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The Eddington limit is basically where stuff is coming out so fast, you can't add more.

As stars get hotter and bigger they give off bigger and bigger winds, and radiation also has a pressure.

So if you're trying to build a big star, think of it like pouring water(hydrogen) down a pipe into a kettle(sun). In this case the more you add the hotter the kettle

boils and steam starts to come out the pipe. If you add enough, then the force of the steam coming out will be enough to stop any more water going down the pipe.

Thats the Eddington limit, you can't add stuff any faster because its balanced by stuff pushing it away.

There are a few ways you can sort of get around it for a bit with funny geometrys but in the end there is a limit.

This applies equally to blackholes, there is a limit in a similar way to how much stuff they can hoover up per time.

If a star or black hole was at the Eddington limit and was speeding through space (presumably from some gravitational interaction which flung it off) and then collided with another star, what would happen, because it surely could not absorb it because it has reached it's maximum mass. Would it fling it away?

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If a star or black hole was at the Eddington limit and was speeding through space (presumably from some gravitational interaction which flung it off) and then collided with another star, what would happen, because it surely could not absorb it because it has reached it's maximum mass. Would it fling it away?

No - in that case you'd have to look at a while set of things. The star would get stripped of a lot of its material I imagine, but I think the core would probably make it through. It would be like throwing a snowball into a steam jet. If its big enough it will get there before its vaporised.

In general though its pretty hard to get two objects in space to collide head on (space is big man, I mean really big, you may think its a long way down the street to the chemist, but thats nothing compared to space...), you have to be very lucky. Mostly they pass close by and material is stripped and stolen or they end up in orbit or some similar interaction.

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No - in that case you'd have to look at a while set of things. The star would get stripped of a lot of its material I imagine, but I think the core would probably make it through. It would be like throwing a snowball into a steam jet. If its big enough it will get there before its vaporised.

In general though its pretty hard to get two objects in space to collide head on (space is big man, I mean really big, you may think its a long way down the street to the chemist, but thats nothing compared to space...), you have to be very lucky. Mostly they pass close by and material is stripped and stolen or they end up in orbit or some similar interaction.

Is it likely (if it happened of course) that the outer portion of the star being eaten would be stripped off and just left trailing in space?

I knew space was huge, but I was very surprised when I read that when Andromeda and the Milky Way collide, it's likely everything will almost stay the same, apart from there being seemingly more stars in the sky (Andromeda Galaxy stars now in the Milky Way), so the Milky Way would be brighter and probably bigger.

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Wouldnt the star still be totally ripped apart? I thought what would happen is that the black hole would rip the star apart but would be unable to consume most of the material so would eject into space in the form of gamma radiation, a quasar. This is assuming head on impact, if a glancing blow then yes i guess the star would be able to bounce of with some dignity intact.

Regarding the Andromeda and Milky Way collision not sure it would be a case of unchanged. Yes actual collisions would be very rare but there would still be huge upheaval, some stars would be thrown out into deep space, almost all would have their orbital positions changed dramatically, gas and dust would become super heated and new star forming regions created. Potentially a very detrimental period for any existing life in either galaxy. However potentially a period of new planet creation and therefore life emergence.

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My understanding is, I deed, that collisions of galaxies are quite common, but as galaxies are almost empty, the stars in them hardly ever collide. There is som e evidence of rare occasions in which a star is being ripped apart and swallowed by a black hole, one of the most energetic events we have ever observed (in this case about 10% of the total mass of the victim star is converted I to energy according to E=Mc2).

Regarding star collisions: even in the center of our Milky way stars circle ach other.(and the super massive black hole that is there), there are some nice video clips on TED that illustrate this. And even in the center of a globular cluster the stars are several hundreds of AUs apart! It is quite empty out there!

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I think I'm a bit lost with the Eddington limit business but at least my original question is answered.

Stars do, as far as we know, have a limited size.

NOT SO! Look at Madona and....well Ophra and...

Hey Mr Q, they are talking about other kinds of stars, like the ones up in the night sky?

Ohh, well then, never mind :grin:

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I recommend Arthur Eddington's book "The Internal Constitution of the Stars", first published in 1926 but still available. Obviously out of date (for instance, when Eddington wrote it no-one knew for sure how stars were powered) but beautifully written, and with lots of insight.

In chapter 1 he imagines a physicist on a cloud-bound planet (such as Earth!) calculating from first principles the radiation pressures and gas pressures of globes of gas of increasing mass. For small globes, gas pressure dominates and radiation pressure is very small because the globe is cold; for large globes, radiation pressure dominates and blows the star apart. But for globes of mass 10^30 to 10^32 kg, gas and radiation pressures are comparable, and it is in this ranges of masses that "we may expect something interesting to happen". Eddington goes on to say that "What 'happens' is the stars". In other words, he was able to show, using fairly basic physics, that stars must have masses within this range. He wasn't far out.

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Interesting stuff. Not easy to get your head around though, you would think that if 2 bodies based on equilibrium collided that one of the objects equilibrium would be destabilised (presumably resulting in some form of supernova) before a larger bodie based on equilibrium could be created. Unless of course im thinking of it in the wrong way and its more a case of one of the bodies sucking material off the other, but surely if this is the model it would still violate the Eddington limit?

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The Eddington limit applies normally to gas and dust accretion - usually calculated on hydrogen atoms - a large body could probably fall into an Eddington limited star and at least partially survive.

Poking around the equations a bit more... the equation is

LEdd = 4πGMmpc / σT

mp is the mass of a proton

σT is the scattering cross section of an electron

so you can see it mostly applies to protons and electrons being thrown out, not stars or other bodies.

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It is very hard (impossible) to visualise or imagine isn't it!

I vaguely remember reading something that implied that, although they are massive, and extremely large in size, their overall density is very low. Not sure if anyone has any info on that?

Stu

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A quick back-of-envelope calculation suggests the sun has an average density of about 1.5 kg per cubic metre. For comparison, the air we breathe is about 1 kg per cubic metre. The core is obviously very much denser than that. On the other hand, a red giant must be a pretty good approximation to a vacuum. For instance, the sun will one day be about x100 in diameter, so about a million times less in density than it currently is.

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Thanks both. Richard, I think red giants must be what I recalled, couldn't get my head around the density being nearly a vacuum, yet they are so massive (in the true sense), and immense.

Stu

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I still can't get my head around just how big the stars really are! They just look so tiny in my telescope and yet they are incomprehensibly massive!

It's the massive distance they are from us (things, as they get further away from us appear to shrink in size). Really want to blow your mind? Think of the amount of energy radiated in the visible part of the electromagnetic spectrum needed for that distant star to be seen by us at such a great distance! And by the way, it takes a telescope many meters in size to finally see the star's disc (of the most massive stars)other than just a pinpoint of light and even then it's size (disc) is barely detected.

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Back to a slightly more simplistic view, I find the attached amazing, and very useful for putting varying planet and star sizes in perspective.

d4b78066-2ff6-03b7.jpg

Stu

I like this, it is a good illustration and at least it enables you to try and comprehend relative sizes. In comparison to VY Canis Majoris, the sun would not even equate to a smalle speck of dust, and, I suspect is actually smaller than that! Amazing.

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I wouldn't worry Ganymede, although it helps to give a comparison, there's no way you can really have any real perspective of just how big they are. All I know is that they are, well, really big!

Stu

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I wouldn't worry Ganymede, although it helps to give a comparison, there's no way you can really have any real perspective of just how big they are. All I know is that they are, well, really big!

Stu

That is certainly true but then think just how small these stars are in relation to the Milky Way; how small a part that is in the local group; how small the local group is in comparison to the universe ........... it just goes on and on .

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  • 2 weeks later...

All I can say to the pics are mega wow, the sizes of each does not compare to yet many super giants out there, just looking in comparison to earth and Canis Major. It would take Christopher Columbus many many years to circulate that globe, prob b dead and buried hundreds of times over :tongue:

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