Story of a star: Part V
Where we talk of burning, aging, and hint at the inevitable consequence of the star's journey
We know from the previous episodes that stars need some form of energy to counter the gravitational force that would tend to make the structure collapse.
In the case of protostars, the responsibility for such a scenario lies on “controlled collapse”; in the case of stars, the main source of energy is thermonuclear reactions, which have timescales of the order of tens of billions of years.
Then, you can easily understand why the main sequence is the configuration in which stars live most of their lives.
Thus the main sequence will also be the main subject of this episode.
Ready, set, go
You already know that the zero-age main sequence is a diagonal line in the H-R diagram: from this line depart the tracks for stars of different masses. In particular, stars above 1.2 solar masses have convective cores and are dominated by CNO reactions (we talked about those here), whereas stars below this mass are dominated by pp chains1, and have radiative cores2.
What happens to a low-mass star?
Well, it will burn hydrogen: by doing so, it will have more and more helium, which will make the mean molecular weight heavier. This leads to a compression of the core because the temperature or the density (or both) of the core does not increase as much as to counter the weight of the outer layers of the star.
Of course, with the core compressed, density does increase, and this results in a gravitational potential release3 that increases the temperature of the gas: now more gas undergoes thermonuclear reactions.
With the increase in temperature, it also increases the rate at which the pp chain occurs: this results in the temperature increasing, along with the radius.
Ok, stars burn hydrogen in their main sequence lifetime.
What happens when hydrogen ends? Well, for low-mass stars, the pp chain stops in the core, but the high temperature all around the core enables the envelope of the core to burn hydrogen: there is an onion-like configuration, with a core now completely made of helium, and an outer shell made of hydrogen, that as soon as becomes helium increases the core.
The reactions in the shell produce more energy than those in the core: this leads to an increase in luminosity. This does not lead to an increase in temperature: what happens is that the temperature decreases slightly, because the energy produced makes the envelope expand, thus cooling the effective temperature.
This new phase ends when the helium core gets too massive compared to the total mass of the star. This limit is known as Schonberg-Chandrasekhar limit, and it is related to the fact that hydrostatic equilibrium depends on the molecular weight of the core and the envelope. To show the results would require a long (very long) amount of calculations, so if interested you can look it up (here, for example).
What is the order of magnitude of such a limit? In the case of a star with a chemical composition similar to that of the Sun, the limit would be reached when the isothermal core exceeds 8% of the star’s total mass.
What we said until now assumes that the gas in the core of the stars can be regarded as ideal; but when the density becomes sufficiently high, the gas becomes degenerate, meaning that the pressure is not controlled anymore by the temperature of the gas, instead the dominant factor becomes the quantum nature of electrons, that now occupy all the space available according to quantum mechanics.
This complication in our story has the visible effect that the Schonberg-Chandrasekhar limit gets shifted up at 13% (in the case of a star of the Sun’s mass).
What happens to a high-mass star?
The process is somewhat similar to the low-mass stars, but the presence of a convective core mixes things (the elements in the outer envelope and those in the inner core), making the chemical composition of the star homogeneous. The convection zone disappears eventually before the total depletion of hydrogen, and the stars encounter contraction like the other low-mass stars.
What awaits in the future?
In the next episode, we will continue to follow the life of stars in the main sequence and also encounter a new group of stars: globular clusters.
See you soon! (and if you liked subscribe!)
remember, CNO reactions are driven mostly by high temperatures, whereas pp chains are related to (relatively) lower temperatures.
below 0.3 solar masses actually, the star is entirely convective.