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What happens to stars smaller than ours?

  1. Jun 19, 2004 #1
    What happens to stars smaller than ours? (eg - red dwarfs) Do they swell to red giants and lose their outer layers or do they simply shrink, getting hotter as they do so to form white dwarfs?
  2. jcsd
  3. Jun 19, 2004 #2
    There used to be a number of good programs available for simulating stellar evolution theory based on selected protostar mass. They seem harder to find online now.

    link to Cornell U. stellar evolution applet for Astronomy course --->

    (don't forget to "clear" before repeating the same size star)

    Do the yellow star (closest to 1 solar mass) case, then compare to one of the red dwarfs and maybe a green/blue star (much larger than 1 solar mass).

    As the text states, red dwarfs tend to stay on the main sequence (uniform hydrogen->helium production at the stellar core) until they run out of hydrogen fuel. If the protostar mass was way too small, the protostar won't make it to the main sequence at all (it becomes a brown dwarf star or planet).

    Historically, the mass bound in a protostar has been considered the primary determinate of stellar evolution. But there are other factors involved too: initial content of elements other than hydrogen, layer mixing, instabilities (both periodic and catastrophic) and so on.

    Just for completeness: protostars "condense" inside enormous gas cloud nebulosities, the process of which is still being investigated. It seems to start with separation from the enveloping nebula followed by collapse under gravitation until pressure balance is achieved.
    Last edited: Jun 19, 2004
  4. Jun 19, 2004 #3


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    That site and simulation was interesting, but on the ones I tried, 1.5 Ms to 15 Ms, it ended with the giant stages and didn't show the expulsion (to planetary nebulae) and the move from there to the bottom and left to the white dwarf stage (for those that wouldn't supernova; up to ~ 8.0 Ms). ..(?)
  5. Jun 19, 2004 #4
    Alas! No one has worked this into a comprehensive demonstration of stellar evolution, including all the finales. At least no one has placed such a demonstration online and advertised it AFAIK. Now that would be really neat!
  6. Jun 20, 2004 #5
    So, what does happen to a red dwarf? does it run out and shrink to white dwarf, swell and lose its outer layers, etc?
  7. Jun 21, 2004 #6
    I checked with my textbook (Kartunen et al.,Fundamental Astronomy,Springer (1987). It's not so current, but things probably haven't changed that much concerning low mass stars.

    I chirped before:
    I must alter that answer a bit. There is a threshold of opacity about 26% of a solar mass.

    Below 26% of a solar mass on the main sequence ( a very red dwarf star), the entire star is opaque to the radiation generated in the core and the material is entirely convective (hotter material circulates with cooler material). Therefore, all of the hydrogen gets a chance to convert to helium in the core and the process runs very, very slowly and smoothly. As the very red dwarf slowly contracts, the surface heats up slowly, which means it crawls a bit up the main sequence. When practically all of the hydrogen has been converted the star becomes a degenerate helium white dwarf (not necessarily "white"-colored) star, gradually sliding horizontally off the main sequence to higher temperature (but not higher luminosity).

    At and above 26% of a solar mass on the main sequence (less red-orange-yelloworange-yellow dwarf star) the core is transparent and transfers its radiation to an opaque convective outer envelope. Only the hydrogen in the core gets converted to helium in this phase. A helium core develops and eventually shrinks to degeneracy (can't expand even with higher temperature). The outer envelope expands and the star leaves the main sequence along an upward track. The lower part of the envelope is heated up and there starts a shell of hydrogen->helium processing above the helium core. The outer atmosphere expands and the temperature moves rightward to red temperature with higher luminosity( red giant phase). Next, Helium->Carbon conversion ignites in the core. A helium flash occurs, throwing the core out of degeneracy. Later, the outer envelope gets ejected and may form a planetary nebula around the star. The remnant of total {EDIT}carbon and {/EDIT}helium produced becomes a white dwarf. All this proceeds at a much faster rate than the (boringly) slow process in the first case.

    However, at about 150% of a solar mass, another threshold is crossed. Things get much more complicated, with hotter interior temperatures, more elaborate phases of nuclear conversion and much faster evolution. And the Chandrasekhar limit has been crossed: the star must ultimately blow up. But this is no longer about red dwarf stars.

    By the way, a star must have mass at least 8% of a solar mass to make it to the main sequence at all.
    Last edited: Jun 21, 2004
  8. Jun 22, 2004 #7


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    Mostly agreed with a post above except for a little expansion and small correction:

    Stars from the 0.08 Msun minimum threshold (to form a star at all) up to stars of about 1.25 Msun have a core temp. < 16 million K and fuse H to He mostly by the Proton-Proton Chain process. Our sun's core is ~14.5 million K. Above 16 million K, the predominant H fusion process is the CNO Cycle. Here we are talking about Main Sequence stars, before He fusion starts at much higher temperatures; approaching 100 million K. He burning is by the Triple-Alpha process, and all three of these fusion reactions are nicely explained at:


    At 1.5 Msun mentioned in the quote above, the star would have a core temperature of about 19 million K (and the CNO Cycle), but the mention of the Chandrasekhar limit and its result (supernova) is not correct. Here's why, and I'm posting it because the "Chandrasekhar limit" has been mentioned here on PF forever but often in the wrong, or mis-understood, context.

    When Chandrasekhar was on the boat to England in 1930-x (?) he calculated the effects of gravity against electron degeneracy pressure and came up with the famous limit of ~1.44 Msun above which any mass, including White Dwarf stars, would further collapse. In most cases we know of today this would be into a neutron star. He also calculated another "limit" of ~3.2 Msun above which any mass would collapse again; black hole. But, these masses were not, and had no direct relation to, a star's initial, or main sequence mass.

    As main sequence stars evolve, and no two are exactly the same, they go through various fusion reactions dependant upon core temperatures and various degrees of expansion into giants / supergiants, etc. Almost all star types go through some process of mass ejection, mass loss CME's, etc. even including mass blasted away by way of a supernova (either type). It is only the mass of the stellar remnant (core) after mass ejection that we need to consider as to when Chandra's Limit(s) would apply. As a general rule, which I happen to hate, a star of less than about 5 Msun would either never form a core of >1.44 Msun or would eject enough mass so that any remaining core would still be less than that limit. A lot of this was posted in the recent topic about Planetary Nebulae.

    Stars from ~ 5-10 solar masses (main sequence) would have a remaining core of 1.44 or more and collapse into neutron stars. Above about 10, there would usually be enough mass in the remaining core to go supernova, and the core after even that blast would still be over 3.2 solar masses and do the black hole thing.. :eek: Most books for this purpose usually just state an original, main sequence mass of 8 solar masses or less which would end up as white dwarfs, not neutron stars.

    "General rules" don't cut the mustard. From one site we get:
    "The stars that eventually become neutron stars are thought to start out with about 15 to 30 times the mass of our sun. These numbers are probably going to change as supernova simulations become more precise, but it appears that for initial masses much less than 15 solar masses the star becomes a white dwarf, whereas for initial masses a lot higher than 30 solar masses you get a black hole instead." [End edit]
    Last edited: Jun 22, 2004
  9. Jun 22, 2004 #8
    Thank you for the correction about the Chand. limit, Labguy, and thanks for joining the discussion.

    I fooled myself reading the Karttunen text's double-page evolution tree diagram and accompanying text in composing my reply. It shows a branch at M >=1.5 solar mass but only carries it forward to finale for 3<=M<15 and M>15 cases. As shown, both subcases end up supernovae.

    I suppose a more thorough and up-to-date chart would require a lot more pages.
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