Is there a upper limit to star size?

Is there a upper limit to star size? Any reason why a galaxy can’t orbit a supermassive star instead of a supermassive black hole? Is there a upper limit where the star will collapse into a BH just because of it’s size? Or just a large enough star will burn through it’s fuel so much faster, so they don’t last long, or blow themselves apart?

Both.

The bigger a star, the hotter it is, and the faster it burns through all its hyderogen, then helium, etc. Any such star of respectable age has gone through he main sequence (google “star main sequence”) and is now a white dwarf.

If I remember my physics correctly - A collection of matter so large that it’s escape velocity at the surface is greater than the speed of light is a balck hole by definition. In fact, the center of a supernova becomes a black hole, or a neutron star (solid wall-to-wall neutrons, as opposed to atomic nuclei surrounded by electrons in mostly empty space) are the result of the implosion effect. The blast squeezes them like superman making diamonds… If the squeezed mass is big enough, it becomes a balc hole.

IIRC, there’s some sort of physics that says the resulting compression does not stop. Unlike water, a black hole’s contents can keep squeezing and contracting until it becomes a sinlge point - a singularity, like trying to find 1/x as x->0.

OTOH, you can have red giants with a tenous atmosphere of glowing hydrogen much bigger than earth’s atmosphere. “The Mote In God’s Eye” mentions this as a plot device, the only hyperspace exit from the moties’ world is through the atmosphere of such a star; any attempt to use this route without the protection field caused the ship to melt.

Chiming in here…

The gravity of any massive body above The Chandrasekhar Limit will collapse it into a black hole, unless there’s another force working against it. However, fusion working in the core of a stare will do just that - by generating new heat, fusion pushes the substance of a star back out, expanding it against gravity, so stars don’t collapse into black holes until their substance has been converted into elements that don’t fuse particularly easily, (higher up than helium, maybe higher than oxygen.)

I do believe that the more massive a star, the more quickly fusion will run its course - I have no idea how massive a star would have to be to make it die and collapse in 100 years, or in a single year. We don’t exactly have the opportunity to observe stars like that during their lifetimes, because there aren’t many situations that would cause them to form in the currently observable universe.

A recent Scientific American article covers a lot of these questions.

The upper limit for a star is about 150 solar masses. A highly theoretical and never seen dark matter star might grow to 1000 solar masses. They all would be short-lived and collapse into balck holes.

Much bigger than earth’s orbit hardly describes a dark matter star. At 200,000 times the sun’s diameter, it would reach 3% of the way to Proxima Centauri.

You may enjoy this.

I see you have the same keybraod I do.

This takes it further to largest known star- see diagrams to right

Yep. This is the one I was looking for. Google desktop search is great.

Distinguish between the stars of greatest diameter, the hypergiants – see Astro’s and adhay’s posts above – and most massive star, which may not be as distended. There is a definite size limit for massiveness, though exactly what it is, is not entirely clear. See “Eddington Size Limit” section at my link.

Only moderately small stars such as our Sun will become white dwarfs, actually. A few times the mass of the Sun, you get a neutron star instead, and after about 10-20 solar masses, you get a black hole (the precise number isn’t known, since it depends on how much mass is blown away in the supernova, and our models of supernovae are still a bit shaky). It’s possible that there’s also another intermediate stage between neutron stars and black holes, called a quark star, but those are still theoretical, and it would probably only be a narrow window of initial conditions that would lead to one.

Animation, with music. It leaves out VY Canis Majoris as the animation was made before the star was measured, or something.

I think the short answer is that there is a limit known as the Eddington limit, because the radiation and wind blowing out of a star will overwhelm infalling mass at this point. I think also recent work has found that this is more complicated because there will be instabilities leading to a loss of spherical symmetry such that the star will have side-by-side regions with very different vertical velocities.

The Eddington limit is for objects that gain their energy from accretion of infalling mass, such as quasars. It’s not relevant for something powered by internal fusion.

Do you have a cite for this? Or an explanation? Because on the face of it, that first sentence is off by 6 orders of magnitude.

:confused:

Those numbers are taken directly from the cite that is in that post.

What is termed the “Eddington limit” in the (limited) material I’ve read suggests that as stars become more massive, the pressure and hence temperature of the core increases exponentially*. This in turn causes more intense and greater fusion reactions, with a consequent outward radiation pressure. At a point somewhere around 120-150 solar masses, long term stability is undermined by that pressure surpassing the ability of gravity to hold the star intact – probably meaning that the “stellar wind” will be such as to physically blow off from the star’s surface enough mass to bring the star back below the Eddington limit. Stars in the early phases of their evolution are not bound by this limit, as they have not yet heated up enough – think of it along the same lines as driving a car which has effectively lost its coolant. If done for extended periods, it will seize up from overheating, but it can be ‘safely’ (acceptable risk) driven for a couple of minutes to get it into a position where it can be repaired, since it wll not have heated up to an unacceptable level during that brief period of operation. Likewise stars can exist with a mass above the Eddington limit – until they heat up enough for it to come into play.

And all this is very much subject to Chronos’s caveat, which I hope he’ll explain in more detail.

  • used metaphorically, but I suspect it’s literally true.

“The Eddington luminosity (also referred to as the Eddington limit) in a star is defined as the point where the gravitational force inwards equals the continuum radiation force outwards, assuming hydrostatic equilibrium and spherical symmetry. When exceeding the Eddington luminosity, a star would initiate a very intense continuum driven stellar wind from its outer layers. Since most massive stars have luminosities far below the Eddington luminosity, however, their winds are mostly driven by the less intense line absorption. [1]”

This is from Wikipedia, but plenty of other sites say similar things. Where are you getting this?

Huh, I’ve never seen it used in that context before, but on thinking about it, I suppose it would be applicable. I end up hearing a lot more about black holes (which are, in practice, often limited by the Eddington luminosity) than I do about supermassive stars.

Quoth Nametag:

Do you mean you’ve heard of stars existing with over a hundred million solar masses? That seems quite the extraordinary claim, to me.

OK, I misread something. When it’s 2 am and one should be asleep, it’s possible to conflate the M in M[sub]o[/sub] with the M that stands for “Mega.” :smack: