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Neutron star questions

  1. Aug 10, 2015 #1
    Dear PF Forum,
    I have question concerning neutron star and star just simply out of curiousity.
    1. Can a star reach silicon burning stage then stop. It doesn't continue to iron burning stage.
    2. Once a star reach iron burning stage, there's no stopping it to blast into supernova? Is supernova inevitable for iron burning stage?
    3. Does the density of neutron star equal to atomic nucleus?

    These ones I should have googled it, but can't find a definite answer.
    4. What is the maximum mass of a neutron star?
    5. What is the minimum mass of a neutron star?

  2. jcsd
  3. Aug 10, 2015 #2
    The accepted range for neutron star masses are between 1.4 and 3.0 Solar masses
  4. Aug 10, 2015 #3
    Once a star starts to burn iron, it's actually losing energy. Outward pressure from left over energy of nuclear fusion is what keeps the star from imploding. When you fuse hydrogen to hydrogen you get a lot of energy. Helium and helium make less energy. Once you get to iron, fusion doesn't actually provide any energy anymore, it starts drawing it away. So not only has the star run out of fuel to sustain it's outward pressure, but the remaining fuel is actually cooling the star and decreasing the pressure. This causes the collapse and explosion.
  5. Aug 10, 2015 #4
    Hydrogen to Helium, you mean?
    The pressure increase you mean?
    So, once iron burning, the fate of the star is sealed to be supernova?
  6. Aug 10, 2015 #5


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    Yes, and very quickly. Once the core starts burning silicon, these reactions play out within a matter of a few days.

    For a star with an initial mass 25 times that of the sun, silicon burning might last 5 days:

  7. Aug 10, 2015 #6
    I spoke with ambiguity, I meant hydrogen to hydrogen to mean H + H = He.

    And no, the pressure decreases, the supernova isn't caused by increasing pressure in the core, it's caused by a rebound. Imagine holding a stone at the surface of water, now let it go. The rock pushes some water out of the way or splashes, but that's not important in my analogy, what's important is what happens next. You have an area where water used to be, where there is none now, so water from all directions rushes in to fill the void. As the water does this, obviously too much water rushed in at once and it's momentum provides the energy to push a column of water straight up.

    That's analogous to the supernova. The core cools while it burns iron, causing it to contract. The guts of the star rush in to fill the void left by the shrinking core, bringing the weight of the star with it. All of this energy hits the core, which is compressed into pure neutron matter, which can not be compressed anymore, so the rest of the star coming down on it bounces, with extra energy created by conversion to neutron matter. That bounce rushes to the surface like in the water analogy and when it reaches the surface of the star it erupts.

    It might interest you to know that there is a time delay of about three hours from when the core actually implodes and when the surface explodes. It's measurable because the collapsing core creates both energy and neutrinos. The energy propagates through the matter, the neutrinos fly straight through it at nearly the speed of light. Neutrino telescopes detect supernovas before telescopes can.
    Last edited: Aug 10, 2015
  8. Aug 10, 2015 #7
    Can silicon burning stop because of insufficient mass?
  9. Aug 10, 2015 #8
    Iron fusion absorbs energy, right?.
    - Absorbing energy, cools the core.
    - Cool contracts the core.
    - Contracting core fuses iron quicker, and on, and on, and on, until BLAMM!!
    I think the pressure is increasing. It's the triumph of gravity force over nuclear force. If you call "iron burning" is nuclear force, because it absorb energy.
    I just want to know, in some white dwarf, the fusion stops at Hydrogen burning, sometimes helium burning, sometimes for heavier star -> Carbon burning then stop.
    I just want to know whether there is a sillicon white dwarf. Or if sillicon start to fuse, then it goes unstoppable to iron burning.
    Or whether a star can contain only iron, the fusion stops at iron and doesn't produce supernova.
    I like two marbles analogy. Hold two marbles up and down ##_0^0## drop them. When lower marble hits the ground it bounces up and hit the upper marble which goes down and bounce the upper marble higher. Clifford Johnson has a video in youtube showing this.
    Yeah about three hours. I once saw Michio Kaku (or Neil Degrasse Tyson) video, showing neutrino observatory 100 metres underground. - What sane people builds an observatorium 100 metres underground - it's said.
  10. Aug 10, 2015 #9


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    No. If the nuclear reactions stop for any reason, gravity causes the core to shrink, raising its temperature until the reactions start again. This process will continue until there is no more silicon left to burn. Once that occurs, final core collapse is imminent.

    Throughout their lives, stars operate by balancing the contraction of their mass due to gravity against the tendency of the hot gas to expand. Once the nuclear fusion at the core is not able to supply sufficient heat to keep gravity at bay, the core shrinks until either new fusion reactions can supply the necessary energy to prevent collapse, or the core collapses completely.

    Smaller stars, like the sun, do not have enough initial mass to become supernovas. Instead, their cores fuse into carbon, after which the core becomes stable enough to keep from collapsing any further, because gravity is not strong enough to overcome the forces present in the carbon nucleus.
  11. Aug 10, 2015 #10
    So the boundary is carbon? What about nitrogen oxygen, if the star produces oxygen, it will not stop until supernova?
  12. Aug 10, 2015 #11


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    There are a whole host of different nuclear reactions which occur at each stage.

    Please read the Wiki article below about supernovas, along with any other articles to which it refers.


    Core collapse supernovas are Type II.

    It's going to take more than a few minutes to read and digest all this material. It forms a good part of a course in stellar astrophysics.
  13. Aug 10, 2015 #12
    Thanks for the link SteamKing, but it's not the supernova that I want to know, it's the element that stop the fusion. Perhaps for a cetain mass, carbon or oxygen? Then there's a carbon star or an oxygen star. What is the heaviest element for a stable star. Oxygen?
  14. Aug 10, 2015 #13


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    AFAIK, white dwarf stars, like what our sun will eventually evolve into, are composed mostly of carbon. The star loses its envelope, and the carbon core eventually cools off, leaving a rather dense cinder, which is about the size of the earth, but with a mass about the same as the sun's.

    If this white dwarf star has a younger star companion orbiting close by, the stellar remnant can accrete hydrogen and lighter elements on its surface, which build up until fusion occurs. This will, at the least, create what is known as a nova (rather than a supernova), which becomes temporarily brighter than original. If enough material collects suddenly, the new fusion reactions will be strong enough to destroy the star completely. This is a Type I supernova.

    If a star is massive enough initially to fuse its core into oxygen and heavier elements, the evolutionary processes in the star will not stop until core collapse eventually occurs, resulting in a Type II supernova.
  15. Aug 10, 2015 #14
    Size like earth, mass like sun, carbon high pressured. I once read, is that really a diamond?
    Yep, Type I supernova., Standard candle?
    Now, THAT'S what I want to know. So it is oxygen.
    I didn't know what should I put my title in this question that Greg Bernhardt gave me a general warning: Wrong Title! :smile:
    With all these questionings, all I want to know is just: Oxygen.
  16. Aug 10, 2015 #15


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    No one knows for sure. As in all stars, there is a mixture of elements in the core. A white dwarf may be predominantly carbon, but other elements, like oxygen, are present. Due to the high temperature in the core and the high gravity, the core material exists in a degenerate state, not the crystalline structure which makes up a diamond.

    White dwarf stars can stay very hot for long periods of time. While the core temperature may be 10 million K, the surface temperature is generally 10,000 K and below to about 4000 K. The reason that white dwarfs are not found with cooler surface temperatures is that the universe is not old enough for these stars to have cooled below this temperature.


    I believe there is some controversy about this, due to recent findings.


    Today, Pluto is a planet. Tomorrow, it's a dwarf planet. Things scientific are subject to change.
    As I mentioned above, stellar interiors are a mixture of every element which has been made in the star, starting with hydrogen and running down the list to oxygen or whatever.

    Carbon burning sometimes leads to oxygen, sometimes to other elements like neon or magnesium. It's a complicated process. Sometimes, in the same star, different fusion paths are taken at the same time, leading to different end products.

    Stars with different initial masses take different evolutionary paths. That's why I suggest you study more and make hasty generalizations less.

  17. Aug 10, 2015 #16
    Yeah, thanks to Neil Degrasse Tyson.
    Thanks for the link.
  18. Aug 10, 2015 #17

    Ken G

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    I actually don't think the iron ever fuses in the star, not until the supernova itself. It is true that the supernova is caused by heat loss from the core, but it's not due to iron fusion, it's the opposite of that-- the temperature gets so high that iron is "photodisintegrated", which more or less undoes the heat-releasing fusion processes that led to the iron in the first place. Neutrino escape is a also a key heat-loss mechanism. But the basic idea is right-- when a core starts a runaway process of heat loss, it is doomed to collapse and supernova. A minor point: all supernovae are core collapse except type Ia (and others of the same ilk). It's not as simple as saying that type II are core collapse, type II just means there is still hydrogen in the stellar envelope when it core collapses.
  19. Aug 10, 2015 #18


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    When silicon burning takes place in the stellar core, nickel-56 (HL = 6 days) is produced. This is a radioactive isotope which decays first into cobalt-56 (HL = 77 days) and then iron-56.

    When the collapse actually occurs, the iron-56 nuclei undergo photodisintegration into free neutrons and helium-4 nuclei as the rapidly heating core creates much gamma radiation. As the density of the core continues to increase, then any free protons and electrons remaining combine by reverse beta process to create even more neutrons and neutrinos. The neutrinos escape the core, carrying energy away, which accelerates the collapse even faster.

  20. Aug 11, 2015 #19


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    Iron cannot fuse it requires more energy than it can provide. That is why elements heavier than iron can only form in supernova explosions.
  21. Aug 11, 2015 #20
    Or it fuses, but it absorbs more energy and trigger more fusion?
    The r and s process?
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