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Thread: How long does it take a star to emit light after it starts forming?

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    How long does it take a star to emit light after it starts forming?

    (In this form because of the source for the question. Move is no problem.)

    In the sequel to The Mote In God's Eye, The Gripping Hand, they talk about a "protostar" "lighting up", becoming a light emitting star. The events in the novel revolt around this event.

    My question would be how long after the star's mass of hydrogen has compacted enough to cause fusion at the center of the mass and how long after that before the light reaches the surface of the mass? Assume the star is an "A" type. SWAGs acceptable, and TIA.

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    I think I read it takes thousands of years for energy to rise to the surface, and I suppose the surface would heat up slowly, over many years.
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    The star will be shining before the core gets hot enough to induce significant fusion. That incandescence is a result of the heating that happens as the protostar contracts under its own weight. Gravitational energy is being transformed into heat, lots of it. That is what gets the star white hot in the first place, and then the fusion in the core keeps it hot for a very long time afterward.

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    Would it look like a star when it was hot or would we have to wait for the photons from the fusion reaction to reach the surface of the star?

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    Do we have a definition of a star that makes a distinction between the glow from compaction and fusion, or is it even distinguishable? If the star isn't shining yet, would it still be shrouded by the cloud from which it was born, until enough light and solar wind blows it away? Also, what frequency are you wanting, as the surface temp may have it glowing in infrared before hotter temperatures are achieved via compaction, deuterium fusion and protium fusion, I suspect.
    Et tu BAUT? Quantum mutatus ab illo.

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    Ara Pacis, you are using simple terms to describe something that is less simple than that.
    We care about the pressure and temperature in the core as to whether any kind of fusion is going on, and we care about the photosphere as to any description of what we see, but usually with protostars, there is a thick layer of dust nebula around the object that would prevent you from seeing it in the optical range even if the photosphere were bright. That being said, Hornblower is right that the star will be hot enough just from the energy of stuff falling on itself that infrared telescopes would reveal that the non-fusing protostar would be visible in the optical range if only that dust weren't in the way.
    Forming opinions as we speak

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    Yes, there is a very common misconception that the initiation of fusion induces dramatic changes in a star. This is simply not true, whether or not the core is undergoing fusion has relatively little effect on anything else in the star, including how bright it looks and whether or not it has a wind that could blow away a shroud of dust. What's more, the presence of dust would actually make a planet warmer than without the dust, even though the warmth would come in the infrared rather than the visible.

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    Visible in the infrared before it's own light makes it to the surface then. SWAG on how long it would take that first photon to emerge?

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    Quote Originally Posted by Noisy Rhysling View Post
    Visible in the infrared before it's own light makes it to the surface then. SWAG on how long it would take that first photon to emerge?
    From the core?
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    https://www.quora.com/Why-does-it-ta...e-of-the-Earth

    This may help...it says upto a million years.
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    Quote Originally Posted by Frog march View Post
    From the core?
    Seems the most likely place for the fusion to start, yes? Most pressure? I'm asking because I don't know.

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    Quote Originally Posted by Frog march View Post
    https://www.quora.com/Why-does-it-ta...e-of-the-Earth

    This may help...it says upto a million years.
    "This requires a bit of physics!" I'm screwed.

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    Quote Originally Posted by Noisy Rhysling View Post
    Visible in the infrared before it's own light makes it to the surface then. SWAG on how long it would take that first photon to emerge?
    I'm not sure there's much value in distinguishing photons that can be traced back to fusion, versus photons that simply trace back to the star being hot. After all, what fusion mostly does is deposit heat into gas that is already quite hot, and hot gas makes photons regardless of what is keeping it hot. It sounds like what you really want to know is the time it takes light to diffuse out from the center of the Sun, regardless of whether fusion is occurring or not, and that time is often claimed to be about 100,000 years. By the way, this time should not be confused with the much longer "Kelvin-Helmholtz time", which is the time it would take for the appearance of the Sun to change significantly if fusion were to cease forever. That time is several orders of magnitude longer, because most of the heat inside the Sun is in the gas, not the photons, and that heat would continue to replenish the photons for a long time, even if there were no fusion.

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    Quote Originally Posted by Frog march View Post
    https://www.quora.com/Why-does-it-ta...e-of-the-Earth

    This may help...it says upto a million years.
    Beware of a few errors or points in need of clarification in that article. The million year time it refers to is not the time for light to diffuse out, it is the time for a significant fraction of the internal heat to escape, which is much longer (see my last post). They should really have explained that, the way it is written gives you the idea that we don't know how to get these numbers right even to a factor of 100! Also, the article says that if you change the mean free path by a factor of 3, the time increases by the square of that, but that's wrong. It would only increase by a factor of 3-- the number of steps is what increases by a factor of 9, but each step takes only 1/3 as long, so the net time is 3 times longer. Beware casual websites!

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    How long would a 2 solar mass protostar spend on Hayashi and Henyey tracks respectively?
    What track does a protostar follow before reaching Hayashi track?

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    Quote Originally Posted by Ken G View Post
    I'm not sure there's much value in distinguishing photons that can be traced back to fusion, versus photons that simply trace back to the star being hot. After all, what fusion mostly does is deposit heat into gas that is already quite hot, and hot gas makes photons regardless of what is keeping it hot. It sounds like what you really want to know is the time it takes light to diffuse out from the center of the Sun, regardless of whether fusion is occurring or not, and that time is often claimed to be about 100,000 years. By the way, this time should not be confused with the much longer "Kelvin-Helmholtz time", which is the time it would take for the appearance of the Sun to change significantly if fusion were to cease forever. That time is several orders of magnitude longer, because most of the heat inside the Sun is in the gas, not the photons, and that heat would continue to replenish the photons for a long time, even if there were no fusion.
    Yeah, that's pretty much what I was looking for. Thanks.

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    Quote Originally Posted by Noisy Rhysling View Post
    Visible in the infrared before it's own light makes it to the surface then. SWAG on how long it would take that first photon to emerge?
    The cloud of star-forming material will have been emitting since the Big Bang. It'll warm up as it collapses, shining brighter and hotter as radiation of heat allows it to compact further into its own gravity well, until fusion starts up and reaches levels sufficient to balance the loss of heat and things settle into equilibrium as a star, which itself will take some time after fusion starts. Any criteria about when to start counting photons as being "from a star" would be purely arbitrary.

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    Quote Originally Posted by chornedsnorkack View Post
    How long would a 2 solar mass protostar spend on Hayashi and Henyey tracks respectively?
    Here is a graph showing the approximate time.
    https://en.wikipedia.org/wiki/Hayash...ion_tracks.svg
    What track does a protostar follow before reaching Hayashi track?
    I would guess somewhere low and to the right, and rapidly go up and left to reach the Hayashi track high on the chart.

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    Quote Originally Posted by chornedsnorkack View Post
    How long would a 2 solar mass protostar spend on Hayashi and Henyey tracks respectively?
    What track does a protostar follow before reaching Hayashi track?
    At https://universe-review.ca/I08-25-premainseq.jpg, you can see some evolutionary tracks with ages superimposed. The answer seems to be, a few million years on the Hayashi track, and maybe 2-3 times longer on the Henyey track. Prior to the Hayashi track, the star is still collapsing, so it isn't in an equilibrium configuration and doesn't really have a track.

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    Quote Originally Posted by Ken G View Post
    At https://universe-review.ca/I08-25-premainseq.jpg, you can see some evolutionary tracks with ages superimposed. The answer seems to be, a few million years on the Hayashi track, and maybe 2-3 times longer on the Henyey track. Prior to the Hayashi track, the star is still collapsing, so it isn't in an equilibrium configuration and doesn't really have a track.
    But it has a luminosity and temperature - so obviously it has a track even in non-equilibrium configuration.
    How fast does a star evolve to Hayashi track? What is the free fall timescale at the start of Hayashi track?

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    Quote Originally Posted by chornedsnorkack View Post
    But it has a luminosity and temperature - so obviously it has a track even in non-equilibrium configuration.
    How fast does a star evolve to Hayashi track? What is the free fall timescale at the start of Hayashi track?
    Note that it has a luminosity and temperature even when it's just a cloud of gas and dust, or even when it's multiple such clouds. Prior to some point, the evolution of the system will depend too much on the initial state (geometry, velocities, variations in composition and temperature...) of the cloud for any kind of consistent track to be defined. It's only when it mostly reaches an equilibrium condition that behavior becomes consistent enough to define tracks followed by systems distinguished by mass.

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    Earlier in this thread Ken G mentioned that the onset of fusion in the core makes very little immediate change in the state of the star. Out of curiosity I calculated the ratio of fusion power to mass for the Sun, working from some 4 million tons of matter per second transformed into radiant energy and 2 x 1027 tons of total mass. It works out to about 5 watts per ton, which is a tiny trickle compared to the total heat bottled up in the Sun at a temperature of millions of degrees toward the core. It's not enough to heat a cup of coffee, let alone heat a ton of stuff rapidly. It keeps the Sun hot because the internal heat at any given temperature goes up as the cube of the linear size, while heat loss through the surface only goes up as the square.

    Perhaps thermonuclear bombs make some of us think of the onset of fusion in a protostar as a cataclysmic event that suddenly heats the star. I would say that comparing the onset of fusion in a star with an H-bomb is like comparing the internal heating of a compost heap with a firecracker.
    Last edited by Hornblower; 2017-Mar-06 at 01:24 AM. Reason: Fix a typo

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    Photons is photons.

    Next!

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    Quote Originally Posted by Hornblower View Post
    Earlier in this thread Ken G mentioned that the onset of fusion in the core makes very little immediate change in the state of the star. Out of curiosity I calculated the ratio of fusion power to mass for the Sun, working from some 4 million tons of matter per second transformed into radiant energy and 2 x 1027 tons of total mass. It works out to about 5 watts per ton, which is a tiny trickle compared to the total heat bottled up in the Sun at a temperature of millions of degrees toward the core. It's not enough to heat a cup of coffee, let alone heat a ton of stuff rapidly. It keeps the Sun hot because the internal heat at any given temperature goes up as the cube of the linear size, while heat loss through the surface only goes up as the square.
    Yes. On the other hand, free fall timescale of Sun is in the region of hour.
    So, if a large amount of cold gas - cold because it is in free fall and its gravitational energy is being converted to kinetic energy of inward motion - comes to a sudden stop and it weight finally meets gas pressure, converting the kinetic energy into heat, how fast does the previously cold and dark cloud evolve into a shiny star?

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    Quote Originally Posted by Hornblower View Post
    Perhaps thermonuclear bombs make some of us think of the onset of fusion in a protostar as a cataclysmic event that suddenly heats the star. I would say that comparing the onset of fusion in a star with an H-bomb is like comparing the internal heating of a compost heap with a firecracker.
    What's more, fusion in a star has essentially the opposite effect of fusion in a bomb. In a bomb, the fusion runs away, greatly shortening the timescale for the bomb's structure to change drastically. In a star, fusion is self-regulated to greatly increase the timescale for the star to change. The star goes from changing significantly on timescales of millions of years to billions of years when fusion begins, so it's like an anti-bomb. That's something to ponder the next time we all enjoy a sunny summer day-- in the wrong hands, the same basic process that has kept the Sun so constant over a million years of human evolution could also cause our extinction.

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    Quote Originally Posted by chornedsnorkack View Post
    So, if a large amount of cold gas - cold because it is in free fall and its gravitational energy is being converted to kinetic energy of inward motion - comes to a sudden stop and it weight finally meets gas pressure, converting the kinetic energy into heat, how fast does the previously cold and dark cloud evolve into a shiny star?
    The free-fall time at the current density is short, that's true, but in general the free-fall time is proportional to the density to the -1/2 power. So when the star first started collapsing, its density was much lower, and the time much longer than an hour. It starts out pretty slow, with timescales of millions of years, but then proceeds faster and faster as the contraction proceeds.

    Incidentally, when the contraction first begins, the star does not heat up because light so easily escapes. So what actually stops the free fall is not just the pressure stepping up to meet the gravity, it is the trapping of light inside the contracting object, causing the contraction to be adiabatic, since the object is then trapping its own light, allowing it to heat up as it contracts.

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    Quote Originally Posted by Ken G View Post
    Incidentally, when the contraction first begins, the star does not heat up because light so easily escapes. So what actually stops the free fall is not just the pressure stepping up to meet the gravity, it is the trapping of light inside the contracting object, causing the contraction to be adiabatic, since the object is then trapping its own light, allowing it to heat up as it contracts.
    Does it mean that the star only stops being transparent when it reaches Hayashi track?

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    Yes, when the star stops being transparent, it can start to set up its own internal force balance, and quickly becomes fully convective as it sorts out its thermal structure.

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    Quote Originally Posted by Ken G View Post
    Yes, when the star stops being transparent, it can start to set up its own internal force balance, and quickly becomes fully convective as it sorts out its thermal structure.
    What's "quickly" here?

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    Quote Originally Posted by John Mendenhall View Post
    Photons is photons.

    Next!
    Makes me wonder about the visible spectrum.

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