Huge Stars, How do they get so Huge?

Betelgeuse, Compared to our Sol, is huge IIRC.

The cloud of gas that creates a star must swirl in under gravity to build a mass that will ignite fission, which will create a burst, and a solar wind, which will drive the remaining gasses and matter away.

If this is correct, how do some stars have such a large mass, while others like our sun are so much smaller?

Mind you I am no physics master or anything, but I do understand, the point of ignition should be at relatively the same point for each and every star.

So, How does a star get so much bigger, why aren’t all stars within a narrow band of size? Is Betelgeuse just so much hotter and active that with the same mass of our sun it balloons out?
This clip does not include Betelgeuse, but helps illustrate what I am talking about.

Okay, this one is fairly simple – but long and drawn out.

A star’s diameter is determined, more or less, by the balance between the gravitational attraction of the mass of the star to itself – i.e., each atom of it is being pulled toward the center by the combined mass of all the rest of it (technically not quite true, but a good first approximation) – in balance against the outward pressure of heat and light (and the tendency of things under pressure to expand and relieve the pressure) generated by the fusion at the star’s core. I.e., a protostar collapses in on itself, heating up the core by the loss of potential energy, until it reaches the temperature and pressure where nuclear fusion of hydrogen into helium can begin. At that point it achieves a balance, and moves onto the Main Sequence. Its total mass determines how hot the core gets, how fast the reaction occurs, and hence what the temperature of the visible surface is.

When a star has ‘burned’ a large fraction of the hydrogen at its core into helium (by fusion, not oxidation, of course), the reaction damps down, since helium will not fuse at the temperature and pressure hydrogen will. So the core contracts, the outward pressure being reduced. The core gets hotter and under more pressure, until finally it reaches the point where three helium atoms will fuse to carbon. (If two heliums fuse to form Beryllium-8, it breaks down as fast as it’s produced, so the jump to three is required.) Withe the core fusing helium to carbon and a shell of hydrogen around it being fused to helium, the star as a whole heats back up – and the radiation pressure and tendency to expand call for a new equilibrium, one where the outer edges of the star are pushed waaay further out from the core by the new higher pressure. This produces a giant star, one with a very low average density (though the density of the core is enormous) and a huge diameter.

The expansion is repeated several times as the carbon itself fuses (producing neon IIRC), etc., ending up with silicon fusing to produce iron in the final step. Each step results in a core reaction surrounded by a shell of the previous reaction, surrounded by another shell, etc., with greater heat and pressure required at the core for each reaction and hence a new equilibrium producing a more distended star. Giant and supergiant stars are the ones where the core has moved to helium burning or a later step in the sequence, with an outer, visible surface that is usually relatively cool (hence red colored) and at an enormous radius from the core, which is indescribably hot and dense.

IANAAstronomer, and hopefully one will be by soon to elaborate, but here’s my understanding of it. A cloud of cold gas sitting in interstellar space won’t necessarily collapse; as long as it’s above absolute zero, it has some small amount of pressure, and if this pressure is stronger than the force of gravity pulling the cloud together, the cloud won’t collapse. The amount of gravitational self-attraction is dependent on the initial density of the cloud, and the pressure depends on both the density and temperature of the cloud. It turns out that clouds above a certain mass will collapse under their own gravity, and clouds below a certain mass won’t; this is critical mass called the Jeans mass. Importantly, the Jeans mass depends on the temperature and density of the initial cloud; smaller stars can’t form from clouds that are sufficiently hot or not sufficiently dense, and you only get big ones.

ETA: Polycarp’s post is entirely correct, and explains why stars like Betelguese get physically large later in life. However, I think the OP was asking how Betelguese got so much mass in the first place.

There are lots of stars that are much, much larger from the get-go, having significant more mass than our Sun. Class O stars, which only comprise something like 0.00003% of the local neighbourhood, for instance, are at least 16 solar masses, and at least 6.6 times the Sun’s radius. Zeta Orionis is about 20 solar masses, and is something like 20 Solar radii. Alpha Orionis, or Betelguese, is about the same mass (20 times the Sun), but about 900 Solar radii.

These stars simply start out in a larger cloud of gas. Their greater mass causes more fusion to occur, so that they fuse their fuel to occur. Betelguese is thought to be only a few million years old, and will move on to the post-hydrogen fusion stages that Polycarp describes much sooner than our Sun will.

It started in the 1980s when Coke and Pepsi switched from real sugar to corn syrup. That is what’s causing obesity in people as well as stars. It’s the corn syrup :slight_smile:

Uh, that should have been that they fuse their fuel much faster. Brain/hand interface not working today, apparently.

Paging Chronos.

I’m not fully up on my astrophysics, but I don’t think that this is right in general. Stars forming out of gas clouds will not generally create a solar wind strong enough to drive some of that gas away. And they will not ‘ignite fission’, but fusion.

AFAIK, the only limit on stellar mass in the early stages is how much matter was in the cloud of gas, and how many foci of mass happen to develop far enough away that they can each form as seperate stars. (As in, a part of the cloud that becomes dense enough to gravitationally attract more gas to itself.)

Bursts of stellar energy driving matter away from a star is the sort of thing that happens later on in a star’s life cycle.

Star were thought to have a maximum mass of about 120 solar masses. However, astronomers have found a star dubbed LBV 1806-20 which they believe to be 150-200 solar masses. Obviously this is a problem for the theory so study continues.

I believe (been awhile since I read about this so not sure) the reason for the maximum mass is as the star grows more massive the fusion reaction gets more energetic (or rather lots and lots more hydrogen per second is fused). However, the star is also very big and the surface, being far away from the center, is under less gravitational attraction. At some point the output of the star is so high and the gravity at the surface so (relatively) low the star blows off any more matter that might try to cling to the star thus marking a maximum mass.

VV Cephei A is a good deal bigger than even Betelgeuse. I think it is the largest in terms of diameter sun we have ever found. Placed in our solar system it would reach nearly to Jupiter.

To the OP here is your answer:


MikeS already said everything I would have, so I didn’t see the need to add more.

Here’s a cool collection of images comparing various planet/star sizes. VV Cephei A is the second largest star in it; VV Canis Majoris being the biggest.

Mind you, when I was a wee-un in school. We were given the treat of how a star is born via a science film. It showed the process as a cloud of gasses and dust swirling into the center. A large ball forming, glowing, then POOF! It ignites, blowing the remaining gasses and dust back away from the newborn sun, to start forming planets, moons, rings, comets and asteroids.

I take it this was more or less a theory designed for young minds and not totally correct.

To make sure I understand better: The gas and dust falls in creating the form which will ignite at a predetermined heat, mass, density. But it will not be a POOF ignition. furthermore the ignition will only be strong enough to balance the star from collapsing further. Merely slowing the flow of other gasses from the cloud. So then the star will continue feeding on this matter until it is gone, continuing to balance itself, heating up and expanding further.

Is this a good way to look at it?

Wheaties, every morning :smiley:

What I don’t understand is, how can a star likely VY Canis Majoris, which has a mass 30-40 times that of the Sun have a radius ~2000 times greater than that of the Sun–I mean for comparison purposes it’s as big as a sphere that extends all the way out to Saturn’s orbit!! I don’t even need to do the math to know that it must be MUCH less dense than our sun. It seems like it must be extremely diaphanous–nearly a vacuum–throughout most of its volume. How far out from its core is such a star capable of any nuclear fusion? And why is it so much larger (by volume) than our Sun?

Fusion occurs in the core of stars. The glowing surface we see is heated by the reaction in the core. And the star is the diameter we see because of the pressure exerted by the heat, light, etc., radiated out from the core (and convection, etc. – allow me “radiated” as a generic term). Think of it as internal pressure blowing up a balloon.

Stars that are higher in mass burn hotter and faster, producing the upper range of the main sequence. Stars that have progressed to helium, carbon, etc. burning have a much hotter core – and one or more shells around it fusing the lighter elements still present there. This produces immensely more radiation pressure, pushing the visible surface out much further – resulting in the giant stars of the lower portions of the pictures in Der Trihs’s link. Using the balloon analogy, think of it as a balloon inflated to a much greater extent. The outer layers of a giant or supergiant star are far less dense than Earth’s atmosphere – someone described them as a “red-hot vacuum”, not quite accurate but evocative of the rarefication of the gas/plasma involved.

Remember that the idea that giant stars are less dense than the Sun, while true, is describing average density – their cores are much denser than the Sun’s, but they are so distended by the radiation pressure that the average density of their enormous volume is extremely low.

Does that provide a useful answer to what you asked?


How does the actual fusion starts from the initial cloud of dust and gas, is it known to be an explosion, maybe similar to a nuclear bomb, or would it be more gradual, like over a period of time, and if so, how long from start to full fusion, hours, days, or years?

See my cite in post #9.

Remember the star is heating up all along. As you compress stuff it gets hotter. Even before fusion starts the star is getting quite hot in the center. Indeed it has too in order to initiate fusion. This heat is resisting collapse all the time. The stuff wants to cool off/lower density but gravity overpowers this.

Then note that deuterium starts before full-blown hydrogen fusion initiates. Deuterium can be fused at much lower temperatures/pressures than “normal” hydrogen fusion does. My cite notes this slows collapse/accretion but does not stop it so no blowing anything off for that.

As such there is a more linear/smooth progression to fusion starting and not one massive nothing and then a bang when it all starts. I do not know what kind of a shock the star experiences once fusion initiates but I am guessing it is not all that bad since clearly stars do not explode during their formation. Might it blow off some of its surface? Maybe but I am guessing a lot of that just falls back to the star eventually. If not I suppose it gets added to the rest of the solar system and planets and what not scoop it up.

Actually, thinking about it some more, I am not sure the star blows any of its mass off. Here’s my thinking, sonmeone stop me if I am wrong:

The star is continually gaining more mass. As mass increases temperature and pressure increase in the core. At some point you get enough mass to tip the star into fusing deuterium then hydrogen. There is, I presume, a fairly sharp line that delineates where this occurs. If the star blew off part of itself it would decrease its mass shortly after it had just enough to start fusion. So, if it did that, seems you would get an on/off cycle of fusion. Enough mass to fuse, blow off material so not enough mass, mass comes back start, blow off material and stop (ad nauseum).

Seems whatever outward pressure is exerted it simply cannot be enough to stop accretion of mass else every star would be just barely big enough to be a star as it pushed any other matter coming at it out of the way.

Clearly these things do no happen so I am guessing stars have a relatively smooth and non-violent start.