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Why stars are different sizes

  1. Feb 21, 2009 #1
    Why are stars different sizes? Are there stars made up of something other than hydrogen? It was my understanding stars are made of hydrogen but if all of them where made of this then they should all start fusion whey they reach critical mass/density making them all relativley the same sizel, so this can't be correct. Can someone please correct me?
  2. jcsd
  3. Feb 21, 2009 #2


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    The size of the cloud that starts self-gravitating and pulling itself together is determined by prior conditions (like the density of the star forming region and pressure waves traveling thru it and competition from other wannabe stars.)

    In other words the size of the gradually collapsing cloud is determined by random prior conditions long before any fusion starts.

    If the proto star is too big then when fusion starts it can blow away the outer layers so that fusion does provide a not-very-exact upper limit----like on order of magnitude 100 solar.


    BTW if I remember right, the sun is a bit bigger than average. An average mass star in our galaxy is probably about half solar mass.
  4. Feb 23, 2009 #3
    that tells me why some clouds are bigger than others, but that is obvious, because some just are. but why are some stars bigger than others? doesn't fusion allways start when a proto star reaches a certain density? if so then would'nt all stars be the same size?
  5. Feb 25, 2009 #4
    A huge irregular cloud of gas held together by dark matter collapses at several points into massive stars. Within this “nursery” some of the massive stars will naturally form before others. These first stars are often of 100 solar masses or more. They radiate enormous energy that disturbs the cloud around them as gas “tendrils” form. These tendrils then collapse into smaller stars. The effect of this radiation can be imaged in infrared.
    http://www.ipac.caltech.edu/Outreach/Edu/sform.html [Broken]
    These huge stars also rapidly convert their cores to iron and then explode within a few million years. This explosion causes a shock wave that further perturbs the surrounding cloud, causing formation of more numerous but less massive, stars.
    Last edited by a moderator: May 4, 2017
  6. Feb 25, 2009 #5


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    Hi disco,
    what Arch said is good. You might also want to look at WikiP articles about
    "... It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star, which then develops into a main sequence star. ... Any disturbance to the cloud may upset its state of equilibrium. Examples of disturbances are shock waves from supernovae; spiral density waves within galaxies and the close approach or collision of another cloud. If the disturbance is sufficiently large, it may lead to gravitational instability and subsequent collapse of a particular region of the cloud..."

    and T-Tauri star
    "T Tauri stars are pre-main sequence stars – the youngest visible F,G,K,M spectral type stars (<2 Solar mass). Their surface temperatures are similar to those of main sequence stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract towards the main sequence, which they reach after about 100 million years..."

    In my earlier post I was trying to think of a good non-technical word for a protostar and I said "self-gravitating cloud" meaning a sub-region of the larger star-forming cloud which because it happens to have higher central density is destined to collapse. I should have just said protostar, like this:

    A protostar begins as just a section of the larger cloud which happens by random disturbances to accidentally have higher central density, a threshhold gradient or degree of concentration which destines that section of cloud to fall together by its own gravity.

    Protostars can have all different masses.

    Fusion is not the determining event you seem to think.

    Typical stars (< Solar mass; F G K M) begin shining long before they start fusion.

    Gravity (not fusion) governs a star's early life.

    Gravity can accomodate a whole range of masses of protostar sections of cloud, from a small fraction of solar, to half solar (very common) up to several times solar.
    All different masses of protostar can self-gravitate and collapse.

    And the gravitational energy released by collapse heats them. And they start glowing and enter the T-Tauri stage and make a kind of wind that drives surrounding gas away and gives them sharper definition, and they become visible by telescope etc etc.....and yet fusion hasn't started yet.
    Last edited: Feb 25, 2009
  7. Feb 26, 2009 #6
    Yup. That explains alot, I thought all stars were powered by fusion. Which is why I thought they would all start luminosing at the same stage. But since they aren't I understand why they are diff sizes, because they are doing different things and emitting light for different reasons.

    So, how does "gravitational energy released as the stars contract" cause a start to emit light? is it the friction between the molecules in the star interacting through friction and then absorbing electrons, then reemitting them? Or something simillar?
  8. Feb 26, 2009 #7


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    Eventually all stars are powered by fusion. It is only at the very early stages that they begin to shine, before fusion starts, simply due to the fact that falling together makes them very hot.

    Jillions of tons of crud falling down from the sky on the rapidly growing protostar nucleus releases a lot of plain old mechanical energy.

    Gravitational energy like you feel on your toe if you accidentally drop a rock. You can call it friction between the molecules of your toe, which are made to bump together by the energy released by the rock. What you say about friction is not a bad way to think about it.

    You can calculate how much gravitational energy is released by the falling together of a star. It's impressive.

    Stars which have begun to shine and become visible like that are called T Tauri. And when they begin to shine they produce a wind, like the solar wind, called a T Tauri wind. It blows away some of the surrounding matter and helps to clear the space around the star.

    Eventually the T Tauri radiates off its initial load of heat (from the gravitational energy of formation). then it would cool down and get more compact, except then fusion starts
    Last edited: Feb 26, 2009
  9. Feb 27, 2009 #8
    well then if they are all eventually fuled by fusion like I thought how can they be of such hugly different sizes? wouldn't they allways start fusion when they are say the size of our sun? Making it impossible for them to reach the size of super giants?
  10. Feb 27, 2009 #9
    Imagine you have a very large young star which hasn't started fusion yet. It's perhaps 10 solar masses, but isn't dense enough yet to begin fusion. Eventually, after the heat from compression radiates away it is cool enough to collapse further, and begins fusion. Fusion doesn't begin merely from large mass, if it did the galaxy as a whole would be one big fusion powered star. Rather fusion starts when there is high density and heat. It is possible for an early star to have huge mass but not yet high density or heat, and thus no fusion.

    Now the question is, why would a large young star suddenly lose mass when it starts fusion?
    Last edited: Feb 27, 2009
  11. Feb 27, 2009 #10
    Here’s a link to Astronomy Magazine where the dynamics of massive star formation are discussed.

    So the theory is that massive stars form from the collision of protostars or directly from accretion. The explosive force of fusion won’t stop accretion until it overcomes the gravity of the mass of the cloud that is forming it. A star the size of the Sun wouldn’t produce enough energy to resist a collapsing 100 solar mass cloud of gas.
    If you look at the dynamics of the stellar nursery, then you see that processes conspire to create not only fusion and gravity, but also turbulence from shock waves that cause different sizes in the clouds that eventually form stars.

    Also, we haven’t discussed this, but a star's size (i.e., volume) will evolve over its lifetime. Our Sun will eventually swell to enormous size as fusion changes the makeup of its core and then shrink to the size of the Earth. So star size is also affected by which elements are undergoing fusion.
    Last edited: Feb 27, 2009
  12. Mar 1, 2009 #11
    So several young protostars can collide with one another forming a more massive star? That makes sense. Also Stars can still acquire mass from from it's parent cloud even after it has started fusion? Hmm... I would think the solar wind if not the initial explosion from fusion starting would blow off the remaining cloud....

    Ok I learned in physics class things have a center of gravity. so if we have a newborn star and it's surrounding parent cloud would it still act as a whole with the star and cloud sharing a comon center of gravity? Which could theoreticaly be stronger than the force of the solar wind which would cause the star to continue to acrete matter?
  13. Mar 2, 2009 #12


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    Here's a way to think of it.
    Theoretically a gas giant like Jupiter that was 75 times Jupiter mass could start fusion and be a real star.

    In fact a star has been observed with mass of about 100 Jupiters. This is about 1/10 the mass of our sun.

    I wouldn't imagine the start of fusion as an "explosion". Fusion is very slow. A gas-ball before fusion starts is already glowing hot from the gravitational energy. T Tauri stars are already visible even though fusion has not started. They already glow.

    Because fusion is limited to occur only in the core where there is high temp and pressure, and because it is very slow, the fusion when it starts may only add a small percentage to the surface temperature. Fusion is not a runaway chain reaction like fission can be.

    You wouldn't always be able to tell the difference between a gas giant that was producing some fusion power and a gas giant that was not. It might simply have a hot core as a souvenir of when it collected together.

    So whether or not fusion is occurring will not always be decisive of how massive a star gets. It's destined mass will be determined by a lot of other factors.

    So it slowly begins some fusion in its core. So what? If there is a lot of crud around that wants to rain down, it will continue raining down on the sucker. Stellar wind notwithstanding.

    In really massive cases like 100 solar, a star's wind pressure and light pressure does interfere with growth. Eddington calculated a limit to growth like that, called, naturally enough, the Eddington Limit. But not to worry about that until things get seriously massive.
    Last edited: Mar 2, 2009
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