The stellar evolution. Low Mass stars

[ehabmozart]

Well, this topic is driving me crazy so any answers or replies should preferably be detailed and in an easy way.. My question is i want a sum up of the evolution of a low mass star... I will give my answer and tell me where are the dropouts.. Let me begin,With a protostar accreting hydrogen in space to somehow the size of sun, the star is said to be born. While it keeps accreting the teperature rises and g increases.. My question here is why temperature increasing before it has even started fusion. OK.. so once into fusion of hydrogen i know that the helium produced is more dense and will occupy less space. Over time this leads to the core contraction, and the g and pressure values increase enourmously. When hydrogen is over in the core and everything is filled with helium, it is said in my book that the hydrogen envelope will fuse much faster with the greater pressure leading to the star expansion.. Is it true? If not, what leads to the expansion of stars... I mean, i read in many sites that "This happens because the shell of hydrogen follows the core of helium as it collapses inwards. The shell of burning hydrogen then increases the energy output of the star, and thus its luminosity and size. " This was non sense for me.. I mean i need a detailed expanation of this... Anyway, the sun is said to be a red giant at that time. Are red giants actually having a higher temperature because of the more fusion or a low one due to its big volume.. What about its luminosity...Finally, after the red giant phase ends, what happens. Does it contract and we call it a panatery nebula.. I mean, when does the fusion of helium begins.. During the red giant phase... I guess that's all what i need... Am really sorry for elongationg but i am really keen cs am having a really really tough exams within days.. So whoever reads it, it is urgent and i need the answers ASAP... Detailed answers :) I really thank in advance from the bottom of my heart whoever replies and dominates... Thanks a lot!

 

[Drakkith]

While it keeps accreting the teperature rises and g increases.. My question here is why temperature increasing before it has even started fusion.

It is a result of the collapse of the gas itself. When you compress a gas it increases in temperature. The collapse occurs faster than energy can be released through radiation, and the gas heats further and further.

Answers for everything else can be found here: http://en.wikipedia.org/wiki/Red_giant

 

[ehabmozart]

THanks .. i went through that link before.. I need replies from people.. I mean the topic in the link is not satisfying me.. Thanks again for contributing

 

[mfb]

Are red giants actually having a higher temperature because of the more fusion or a low one due to its big volume

Fusion occurs in the central part, which can be hot, dense and produce a lot of heat. This reduces the density of the outer parts, and the temperature of the surface (-> red color), even with a higher total luminosity.

 

[Drakkith]

THanks .. i went through that link before.. I need replies from people.. I mean the topic in the link is not satisfying me.. Thanks again for contributing

I wrote a pretty good reply before realizing I was just saying the same thing the article was. Perhaps you could point out where the article stops helping you?

 

[Drakkith]

When hydrogen is over in the core and everything is filled with helium, it is said in my book that the hydrogen envelope will fuse much faster with the greater pressure leading to the star expansion.. Is it true? If not, what leads to the expansion of stars... I mean, i read in many sites that "This happens because the shell of hydrogen follows the core of helium as it collapses inwards. The shell of burning hydrogen then increases the energy output of the star, and thus its luminosity and size. " This was non sense for me.. I mean i need a detailed expanation of this...

Once the core stops fusing hydrogen the star begins to collapse, drastically increasing the temperature inside the star. Once the area of the star right outside the core reaches a high enough temperature it too starts to fuse hydrogen. I believe the shell is driven to a much higher temperature than the hydrogen burning core was, leading to a much higher energy output that puffs up the stars outer layers.