Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The sun

  1. Jan 28, 2005 #1
    Was the sun ever a blue star in the early stages of its life?

    does this mean for a proportion of the time of the 5 initial billion years of the suns lifetime, the planets were cooking because of the suns initial powerful (blue) heat?
     
  2. jcsd
  3. Jan 28, 2005 #2

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, the sun never achieved a hot enough surface temperature to be a "blue" star. What spectral type a star will be when in the main sequence* depends on its mass. I don't remember the details, stars forming from more massive clouds heat up more when those clouds collapse due to greater grav. potential energy, and also have more material for nuclear fusion.



    *(i.e. the portion of its life cycle in which it spends most of its time after formation, up to the cessation of core hydrogen "burning")
     
  4. Jan 28, 2005 #3

    But doesnt all main sequence stars have the blue star status as well in their young life? I know this is basic of stellar astrology, so i think i am missing out on something..
     
  5. Jan 28, 2005 #4

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The life history of a star is determined by the mass of material in the initial gas cloud. Not all stars are created equal.

    LOL, sorry.
     
  6. Jan 28, 2005 #5

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    In short, NO.

    It is not basic...Oh, and just to let you know, the science of celestial bodies, the universe, etc is correctly called "astronomy". Astronomers will get mad at you if you confuse their profession with astrology :wink: , because astrology is not a science. It is all of that horoscope nonsense. I just thought I'd point it out, so that you don't have to face the wrath of the disgruntled astronomer in the future.
     
  7. Jan 28, 2005 #6

    DB

    User Avatar

    Hey cepheid, I have read that all stars start out fusing just hydrogen and helium, and that massive stars reach the "heavy metals" fusion, while main sequence stars end at around calcium. (all once the hydrogen has run out) So would that mean that the difference between spectral class star formation is mass and density of H-He?, and not the more material at the begining of stellar evolution?

    Thanks
     
  8. Jan 28, 2005 #7
    OUCH. a thousand apologies to people. 'Astronomy'

    i actually remember my teacher talking about this, and i still say it, lol...

    And when i meant basic, i meant basic as in you could look it up and see it upon a encyclopedia. I know they generalized and said all main sequence stars go through blue colour to red.

    Again, my apologies..
     
  9. Jan 28, 2005 #8

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    The core temperature of a newly formed star is determined strictly by its mass. Puny, low mass stars glow dark red, like a stove element. Behemoth, overweight stars glow blue, like a blow torch flame. Medium size stars, like the sun, are yellow.
     
  10. Jan 28, 2005 #9

    DB

    User Avatar

    Yes, and all initially fuse hydrogen and helium no matter what class right?
     
  11. Jan 28, 2005 #10

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This does not read quite right. Stars fuse Hydrogen into Helium. So 2 atoms of H are combined to form a single He atom and a bit of energy. Later in the life of the star 2 He atoms will combine to form a atom of Carbon and a bit of energy but less energy then released in the H+H -> He reaction.


    Stars do not form elements heavier then Iron in a fusion process since there no release of energy in their formation. The heavier elements are formed in the explosion associated with a nova.

    The color of a star is determined by the amount of energy being generated by the fusion process. The more mass that is present, the more atoms that are fusing and therefore more energy is released and the star is hotter. In addition more mass means more gravitational force to drive the fusion reaction so when a large mass of H is involved in the formation of a star it will burn hotter and faster. Once the point has been reached that the main reaction occurring is the He + He -> C the star will no longer be producing sufficient energy to balance the force of gravity. This leads to a collapse and a nova event.

    Generally speaking large hot stars have a shorter life cycle then smaller cooler stars.
    I am sure that a bit of Googling will turn up many detailed descriptions of the stellar life cycle, that task should be high on your list if you are indeed interested.
     
  12. Jan 28, 2005 #11

    DB

    User Avatar

    Thanks Integral, I have google many topics on stellar evolution, though when cepheid said that stars forming from more massive clouds also have more material for nuclear fusion I became doubtful. These clouds are consisting only of hydrogen and helium. Yes stars fuse H+H --> He, but aren't most proto-solar nebulas consisting 75% H and 25% He ? And once they colapse and form a star, wouldn't they already be made up of 25% He aswell as H+H ---> He reactions occuring?
     
  13. Jan 28, 2005 #12

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I would think that there are several factors driving the various reactions. First, of course, is availability. Given the proportions you give, clearly H+H will be the dominate reaction simply because the odds of a He+He collision is smaller. The next factor would be the requisite pressure to drive the reaction. It would seem to me that the core of a star is will have more reactions with the heavier atoms then then the outer shell, therefore the various regions of a star must consist of different materials and reactions.

    What are you doubtful of? I see no problem with his assessment.

    The more material (H, He what ever the ratio) that is present in the initial cloud the greater the mass the hotter the star. Because 25% (I will use your numbers) of the mass is He does not mean that 25% of the reactions will be He + He.

    Consider a room with 100 people, 75 in red shirts, 25 in green shirts, turn out the lights and mill around until throughly mixed, then each person grasps a neighbor and turn on the lights. How many, pairs of green shirts will there be? Certainly not 12.

    This is a readily computable number. Which I will leave as an exercise for the reader :wink: If you can't do it I am sure that someone else will provide the answer soon.
     
  14. Jan 28, 2005 #13

    Labguy

    User Avatar
    Science Advisor

    Yes, all reactions are, at first, H to He in stars initially made of just H and He. But, the Main Sequence ends at H burning. When temperatures are ~100 million K, the He can fuse (mostly to C) and the star leaves the main sequence. In "average mass stars, the H burns mostly by the Proton-Proton Chain process;
    http://fusedweb.pppl.gov/CPEP/Chart_Pages/2.TwoFusionReactions.html

    In more massive stars, the H burns predominately by the CNO cycle if even small amounts of these "metals are present; http://csep10.phys.utk.edu/astr162/lect/energy/cno.html
    The He can burn only by the Triple-Alpha Process;
    http://csep10.phys.utk.edu/astr162/lect/energy/triplealph.html

    True. In fact, none of the He will fuse below ~100 million K as mentioned above, regardless of the H to He ratio in the initial protostar.

    This paragraph is true except for the last sentence. When the H burning stops in a core, the star will still burn H in a shell around the He core and if the mass is greater than about 0.5 solar masses there will be enough gravity to compress the core (raising temperature) to where the He starts fusion. This starts as a rapid "Helium Flash" and some outer layers of the star's material is ejected. The He burning will (can) stabilize but the star will soon start to expand to the giant stages, not yet a collapse or nova event.

    Right. For a fairly good "timeline" of these processes, go to: http://www.astronomynotes.com/evolutn/s2.htm and keep clicking the "next" until the end. Another one just as good is found at: http://nrumiano.free.fr/Estars/birth.html . Again, just follow the progression at the bottom of each page.
     
  15. Jan 29, 2005 #14

    DB

    User Avatar

    Thanks, great links!
     
  16. Jan 29, 2005 #15

    saltydog

    User Avatar
    Science Advisor
    Homework Helper

    I think the Hertzsprung-Russell diagram is the definitive reference in regards to stellar evolution. Isn't our sun going red-giant next with a diameter to engulf the earth? We'll be long-gone by then though.

    SD
     
  17. Jan 29, 2005 #16

    DB

    User Avatar

    Ya, here you can read my post of our sun's red giant phase, I think it is a pretty accurate description.
     
  18. Jan 29, 2005 #17

    Labguy

    User Avatar
    Science Advisor

    No, our Sun will never advance beyond the stage of burning He to C and O. Not enough mass to fuse to the heavier elements; not even close to Iron.
     
  19. Jan 30, 2005 #18

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    The sun is massive enough to go red giant - in about 5 billion years. It is not massive enough to go past the C/O cycle, as Labguy noted. It will, however, get big enough to incinerate earth and make mars quite tropical, at least for 100 million years.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: The sun
  1. The Sun (Replies: 3)

  2. Sun's intensity (Replies: 9)

  3. Sun rays (Replies: 3)

  4. The walking sun (Replies: 5)

  5. Sun or sol? (Replies: 4)

Loading...