The life of a star is a difficult subject, because modern
science has only examined them for a hundred years or so and they
exist for millions or billions of years. It is like examining
six minutes of the life of a man and then trying to describe his whole
If you only had one man to examine, you would have great difficulties! But what if you had, say, six billion people to look at for those six minutes? And if you were careful about the assumptions you were going to need to make? You might be able to create a decently accurate description of a man's entire life.
Continuing this analogy, you might note that around1% of the people you examine were VERY small, what you might choose to call babies! You might then assume that being a baby might represent about 1% of the lifetime of a man. In your six minutes of watching thousands of babies, you could probably figure out many common patterns, no talking but simple sounds; tantrums, sleep patterns, etc.
You might also note that 10% of the people you see for your six minutes seem nearly full sized but generally very rebellious, what you might choose to call teenagers. You might conclude that this stage probably accounts for around 10% of the man's life.
After getting extensive data like this for many different types and sizes of humans, you might then try to put them in some sort of order. You might choose to believe that the baby stage was first, with the larger and larger stages following afterward, and so on.
When you finally write your research report on what you found in your six minutes, you might definitely describe the entire 75 year lifetime of a man. It's an accomplishment that might have seemed impossible, except for using this approach.
Astronomers and astrophysicists have essentially done the same thing regarding the stars. There are certainly plenty of them out there, more billions of them than we can even count! And, as long as we are careful in assumptions we make, we might be able to describe a decently accurate entire lifetime of a star. Will it be absolutely accurate? Probably not! But it probably will generally be pretty close.
The following discussion of the lifetime of a star is based on this approach, with various additional research and data added in. It has taken most of the past hundred years to get to this general description, which is currently generally considered realistic.
In a second or an hour, those individual Hydrogen atoms would not attract each other very much, but they really would be microscopically closer together. But given a million years, or a hundred million years, they could be attracted together into a big glob. Depending on how much Hydrogen was originally in the cloud, this glob might have different total mass. In this discussion, we will variously consider several different size globs, which we will describe by comparing the total mass with that of the Sun. One will be 0.001 solar mass; another will be 0.1 solar mass; 1.0 solar mass; 10 solar masses; and 1,000 solar masses.
This added energy appears as light and heat, which heats up the Hydrogen in the center of the glob even more, which speeds up more Hydrogen atoms and causes more fusions. This is called a self-perpetuating or self-sustaining nuclear fusion reaction.
Notice, I said CENTER! The outer surface of the Sun, what we see, is not burning at all, and it is not creating any energy either. It is far too cool for that, only being around 6000°C or 11000°F. That's really hot to us, but nowhere near the millions of degrees necessary for fusion to occur. That fusion can therefore ONLY occur near the very center of the Sun (and every other star). The outside surface of the Sun (or any other star) ONLY seems bright to us because the super heat at the center of the star has to move outward to get away from the center, and so the surface is actually heated from below from that heat rising through the star! We can never actually see the nuclear fusion occurring in any star (because it can only occur near the very center of it) and the surface we see is not actually "on fire" or even producing any energy!
The glob that started with a total mass of 0.1 solar mass would create temperatures high enough to start nuclear fusion, which would then be self-sustaining, so this would result in a star. Interestingly, when this fusion starts, the extra heat starts to inspire faster and faster fusion reactions, because the hotter it gets, the more fusions will occur, right? Well, that would be true, and the (new) star would very soon go berserk and be in a runaway, uncontrolled fusion event, and the entire star would use up all of its Hydrogen on a matter of seconds or minutes! If this happened, we would see no stars in the sky!
There is another effect which occurs. There is a phenomenon called Radiation Pressure. In any human experience, this pressure is so small that it can never be sensed or measured. But with the astounding amounts of energy created in the center of a star, this radiation pressure can be very strong. The effect is that this outward directed radiation pressure pushes outward on all the Hydrogen surrounding the area, and it makes more space available (by very slightly making the star a few feet larger in diameter). As this available space slightly increases, the inward directed gravitational collapse pressure gets a little less, which means the temperature can drop a little. This effect would them enable FEWER fusions to occur and so the star would start to slightly collapse!
EVERY star therefore has both of these effects, the fact that additional heat given off from the fusions tend to make the center of the star hotter to increase the rate of fusion, and also the effect of radiation pressure which has the opposite effect. This 0.1 solar mass star would have established a situation of stability, where the two exactly equal each other, which then causes the star to create a moderate amount of radiation, but extremely evenly, and for an unbelievably long period of time! It is possible to calculate how much of the Hydrogen gets used up by such 0.1 solar mass stars, and in the 5 billion years that we know our Sun has existed, such stars would have used up less than 1% of their Hydrogen! They will be able to continue producing energy (including light) for at least another 500 billion years!
Next, if we consider the Sun, a mass 1.0 solar mass star, we know that it has used up roughly half of its original Hydrogen in those 5 billion years. In another 5 billion years, it will run out of Hydrogen as a fuel source, and some interesting things may occur but it figures to then simply "go out" and gradually just cool off. After a while, the surface would get more orange and then red, and then (visibly) disappear, and eventually just be a dark ball of stuff! Note that the expected lifetime of the Sun if less than 1/100 that of a star that had 1/10 the mass. This is due to research done where hotter sources radiate energy far faster than you might first expect! If any source of radiation is doubled in (absolute) temperature then it radiates energy sixteen times as fast! This dependence is as the fourth power of the absolute temperature.
Now, we have no direct way of knowing exactly how hot it is inside the center of any star, even the Sun. The (visible) outer surface of the Sun might be twice as hot, but does that also mean that the temperature of the center is also twice as hot? No one really knows for sure, but a general assumption is that it is hotter. There are some indirect ways of estimating how hot it must be, from the total radiative energy measured (and calculated) from a star and some assumptions based on the spectrum of the star and sometimes some other information.
Now, for our next blob, the one with 10 solar masses, we still have a similar situation, where the initial gravitational collapse caused high enough central temperatures to start fusion reactions. In this case, the central temperature has that extra contribution due to gravitational pressure and so we assume that the central temperature must be higher. Then, the rate of energy production would be much higher than that by the Sun, say 1000 times as much. This would mean that such a 10 solar mass star would use up its Hydrogen in around 1/100 as long as the Sun will (remembering that there was 10 times as much Hydrogen to start with). Such a star would seem hotter and brighter (due to a hotter surface), but it would be likely to exist for only 1% as long as the Sun, maybe 50 million years, a rather short time in astronomical standards!
Finally, we consider the blob that started with 1000 solar masses of Hydrogen gas. The gravitational collapse of this blob would create ferociously high temperatures. This would immediately start creating very large numbers of fusion reactions in its center. The speed of all this developing would not allow the gradual balancing of the temperature of that central region, where the outward radiation pressure eventually exactly matched the inward pressure of gravitational collapse. In this case, the high mass new star would be extremely unstable. It would also use up its Hydrogen fuel at incredible rates! Even if it could somehow remain stable, its total lifetime might only be ten or a hundred years, before all the Hydrogen would be used up. Such a situation seems very unlikely, because the Hydrogen cannot really move around inside the star fast enough to fuel such rapid consumption, another factor that suggests that it will be extremely unstable. Such extreme high mass stars (1000 solar mass) are thought to be so unstable that none could make it through an entire lifetime, even though such a lifetime would only be a few years!
Instead, such high mass stars are believed to all manage to blow themselves apart, almost immediately. The result is that the Hydrogen mass would get blown apart (with fusion immediately stopping). That gas might then re-collapse together, and it could all happen again, OR it might have gotten blown far enough apart to eventually collapse into smaller blobs, to form less massive stars which are more stable. Another result could be that the Hydrogen gas simply wanders around out in space for very long times. Eventually, it would certainly get gravitationally pulled into some existing star, or some newly forming Hydrogen blob, or even some "dead" star that had already burned out, because all those things will always have gravitational attraction.
In the discussions above, we referred several times to a necessary balance between outward radiation pressure and inward gravitationally caused pressure. All stars that last very long have these two balance AS AN AVERAGE OVER TIME (a static balance), but they can still have temporary (dynamic) imbalances. Say that a star briefly gets slightly overheated (due to a combination of extra fusion due to the higher temperature and to an increase in gravitational pressure due to a slight collapse (essentially a slight reduction in the size of the core and maybe the entire star). It might then take a minute, or an hour, or a year, of such excess radiation occurring in the center before the radiation pressure is able to push the surrounding Hydrogen away enough to let it again get a little cooler, where less fusion would then occur. This sequence of "pulsing" could therefore occur over and over and over. The result would be what we call a variable star, one where the amount of light we measure changes, greater and lesser. Nearly all such variable stars have relatively small amounts of such variability, and it seems that most stars have at least a tiny amount of such variability.
Even the Sun shows evidence of a slight variability, on a number if time scales. There is an eleven-year (or more correctly, a twenty-two-year slight variability that is most obvious regarding sunspots, but there seem to be both longer-term and shorter-term variabilities in the Sun, too.
Most of the observed stars which show variability of small amounts seem to have very consistent patterns, where a light-curve can be drawn (brightness versus time) which closely repeats itself. Several categories of such "stable" variable stars have been observed, which have different light-curve shapes or different timescales. There are also "irregular" variable stars, which do not show such repeatability of the light curve, either in brightness or in the time involved for an entire cycle. Such irregular variables tend to have much greater changes in brightness, and therefore total radiative output.
It seems to me that this makes a lot of sense. Small (dynamic) fluctuations in the instantaneous balance between radiation pressure and gravitational collapse pressure might easily (and quickly) cause the opposite (and therefore stabilizing) effect, so a repeatable, short-term variable would result. When those perturbations are much larger (usually only in massive stars which have natural instabilities to start with) then the response figures to be less identical and so less predictable. This would then logically make for a longer-term irregular variable star, with far larger variations in brightness.
Notice that I am avoiding calling all this "burning". It seems that most people, even astrophysicists, refer to this as "burning" Hydrogen, but that is actually now what is happening. The definition of what we call burning describes a CHEMICAL REACTION called OXIDATION, where a fuel combines with oxygen to release some chemical energy. Fusion does not require any oxygen, or anything other than simple heat (very high speed), for pretty much anything to experience such nuclear fusion. Try to NOT call it "burning"!
If the total mass is relatively moderate, the expansion is relatively stable and the star then starts an entirely new phase of its existence, of now using Helium as fuel and creating Carbon atoms in the process. If the mass is really large, this sudden expansion is unstable, and the star essentially blows itself apart, in an event which might be called a "nova" or "supernova", depending on just how massive it was. An important thing to note is that this now sends a lot of Helium atoms off into space! And keep in mind that massive stars have relatively short lifetimes, so in just 10 million years or so, the neighborhood that had only had Hydrogen gas now has a combination of Hydrogen and Helium. Later star formation could then start off with a combination of Hydrogen and Helium, and not just Hydrogen.
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Now, if such a star would just go out and fade away, none of those new atoms would go anywhere, as they would forever be trapped inside dead and dark stars. Fortunately (for us!), a lot of those stars then conveniently blow themselves to bits, which then sends all those elements out into space. As already mentioned with Helium, later globs would include such mixtures of elements in forming a "next generation" of stars. Since the really massive stars live such short lives, like 10 million years, these new atoms then have plenty of time to wander across the Universe to become part of some distant new star formation a billion years later.
It is generally thought that the Universe is around 15 billion years old, and the Sun is around 5 billion years old. At ten million years per generation, a LOT of massive stars might have lived and exploded before the Sun was even a twinkling in the eye! This explains why our Sun contains many elements, which we know from its spectrum.
It appears to be especially important that there is some Carbon in the Sun. Carbon nuclei can act as a catalyst in enabling the fusion of Hydrogen into Helium operate much more efficiently than when only Hydrogen is present, and after a series of fusions occur, there is a nuclear decay which results in a Carbon atom and a Helium atom. Since the Carbon was necessary, but is not "used up" in this process, only a very small amount of Carbon is actually necessary, and the Sun seems to have enough!
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago