Stars

[Ganymede12]

Do stars have a maximum size or are they only limited by the amount of matter they have to form?

[nmoushon]

I would think the latter. The range from the smallest star to the largest is huge so I wouldn't think that there any limit to it. I doubt we would know if there was.

[Ganymede12]

I guess you could say that the limit is the size of the Universe!

[Naemeth]

The amount of matter (hydrogen mainly) they can absorb is the limiting factor. If that star had gravitational interactions with another close star, it could be spun out of orbit, perhaps crashing through Nebulae and other planets causing it to swell because it has more matter available, theoretically of course .

[sologuitarist61]

I am sure that there is an upper limit and a lower one as well.

I am sure that I have read that anything much smaller than the sun would be a 'brown dwarf' and so although still a stellar object, not what we would call a sun. Smaller than that and I think they just become very big planets, like these exo-planets 10 or 20 time the size of Jupiter that are being found.

Going up in size, there is nothing really huge out there so there must be an upper limit. I have read about stars a few thousand times the size of the sun, but I think that is about the limit overwise I would guess that gravitation pull inwards would be greater than the outward pressure from nuclear fusion and that they would probably collapse into a black hole (white dwarf, neutron star?). Doubtless others will put me right if I err in what I have said

[Naemeth]

I am sure that there is an upper limit and a lower one as well.

I am sure that I have read that anything much smaller than the sun would be a 'brown dwarf' and so although still a stellar object, not what we would call a sun. Smaller than that and I think they just become very big planets, like these exo-planets 10 or 20 time the size of Jupiter that are being found.

Going up in size, there is nothing really huge out there so there must be an upper limit. I have read about stars a few thousand times the size of the sun, but I think that is about the limit overwise I would guess that gravitation pull inwards would be greater than the outward pressure from nuclear fusion and that they would probably collapse into a black hole (white dwarf, neutron star?). Doubtless others will put me right if I err in what I have said

You're correct there . Brown dwarfs (between 13 and 80 times more massive than Jupiter) are failed stars that aren't massive enough to be at the temperature and pressure in order to fuse hydrogen, and thus ignite into a star.

[acey]

When talking about how big a star is, we need to be careful whether we mean its mass or its diameter.

A star is governed by two things: gravity and heat. Gravity pulls everything towards the centre, so the star is effectively trying to collpase in on itself. But at the same time this produces heat which makes the interior expand against the gravity. If the two things balance ("hydrostatic equilibrium") then you've got a stable star. If they don't then you've either got a collapsing star or an exploding one. This puts theoretical limits on star sizes.

If a proto-star is of low mass then the core won't get hot enough for nuclear fusion to start. If it is of very high mass then the core temperature can get so great that the star is blown apart. The latter situation is called the Eddington limit, and it's believed to be around 150 solar masses. The radius limit also comes from the requirement of hydrostatic equilibrium and is called the Hayashi limit. It's believed to be about 1500 solar radii.

One of the largest known stars is Mu Cephei, Herschel's Garnet Star, visible with the naked eye and a very attractive binocular object because of its intense colour. If it were placed at the Sun's position it would extend beyond the orbit of Saturn.

http://en.wikipedia.org/wiki/List_of_most_massive_stars

http://en.wikipedia.org/wiki/List_of_largest_known_stars

[sologuitarist61]

A good point well made

[Vince1963]

Hi Richard this is true. I have read the same thing... Acey explained it better though LOL.. But that's what you meant in it LOL

[Folkert]

Star sizes are limited in mass. The lower limit is 0.08 time the mass of the Sun. The pressure of these bodies is insufficient to initiate the PP chain reaction to start "burning". The higher limit is somewhere around 100 Sun masses. Above this threshold the radiation pressure becomes so strong that it blows away the outer layers of the star.

size-wise the limits are a little different. Huge are e.g. VV Cephei, with a radius of 8.8 AU, and Mu Cephei (5.7 AU). The smallest stars are basically not stars anymore, as their burning has stopped: white dwarfs (about earth size), neutron stars (10 km), and ultimately black holes (singularity).

[Auntystatic]

I think the largest star discovered so far is R136a1. Its more than 300 times the mass of the Sun, and twice as large as the generally accepted limit of 150 solar masses. I'll copy and past the next bit from the press release, but I saw it on a sky @ night episode too.

A team led by Paul Crowther, professor of astrophysics at the University of Sheffield, used ESO’s Very Large Telescope (VLT) and archival data from the Hubble Space Telescope to study two young clusters of stars, NGC 3603 and RMC 136a.

"Being a little over a million years old, the most extreme star R136a1 is already middleaged, and has undergone an intense weight loss programme, shedding a fifth of its initial mass over that time, or more than fifty solar masses."

Not only is R136a1 the most massive star ever found, but it also has the highest luminosity too - close to 10 million times greater than the Sun.

[Ronnie67]

In order to know what the largest star in the universe is we would have had to look at all the stars. We haven't come close to that - we haven't even looked at all the stars in our galaxy. There are about 100 billion stars in our galaxy, and about as many galaxies in the observable universe so you might understand why.

The best I can do is tell you what the largest known star is, but to do that I also need to know what you mean by largest. Do we pick the brightest, or the largest in volume, or the most massive (the one with the most mass)? More massive stars are generally less dense so take up a proportionally bigger volume. The most massive known is generally accepted to be the Pistol Star, which has a mass about 100-150 times that of the Sun, and is about 10 million times brighter. About 100 Suns would provide the same amount of stuff as the star, but we would need more that 100 Suns worth of volume to fill in the space it takes up.

[Ronnie67]

The Pistol Star is still very large in volume. It has a mass about 100 times the mass of the Sun and a radius of about 100 million miles (comparable to the Earth-Sun distance, or about 300 times the radius of the Sun). More massive stars are also less dense so take up proportionally more space. Red giant stars (like Betelgeuse and Antares) are also very big in volume.

[Ronnie67]

The largest known star is VY Canis Majoris which is about 2,100 times larger than our Sun which would mean 9,261,000,000 Suns would fit inside. It can be found in the constellation Canis Major and is approximately 4,900 light years away.

[symesie04]

I think the largest star discovered so far is R136a1. Its more than 300 times the mass of the Sun, and twice as large as the generally accepted limit of 150 solar masses. I'll copy and past the next bit from the press release, but I saw it on a sky @ night episode too.

A team led by Paul Crowther, professor of astrophysics at the University of Sheffield, used ESO’s Very Large Telescope (VLT) and archival data from the Hubble Space Telescope to study two young clusters of stars, NGC 3603 and RMC 136a.

"Being a little over a million years old, the most extreme star R136a1 is already middleaged, and has undergone an intense weight loss programme, shedding a fifth of its initial mass over that time, or more than fifty solar masses."

Not only is R136a1 the most massive star ever found, but it also has the highest luminosity too - close to 10 million times greater than the Sun.

I thought that there was some serious doubt on this finding. I believe the validity of the measurements were being questioned.

[JulianO]

Recent video on R136a1

[symesie04]

Interesting. So as mentioned above then it would appear that there is no limit to the mass of a prime sequence star?

[JulianO]

prime sequence? Do you mean main sequence?

And yes - there is a limit, the Eddington limit explain a few posts up. It depends somewhat on composition though, as burning rates are quite dependant on heavier elements being present or not.

[symesie04]

Ooops yes main sequence.

Indeed thats what im trying to get my head around. I thought the Eddington limit limits stars to around 150 solar masses for Hydrogen based makeup. Even if the stars makeup is predominately Helium 300 solar masses surely exceeds the Eddington limit.

[JulianO]

If a star is on the main sequence, then it is burning hydrogen. Helium burning and shell burning take it off the main sequence.

However there are a variety of ways to burn hydrogen. If the star is small then it will use one of the pp reactions. You need about 5 million kelvin to start this off and different variants of the reaction happen as you get hotter.

However overall the rate of burning for PP reactions is ~ n²T⁴ where n is the number density and T the temperature. So as it gets hotter, and denser, the reaction rate speeds up, but as its T⁴ then temperature plays a big part.

For bigger stars, another reaction, the CNO reaction becomes important. This use carbon, nitrogen and oxygen in a catalytic way to fuse hydrogen to helium.It needs about 15 million kelvin to start, so our sun is just able to run this reaction, but it contributes only a tiny bit.

The reaction rate of the CNO though is ~ n²T¹⁷ - so HUGELY dependant on temperature. As the temperature goes up, the CNO generates vast amount of reactions, and so lots of radiation and this is what blows away material that would otherwise contribute to the mass. It would be like trying to top up a firehose with water from a jug.

However, note that the CNO needs carbon as a starting point, so if there is no carbon in the star, it can't react that way until its made some. Carbon is made in large quantities in the helium burning phase, so after the main sequence is done. So for a star to undergo the CNO it needs to be formed from some of the debris of earlier stars.

There is of course much more to all this, and I'm not an expert in it, but basically you need to know quite a lot about the type of star before you can say how big it will get.

[symesie04]

Hmm yes point understood (i think). But as R136a1 is a 1st generation star you would assume it would contain various amounts of materials heavier than hydrogen and hellium including carbon. I thought it was only 3rd and possibly some 2nd generation stars that were "pure" as such. Therefore wouldnt such a massive star with carbon present violate the Eddington limit and start stripping its materail before it could ever aquire 300 solar masses worth?

[JulianO]

Yes - well it depends exactly on the metalicity of the star how fast it burns, and that will influence the Eddington limit. There are Eddington limits for black holes, which limit how much they can suck in, but that's driven by the radiation emitted from the accretion disk.

All I'm saying is there a limit to how big stars grow, its up to experts in the field to make models and predict exactly what the limit is, but its going to be dependant on a number of things. it is hypothesised that population III stars were much bigger than we see today, because of the lack of metals amongst other things.

[symesie04]

Oh boy "population" "main sequence" ill get there

I guess thats one of the beauties of astro physics, there always seems to be something that defies theory.

[Ganymede12]

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.

[JulianO]

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.