Why Are Red Giants More Luminous Than Other Stars?

[ehabmozart]

Well, I've seen in almost all sites and books that Red Giants are more luminous than the star itself... Particularly, let us take our sun as an example... First of all, we know that due to the He- Fusion and the enormous pressure acting on the H- on the surface, it IGNITES or fuses faster... Thus, the energy is vast causing it's Surface area to increase... This is the main cause of y red giants appear more luminous.. However, to my point of view .. L= σ A T^4 ... The temperature will decrease by this increase in volume and the area is increasing.. If we take each by scale. Temperature should rule the way the red giant luminates.. If there is an increase in area, temperature will decrease with the scale of FOUR... Shouldn't it be less luminous ... I need clarification in this part... Thanks in advance to whoever helps!

 

[Drakkith]

I believe the luminosity of the star is a measure of the total energy output, regardless of where in the spectrum it is at. As the surface area increases the same amount of energy must still be released, meaning the star cools off and releases it's energy accordingly. So no matter how big or small the star gets, as long as the reactions inside the star release the same amount of energy then the star stays the same luminosity regardless of it's physical size.

 

[mfb]

If we take each by scale.

This is not the case for red giants. While the temperature might change by a factor of ~2, the radius can increase by a factor of ~100. I did not check the numbers, but they should give you an idea. This could give a factor of 100^2 * (1/2)^4 = ~500 more luminosity.

Edit: Wikipedia gives a factor of ~200 for the sun. And a temperature drop by more than a factor of 10 would make it invisible, therefore the sun as a red giant will have a higher total luminosity.

 

[qraal]

Well, I've seen in almost all sites and books that Red Giants are more luminous than the star itself... Particularly, let us take our sun as an example... First of all, we know that due to the He- Fusion and the enormous pressure acting on the H- on the surface, it IGNITES or fuses faster... Thus, the energy is vast causing it's Surface area to increase... This is the main cause of y red giants appear more luminous.. However, to my point of view .. L= σ A T^4 ... The temperature will decrease by this increase in volume and the area is increasing.. If we take each by scale. Temperature should rule the way the red giant luminates.. If there is an increase in area, temperature will decrease with the scale of FOUR... Shouldn't it be less luminous ... I need clarification in this part... Thanks in advance to whoever helps!

A Red Giant is a bit like a very hot white dwarf (the hydrogen-depleted core) surrounded by a shell of furiously fusing hydrogen, and then a very large, diffuse atmosphere which down-shifts the ultra-high frequencies emitted from the shell-core into a rosy glow. So, in a sense, the core has brightened intensely, but the rest of the Sun dilutes that fierce glow from below.

Read some good papers on stellar evolution and you'll have a better idea of the physics involved. The classics are the paper by Boothroyd & Sackmann (+Kraemer) and the later paper by Schroeder & Connon Smith...

Boothroyd, Sackmann & Kraemer Our Sun. III. Present and Future

Schroeder & Connon Smith Distant future of the Sun and Earth revisited

Plus there's the work by Laughlin, Adams & Bodenheimer on low-mass stars:
The End of the Main Sequence

Explore these and you'll know a lot more.

 

[qraal]

One more reference with a very simple explanation (ok, so there's a bit of maths) is this paper by Adams, Laughlin & Graves...

Red Dwarfs and the End of the Main Sequence

...which explains how the Red Giant inflation is driven by opacity. "Opacity", very roughly, is how hard energy escapes from a star's substance, and can very quite significantly through the body of the star.