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Life & Death of a Star - Confused

  1. Jul 22, 2009 #1
    Hi everyone,

    Firstly i want to say that i've watched some of the "The Universe" episodes on my computer, and i pretty much have basic knowledge about the cosmos. But there's only one thing in particular that i'm confused: the life of a star. I pretty much know the "core" of it, but i can't make any connections between events that happen after or before another event in the life of a star.

    First of all, i know how a star its formed (if i make a mistake anywhere, feel free to correct me :) ) : from dust and gas clouds formed from another star's death, with the help of gravity (and as the matter gets more denser, it starts to heat up). After the star is born, it'll live like this untill it uses all of its hydrogen (by fusing it into helium). With less and less hydrogen the star's beginning to grow in size (more pressure (due to what?, pls clarify here) overcomes gravity), after a critical point the stars violently blows up (does it explode or implode? or both...because it's shrinking in size "while blowing up" and it might look like an implosion too).... now here's the big BIG confusion:

    WHAT happens next? Does it collapse into a black hole? Or the original core of the star remains, known as a white dwarf? What about pulsars/magnetars ...i've heard that they form after a supernova...

    Let's say it transforms into a white dwarf... what is burning now? Helium? I know that some stars after consuming all of their hydrogen, start to convert whatever they have (like helium into the next element in the periodic table and so on... UNTILL iron (where it cannot be fused into anything else))...but when this faze kicks in? After the supernova (with the white dwarf) or immediately before the supernova?

    What's next after the white dwarf faze? The brown dwarf? Those ugly brown stars which emit very little light compared to a normal star (but they live thousands of times more than our current universe age).

    Bonus question: can we find a "spot" in space where the temperature reading is exactly 0 K? (the COMPLETE absence of heat) like in intergalactic space ( 50.000 light years apart from any galaxy)...
  2. jcsd
  3. Jul 22, 2009 #2


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    This is strongly dependent upon the star's mass. Very basically, small stars like our Sun end their lives by inflating into a red giant for a time, then as the remaining hydrogen fuel depletes it slowly shrinks to become a white dwarf. Stars much more massive than the sun, on the order of about 3 times larger, go nova and end up as neutron stars. Very massive stars, some 10 solar masses and up go nova and wind up as black holes.
  4. Jul 22, 2009 #3


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    Welcome to pf, rather than retype a lot of stuff you shoul take a look at http://en.wikipedia.org/wiki/Stellar_evolution and come back with any questions.

    About the last point, apart from the problem of defining what temperature means in empty space, nothing can cool naturally to below about 3kelvin because this is the temperature of the microwave background
  5. Jul 23, 2009 #4


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    The basic idea is that if you have a clump of gas that is dense enough compared to its surroundings, it will tend to collapse in on itself. As it collapses, it gets hotter. If it weren't for nuclear physics and quantum mechanics, this process would continue forever, as the star would just get denser and denser and hotter and hotter.

    But what happens first is that if it has enough mass, it gets hot and dense enough that a nuclear reaction starts at its core. How big this nuclear reaction is, how long it lasts, and how many elements it is able to fuse depend upon how massive the star is. Generally, more massive stars can make heavier elements. In any case, the infusion of heat from the nuclear process halts the star's collapse.

    Now, eventually the star will expend all of its nuclear fuel, and won't be able to make the next more massive element on the list. So the process of collapse continues again: with no infusion of heat, it just gets smaller and smaller and hotter and hotter. Eventually quantum mechanics steps in.

    What halts the collapse is the Pauli exclusion principle: no two electrons can be in the same state at the same time. This means that once the star has collapsed far enough, the electrons are so close together that if they were any closer, some would have to occupy the same state, which can't happen. Thus the collapse halts, and the star just cools off. This is a white dwarf.

    Now, the amount of pressure that these electrons can exert (this is called electron degeneracy pressure) is finite. Eventually the pressure will be so great that the electrons start joining with protons to produce neutrons and neutrinos. The neutrinos escape, but the neutrons stick around, making a neutron star. Just like with the electrons, no two neutrons can be in the same state at the same time, but they can support a larger pressure, and thus more mass. So more massive stars than white dwarfs become neutron stars.

    But again, the amount of pressure they can support remains finite. If the star is too massive, it makes a black hole instead.

    Note that every time the star collapses, that collapse is accompanied by a release of energy. This comes from the loss of gravitational potential energy: by concentrating much more of the mass closer together, there is less gravitational potential energy. For the transition to a neutron star or a black hole, the difference in energy is truly phenomenal, as is the release of energy. Which is why we get such events as supernovas and hypernovas.
  6. Jul 23, 2009 #5


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    There is a tricky little feature of the universe called 'Jeans mass'. It inhibits the formation of supermassive stars. See, for example:
    The Formation of the First Star in the Universe
  7. Jul 24, 2009 #6
    Thanks guys for the helpful replies.

    I still have one more question, regarding helium (and elements heavier than helium untill iron) being fused into heavier elements. Why is it fusing the atoms even further? And not stop in huge nova. Do all the stars behave like this?(fusing helium atoms into heavier elements)(explaining for dummies required ^^)
  8. Jul 24, 2009 #7


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    Normal stars fuse elements up to and including iron. After that, they aren't capable of fusing heavier elements because the energy required to make them is greater than the energy released. All heavier elements are formed in the cores of supernovas, which have the extra energy to form those heavy elements.
  9. Jul 25, 2009 #8


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    It's not that stars are not capable of fusing elements past iron. They certainly are, it just doesn't help them in their quest to halt gravitational collapse, so no appreciable amounts ever have time to get formed.

    The reason a supernova can form large quantities is because, true, that they have a lot of extra energy. But also, there is no impending doom of gravitational collapse to cut the process short.
  10. Jul 25, 2009 #9


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    I don't think that's true. The generation of heavy elements within stars is a thermal process. As such, they can't really produce elements heavier than iron except because they would almost immediately be destroyed in the same hot thermal bath that produced them.

    The reason that they are produced in small but appreciable quantities in supernovae is because supernovae are highly non-thermal processes.
  11. Jul 25, 2009 #10
    I think AGB stars can produce elements heavier than iron via the s-process.
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