Trouble understanding stars

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NebularNeblactics

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ok..ive always loved space and astronomy but i never really got into it like i have wanted. i used to read books about the planets wen i was in elementary school. anyway, the point is that i am very noobish so please dont bash me to brutally :lol:

My question with stars is...wut exactly are they? I mean i guess i know they are huge fireballs floating around...but do they have rock cores underneath all the fire? or would the core look exactly like wut u would see on the outside? Is it even fire that they are made of? i believe they are comprised mostly of certain elements and the elements are reacting in sum way to produce the energy...Is it gravity that sustains these reactions and causes them to occur?

And another thing i wonder is about wen they die. Supposedly they run out of "fuel" and explode...What is their fuel that keeps them "alive". They must be unfathomly massive in order to contain enough fuel, or chemical reactions, to sustain their lives for billions of years...or is the fuel recyclable within the star itself?

Also, I cant understand how they turn into black holes...If a star ejects all its energy and mass..then how does it have such a strong gravitational force that it collapses upon itself...and wut is left to collapse if the material is ejected? Now sum stars dont turn into black holes...i believe they can become brown dwarves or neutron stars...Wut exactly is this? does the star still shine? or is it a rock core just floating around like a planet??


Ok i think i have ranted long enough with my noobish questions and reasoning ;) Any help in understanding would be greatly appreciated thank you :)
 
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SpaceTas

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Start reading again ! It helps a lot getting the whole story together rather than tring to piece it together from the internet. Other's can suggest good intro's to stars.

I like "100 billion Suns" forgot author, fairly old. Also the textbooks by Fraknoi. Also keep an eye out for astronomy clubs, or adult education classes.

But here's is a very quick start and only to your first question: What are stars?

Stars are almost all hydrogen, the lightest element. About 1 in 16 atoms in a star is Helium. In addition there is 1 in 100 or few of something else like Carbon Nitrogen or Oxygen. The mass of stars is huge; millions times that of the Earth, and they are large compared to the Earth (The sun is 300 times diameter of Earth; million times the volume). The surface temperatures run from 3000 deg C to 20,000 deg C. So even on the surface they are very hot. So hot that nothing can exist as a solid. So stars are composed of gas. In the center they are a lot hotter, and the density is very high. This is because of the pressure of the rest of the star. Only in a small (10% of the star) central region is the temperature/pressure high enough so that nuclear fusion of hydrogen into helium can occur. Basically 4 hydrogen's are combined to form 1 Helium. One helium has slightly less mass than 4 hydrogen's. The difference in mass is converted to energy. This is a huge amount of energy which then moves slowly up through the star; heating the star and helping support it's mass and eventually reaching the surface giving us starlight, (sunshine). No other source of energy is large enough to power the stars (sun) for long enough at the power output observed. Even if the sun were composed purely of coal (which it is not) it would burn for only 200 years, let alone the 4,500,000,000 years that the Earth has been warmed by the Sun.

1 line summary: stars are very hot, massive, large, balls of gas, that produce their energy by nuclear fusion.
 
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NebularNeblactics

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thanks for taking the time to reply! and recommending sum literature, i will definately look into it! Is it gravity that causes the hydrogen atoms to form with helium, or is the heat? I think it is amazing how a huge ball of gas can be floating out there being so chemically active for such a long period of time..If you think about it, there is so much helium and hydrogen in the universe that its absolutely mind boggling! Im trying to learn as much as I can and you definately gave me a push in the right direction! i appreciate it! :)
 
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MeteorWayne

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Well, it's both heat and pressure that alloows fusion to occur.

And it's not "chemically" active; fusion is a whole other type of process than normal chemical reactions.
 
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Saiph

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Very good explaination SpaceTas, I also recommend purchasing an actual introduction to astronomy textbook. If you drop by a college bookstore you can usually snag a used copy for a decent price. If your a bit resourceful, turn up right after finals, when all the students are selling their books back. Offer them a bit more than the store is, and you can get an even better deal. I recommend Micheal A. Seeds "Intro to astronomy" it covers the basics in a very clear reader friendly fashion. I actually just read it like it was a book.

I'll help with the second question, star death. The fuel is, as SpaceTas mentioned, the very hydrogen the star is made of. However a star only burns the hydrogen in the inner core, about 10%. There is very little mixing with the rest of the star. When the hydrogen in the core is all turned into helium, smaller stars die right there. They aren't big enough to put the core under enough pressure to fuse helium into carbon. The hydrogen fule is not gone, vanished as energy radiated away. Well, some is (that's how fusion works) but 99% of the mass is still there, analogous to 'ash' of a normal fire, existing still in the heavier elements created during fusion.

So in a sense there is, and there is not, recycling. The star only burns what starts in the core. We know of no natural mechanism to introduce more fuel. However the ashes from one set of fusion can, if the star is large enough, be used in another stage. Large stars burn hydrogen into helium, then the helium into carbon, then the carbon into Iron.

When a stars core fills with atoms it can't fuse, cause it's not big enough, the fusion reaction stops. In small stars the final stages cause the star to swell, and shed the outer layers. This exposes the hot core, and we have a white dwarf. A super hot, but very dense mass of material that's still to light to have fusion. It's essentially a hot rock that's ~10% the mass of the original living star.

In large stars the core is to massive for that, and end up contracting under their own weight while the rest of the star still surrounds them...and shrinks fast. It ends up as a neutron star, or if it's really big, as a black hole.

The rest of the star is still essentially supported by the core when it suddenly shrinks, and It's like pulling the chair out from under someone trying to sit down..the upper parts of the star come rushing in towards where the core used to be. When they get there, they hit all the other in rushing gas...and the pressure is enough to start fusion over again.

This time though, it isn't a controlled, continuous fusion reaction. It's more like a bomb, and the outer regions explode outwards in a massive explosion, a Supernova.
 
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NebularNeblactics

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oh wow. im starting to understand how all this works now. thanks for the detailed and clear explanation saiph. Stars are very fascinating things indeed. They are almost like living things with all the stages that they go through. The picture is becoming clear :D I'm definately going to check out the book you recommended so that i can further reasearch and check out more of this forum aswell!...You guys are very wise in the ways of the universe and hopefully i will be on your level one day!
 
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