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Neutron star temperature

  1. May 16, 2015 #1
    Dear PF Forum,
    I've been searching the answer for this particular question about neutron star in google, but I don't find it, yet.
    1. What is the temperature of a neutron star, right after it is formed from supernova?
    2. Can anybody give me some timeline about neutron star cooling down?
    3. What is the fate of a neutron star? Will the neutron star become a black star after million of year?
    4. Does neutron star still produce its own energy? If yes, where does this come from? Is it gravitaty?
    5. Does brown dwarf still produce energy? Is it Gravity?

  2. jcsd
  3. May 16, 2015 #2


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    1. Around 1012 kelvin, and it falls to around 1011 kelvin within a few seconds. See here: http://www.astro.umd.edu/~miller/nstar.html

    2. The temperature drops rapidly at first, as the enormous thermal energy of the particles creates neutrinos, which carry away lots of heat. Heat loss slows over time, and within about 1,000 years the surface temperature has fallen to a few million kelvin. As the temperature continues to fall, the heat loss slows enormously. In the vacuum of space, heat can only be lost by thermal radiation (unless the temperature is high enough to create neutrinos) and the amount of thermal radiation produced depends on the temperature of the object and its surface area. A neutron star has an extremely small surface area for the amount of thermal energy it has trapped inside it. This means it will take a very, very long to cool down, billions and billions of years in fact (assuming they act similar to white dwarfs, which behave in this manner for the same reasons I mentioned).

    3. I'm not sure what a 'black star' is, but neutron stars should continue to shine billions of years. As they cool, they will radiate less energy and at longer wavelengths, eventually becoming cold, non-radiating objects.

    4. I wouldn't say energy is 'produced'. The collapse process itself converts a great amount of gravitational potential energy into kinetic energy, and then into thermal energy during the formation of the neutron star. After this, the neutron star effectively acts like a large heat sink, radiating away the thermal energy it has trapped inside it. Infalling matter will release a great amount of energy as well. Importantly, a neutron star has no process to convert any type of 'fuel' into energy like stars do in nuclear fusion. The energy it has when it is created is all it will ever have unless more material falls onto it or it merges with another object.

    5. Brown dwarfs do release energy through thermal radiation, as the formation process that creates the brown dwarf (or any stellar object) converts the gravitational potential energy of the collapsing gas and dust into thermal energy. Unlike more massive stars, brown dwarfs never get hot enough for fusion, though.
  4. May 16, 2015 #3
    To Drakith,
    Thank you, thank you.
    I've been searching this answer for six years. Those links only told me, silicon burning, CNO cycle, Pauli Principle, Chandrasekhar, etc..., not a damn thing about temperature! And its fate.
    Furthermore, "black star" is not an astronomy term, what I mean is the star that's not yellow, white, red. It's black because it can't produce light anymore. But it's not a black hole either. Still has escape velocity below speed of light.

    And as for question number 5.
    Brown dwarf, I think, belongs to planet category rather than star.
    But, it's really a typo. What I mean is white dwarf. Do they still produce energy?
    And can you please tell me how long does it take (theoretically) for a white dwarf to become black dwarf? Websites only told me,
    But not the time it takes.
    Thanks for any answer.
    Last edited: May 16, 2015
  5. May 16, 2015 #4


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    I would say they exist in a category that straddles both planet and star. It's hard to call something with 70 times the mass of Jupiter a planet, but hey, if it wants to be a planet, who am I to say no?

    They do not. White dwarfs have run out of fusible fuel and are composed of mainly carbon and oxygen, with higher mass white dwarfs being composed of neon and magnesium. Like neutron stars, they are radiating away heat trapped inside.

    Since the rate at which the white dwarf cools scales with its surface temperature, the cooler it becomes, the slower it cools. So a white dwarf will not reach equilibrium with the background radiation for several trillion years. But the vast majority of this time is spent at very low temperatures. It only takes about 10-20 billion years for a white dwarf to cool from 30,000k to under 4,000k, at which point it is beginning to radiate mainly in the infrared.
  6. May 16, 2015 #5
    Ahh, you again Drakith. Thank you so much.
    Still 1 more question?
    1. Can a white dwarf made from silicon?
    2. What is CNO cycle? Is it Carbon - Nitrogen Oxygen, or Cargon - Oxygen - Neon? Or Alpha Process doesn't allow Nitrogen (7 proton)?
    3. If it's Carbon - Oxygen - Neon, where does Nitrogen come from? Supernova nucleosynthesis?
    4. If anybody not too busy, can anyone tell me what elements lower than iron are produced during stellar nucleosynthesis? BEFORE supernova, I mean.

    Thanks :smile:
  7. May 16, 2015 #6
    Sorry, it's 4 question actually :smile:
  8. May 16, 2015 #7
    Sorry, wrong question:
    If anybody not too busy, can anyone tell me what elements lower than iron are produced during SUPERNOVA nucleosynthesis? AFTER supernova, I mean.
  9. May 16, 2015 #8


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    1. Not sure. I've never read of this being possible, so I assume no.
    2. http://en.wikipedia.org/wiki/CNO_cycle
    3. The CNO cycle uses nitrogen, which, I think, is produced directly in the core by fusion of hydrogen with carbon-12 as part of the CNO cycle. This requires that carbon be present in the star at its birth, and this carbon comes from either supernovas or the material sloughed off by massive stars during certain parts of their lives.
    4. See here: http://en.wikipedia.org/wiki/Stellar_nucleosynthesis (specifically the chart just above the diagram of the proton-proton chain reaction)

    If you want to get into the nitty-gritty details of any of this, please start a new thread on the subject.
  10. May 16, 2015 #9


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    The CNO cycle is present only in large stars which are well into their life spans. A star does not start out its life with a large amount of carbon available with which to initiate fusion via the CNO.


    All stars begin their lives fusing hydrogen into helium, using the proton-proton reactions. This requires a certain temperature in the stellar core to start this process, at least 4 million K:


    As the hydrogen is turned into helium and exhausted from the core, the core contracts, further raising its temperature. When core temperatures reach about 100 million K, then the triple-alpha fusion process takes over, turning three helium nuclei into one carbon-12 nucleus:


    That's where the carbon comes from to fuel the CNO cycle in a stellar core.

    Although the sun's core is hot enough to permit some CNO reactions to occur, the mass of the sun is too small for this process to produce a significant amount of energy. Eventually, via the triple alpha process, after the sun has turned into a red giant star with a helium core, the helium will be converted into carbon. Once the core is turned into carbon, there is not enough mass left to start any additional fusion reactions to make heavier elements, but the core is prevented from shrinking further by electron degeneracy pressure.

    Larger stars progress from the CNO cycle, as they age, into burning the carbon into oxygen, then into neon, silicon, and finally, iron. Each stage takes exponentially less time to complete, so much so, that the conversion of a silicon stellar core into iron can take only a few hours, maybe a day at the most.


    Page 34 of the document above shows the evolution of a 25-solar-mass star in Table 22-1. Start to finish, from when hydrogen fusion starts to when the core collapses, causing a supernova, the whole process takes less than 8 million years.

    Once the core is turned into iron, the star has reached the end of its life, as there is no longer enough energy produced to overcome gravity and prevent the iron core from collapsing. That's when the core collapses into a neutron star and the rest of the stellar envelope is blown away in a supernova explosion.
  11. May 16, 2015 #10
    Thanks Drakith for your invaluable answer.
    Btw, SteamKing it was a wrong question that I asked above.
    What I want to ask is this:
    Is there any elements lower than iron that produced during supernova nucleosynthesis, not stellar nucleosynthesis.

  12. May 17, 2015 #11


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    I think that pretty much all of the elements with atomic numbers less than 26 (which is the atomic number of iron) have been synthesized to some extent during the life of the star up to the point at which the core collapses. The core is still surrounded by an envelope which contains all of these lighter elements, up to and including hydrogen. The envelope is that portion of the star surrounding the core where no fusion reactions take place.

    Stars with different masses create different proportions of elements lighter than iron in their cores. This article on supernovas has an illustration showing the layers of different lighter elements in a star about to suffer a core collapse after iron has formed at the center:


    Some relatively light stars, those with masses between 8 and 10 times that of the sun, become unstable before there is much iron present in the core. These stars have a mixture of oxygen, neon, and magnesium which can collapse because the core cannot generate enough energy to support itself against gravity. A neutron star usually results. Heavier stars than this can either collapse into neutron stars or black holes. There is a range in which the collapse is so violent, no remnant remains.

    In any event, the passage of the shock wave from the sudden core collapse and the generation of large quantities of neutrons lead to fusion of elements heavier than iron in the outer layers of the star:


    In addition to the creation of a large neutron flux, massive quantities of neutrinos are also created during the collapse. Neutrinos only rarely interact with matter, and because of this, they are able to travel directly from the collapsing core into space ahead of the shock wave from the core. On their way out of the star, the flux of neutrinos is so intense and so massive, some of them do interact with the lighter elements in the exploding star.

    From recent studies of the supernova collapse process, it seems that neutrino reactions are responsible for creating certain isotopes of lithium, boron, and beryllium found in the remains of the dead star:


    http://www.ipmu.jp/webfm_send/376 [Broken]
    Last edited by a moderator: May 7, 2017
  13. May 17, 2015 #12
    Sorry, I forgot to add that line in my previous question. I remembered to type that line, but somehow my fingers just typed the question :smile:
    And thanks again SteamKing for your detailed answer.
  14. May 17, 2015 #13
    Thanks for your explanation SteamKing.
    Wow, so lighter elements are really synthesized in supernova. And it's interesting that neutrinos are responsible for this instead of baryon type.
    And such interesting links you give me. I bookmarked them.
    Btw, one more question.
    Can neutrino escape black hole? I have read years ago that neutrino doesn't have mass, in 1980 if I'm not mistaken. And once again in Dan Brown novel, Angel and Demon. What the recent studies say? Do neutrinos have mass?
    If yes (it must be very tiny), can they escape black hole?
    If no, can they still escape black hole? Any proof that neutrinos escape black hole?

    Thanks for any idea
    Last edited: May 17, 2015
  15. May 17, 2015 #14


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    Neutrinos are only responsible for some of the elements. Most of the elements produced in a supernova come from fusion and neutron capture.

    It cannot. Nothing can escape a black hole.

    Since the discovery of neutrino flavor oscillation, they are believed to have a very small but non-zero mass.
  16. May 17, 2015 #15
    Do you have any links to neutron star cooling curves past 1 million years and below 100 000 K?
  17. May 17, 2015 #16
    Last edited by a moderator: May 7, 2017
  18. May 17, 2015 #17
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  19. May 17, 2015 #18


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    This is mostly a guess, but I'd bet you could treat low-temperature neutron stars like very small white dwarfs. I'd expect them to take even longer to cool down than white dwarfs do due to their much smaller surface area and much larger mass.
  20. Sep 25, 2015 #19
    If something gets pulled towards something, It need not have mass. Like photons gets bend behind significant gravity like black hole. So neutrinos WILL get pulled in beyond the event horizon.Besides photons speed is c while neutrinos speed is <c.
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