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# Explain why a star is brightest when it's coolest? Watch

1. So I understand the question is talking about a red/white dwarf, but I only understand half the concept.

So red dwarfs are massive, hence have a massive surface area. This means the temperature is spread out over that area, and so it will be cooler.

But why would it be brightest? Is it because it's letting out lots of energy (due to heat loss due to massive surface area)?
2. Surly you mean red giant? If so, the reason the energy production is increased is that the star has started to fuse helium nuclei, which give out more energy, and I'm pretty sure the nuclei fuse at a similar rate, so the star will be hotter overall, due to the fact that helium nuclei need more energy to be able to fuse, so even though the surface area increases, the star actually gets HOTTER as it gets larger. Correct me if I'm wrong, we don't realy get taught much on this topic at GCSE, heres hoping we do more cool stuff (astrophysics and nuclear physics) at A-level rather than all this boring force based physics we have to do at GCSE
Hope I helped!
3. (Original post by eggfriedrice)
So I understand the question is talking about a red/white dwarf, but I only understand half the concept.

So red dwarfs are massive, hence have a massive surface area. This means the temperature is spread out over that area, and so it will be cooler.

But why would it be brightest? Is it because it's letting out lots of energy (due to heat loss due to massive surface area)?
Red dwarfs are small stars (usually smaller then the sun).. I assume you mean red supergiant?
If so, then..
Brightness of a star, it's luminosity, is the total amount of energy given off by it.

Using the stefan-boltzmann law which states the power output from a black body radiator (e.g. Star) is proportional to its surface area and the fourth power of its temperature. We can conclude that larger surface area = more energy given off per second thus a higher luminosity and brightness.
TL;DR. Yes you're right it's because it's releasing lots of energy because of its size.
4. (Original post by eggfriedrice)
So I understand the question is talking about a red/white dwarf, but I only understand half the concept.

So red dwarfs are massive, hence have a massive surface area. This means the temperature is spread out over that area, and so it will be cooler.

But why would it be brightest? Is it because it's letting out lots of energy (due to heat loss due to massive surface area)?
Yeah, I agree largely with what you have said, although like Devilhenge said, you mean a red giant.

The core will not be cooler, it will be hotter due to the reactions of helium fusion into carbon and oxygen (which are heavier and denser so increasing the gravitational field strength), in addition to hydrogen fusion as well, but like you say, the radius of the star will expand with greater thermal radiation as the pressure at the core is increasing.

This means that there will be a greater power output, and since intensity is power per unit area, the increase in power will mean an increase in intensity. You could consider brightness as intensity, and therefore say that the star is bright.

I think that's right, but my brains kind of had it for today, plus I haven't really taken into account the increased area (due to radius increasing) and that intensity follows an inverse square law. But otherwise this answer might help (based on what I can remember from my A2 physics book).

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5. stefans law relates absolute luminosity to a stars temperature and radius.

Probably you should post the actual question you're looking at.

Ps don't use massive to mean large on physics papers unless you're talking about something with a lot of mass, don't say massive area etc.

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