionless core

10. Helium shell around inert core begins fusion causing utward pressure and restoring

the star to red giant size.

11. Insufficient gravity cannot stop the outward 100,000 mpr expansion. Star lses uter layers leaving a fusionless core behind.

12. That inert, fusionless core is called a white dwarf.


White Dwarf
http://www.eg.bucknell.edu/physics/astronomy/as102-spr00/web_pages/web8.html

 

[Chronos]

By the time they become giants they have burnt up a lot of their fuel and their outer layers have bloated due to less gravity. It doesn't mean that their cores aren't very dense though. It just means their average density is lower than main sequence stars which are still fusing hydrogen and have enough gravity to keep their outer layers compact..

It has nothing to do with less gravity. The core becomes super heated in ancient stars - which means more pressure on the outer layers. Stars are in a continuos state of hydrostatic equilibrium - until the core temperature becomes too hot or cool to remain in gravitational balance.

 

[Radrook]

It has nothing to do with less gravity. The core becomes super heated in ancient stars - which means more pressure on the outer layers. Stars are in a continuos state of hydrostatic equilibrium - until the core temperature becomes too hot or cool to remain in gravitational balance.

No that isn't the way it works throughout the whole process. Neither does the expansion depend only on a superheated core. As explained in my second post, the shells surrounding the core contribute significantly by becoming superheated by pressure and NOT by core fusion itself.

At one point, both core and shell are fusing different elements and contributing to the sustenance of the Red Giant stage.

However, the last stage has the core contributing NOTHING. The helium shell alone then becomes responsible for the runaway expansion in which the star loses all its outer layer leaving behind a white dwarf.

It can't be caused by a superheated core at that stage because the core by that time isn't fusing anything.

Here is the sequence that leads a sun-like star to Red Giant stage.


1. Star fuses Hydrogen to helium in its core

2, Hydrogen into Helium fusion in Core ceases

3. Star compresses

4.. Helium into oxygen and carbon fusion begins in core

5. A layer of Hydrogen that surrounds the core now begins fusing Hydrogen to Helium




6. Both types, core and shell fusion combine and cause the star to grow enormous as their combined outward thrust overpowers gravity.


==========================================

7. Helium fusion at the core stops-outward pressure decreases

8. Gravity causes red giant star to collapse and shrink

9. Insufficient mass to fuse oxygen and Carbon into heavier element = inert fusionless core
=========================================================

10. Helium shell around inert core begins fusion causing outward pressure and restoring

the star to red giant size.

11. There is insufficient gravity to stop the outward 100,000 mph expansion. Star loses outer layers leaving a fusionless core behind.

12. That inert fusionless core is called a white dwarf.


The core's gravity is insufficient to prevent that last stage leading to what is called a planetary nebula. Surrounded by this nebula is the star's former inert core now called a white dwarf star. Such a star is prevented from further collapse by electron resistance to additional gravitational compression. More massive stars that can overcome this resistance wind up as neutron stars or black holes.


White Dwarf
http://www.eg.bucknell.edu/physics/astronomy/as102-spr00/web_pages/web8.html