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Stargazing Star formation question

  1. Feb 17, 2017 at 8:05 AM #1
    I have a question, it has to do with the formation of stars and how it is triggered.

    I was reading that any given stellar body with enough mass would collapse into a star, does this mean that any type of matter you put in space regardless of what it is, given the correct conditions will turn into a star?.
    Even if I put a bunch of water, paper, metal, horses, whatever?.
    Sorry if the question sounds a little dumb.
     
  2. jcsd
  3. Feb 17, 2017 at 9:07 AM #2

    phinds

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    Yes, it is irrelevant what the macro composition of the matter is. If you get enough of it you get a star. If you get enough of it, for that matter, you will get a black hole.
     
  4. Feb 17, 2017 at 10:56 AM #3
    How do you define "star"?
     
  5. Feb 17, 2017 at 2:31 PM #4

    Drakkith

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    There are some caveats here that need addressing.

    1. As snorkack asked, "how do you define 'star'?"

    I am unable to quickly find a rigorous definition of a star, but I believe a star is usually considered a large, luminous object held together by its own gravity and that has the ability to fuse hydrogen in its core (or did so in its past). The last part, the ability to fuse hydrogen in its core, is important because it lets us make a distinction between 'stars' and 'things-that-look-kind-of-like-stars-but-aren't-quite'. Brown dwarfs are formed in exactly the same manner as stars and have the same composition, but are unable to fuse hydrogen (protium) in their cores because they are not massive enough. They also fall into the classification of 'sub-stellar objects', which are objects less massive than stars.

    2. What are stars usually made of?

    The typical star is composed of about 71% hydrogen and 27% helium (by mass). So around 98% of the matter making up a star is hydrogen or helium, the two lightest elements.

    3. Why are these two things important?

    We'll get to that. Right now actually. :wink:

    These two things are important because of how stars get the energy they need to stave off collapse for millions to billions of years. They get it through nuclear fusion. Primarily the fusion of hydrogen. The stage of a stars life during which it fuses hydrogen is known as its Main Sequence Phase and is typically the longest part of a stars life.

    As I said above, stars are composed mostly of hydrogen and helium. These two elements release a lot of energy from fusion and the products of their fusion also release lots of energy. At least up until you reach the elements of iron and nickel. Because of the way the opposing forces of electromagnetism and the strong force interact with nuclei, iron and nickel are the 'end point' for both fusion and fission. Fusing these elements together takes energy instead of releasing it, as does splitting them apart.

    Now, so what? Well, the OP is asking the question, "Does this mean that any type of matter you put in space regardless of what it is, given the correct conditions will turn into a star?"

    Given the definition of a star I gave above, and the knowledge that elements heavier than iron/nickel do not release energy upon fusing, the answer is no. If I put 5 solar masses of the element lead into a small gas cloud, which then collapsed under its own gravity, that material would not turn into a star. Oh it would collapse and shine like a star for a while, but it would not be a star under the definition I gave above. And it wouldn't shine for very long either (relative to an average main sequence star). Fusing hydrogen releases a substantial amount of energy. Far more than the gravitational potential energy released by our lead star over time. GPE is important because it is the sole source of energy available to a non-fusing, collapsing body like our lead star.

    But, if you're definition of a star is different from mine, or if you're creative enough to 'stretch' the definition above, then you might be able to answer with something other than "no".
     
  6. Feb 17, 2017 at 5:59 PM #5

    phinds

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    OK, I stand corrected. A large mass of Drakkiths, say, would not likely become a star.
     
  7. Feb 17, 2017 at 6:06 PM #6

    fresh_42

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    Carbon, oxygen and hydrogen? Should still work. I think the problem starts with metals.
     
  8. Feb 18, 2017 at 1:46 AM #7
    An object that from its formation consists of iron, silicon and oxygen would not fuse protium because it does not contain any - even if it were hot and massive enough to fuse protium if it contained any.
     
  9. Feb 18, 2017 at 6:19 AM #8

    Ken G

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    Remarkably, that's not as true as you might expect. Typical stars like the Sun end up putting about half their mass into white dwarfs about the size of the Earth, and stars made of lead might do that for all their mass. If you calculate the gravitational energy released when they do that, you will see it is somewhat competitive with all the energy they release due to fusion (perhaps about 1/10). So while it is true that the main sequence is the longest lived period while stars are still bright, it really isn't true that this phase is all about releasing fusion energy-- a non-negligible fraction of it is gravitational energy, released in creating white dwarfs, and even more energy would be released that way if shell fusion didn't ultimately expel half the star. What all this means is, if there were no such thing as fusion, we'd still have a sky filled with bright dots-- they'd typically be bluer and have perhaps a ten times shorter lifetime of being bright (so there'd be something like 1/10 as many we could see), but I wager we'd just use a different definition of a "star."
     
    Last edited: Feb 18, 2017 at 6:38 AM
  10. Feb 18, 2017 at 10:07 AM #9

    chasrob

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    I don't understand. I was told that iron and the elements above would release energy upon fusing, but to fuse them takes more energy than was released by the fusion of the element.
     
  11. Feb 18, 2017 at 10:39 AM #10

    Ken G

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    That's what he means-- in the net, fusing iron into something heavier will use up energy, rather than release it, so it's not a nuclear fuel for a star.
     
  12. Feb 18, 2017 at 12:20 PM #11

    Drakkith

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    Yep. :biggrin:
     
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