Why Is the Sun Expanding & Getting Hotter?

[stevendaryl]

I have read popular accounts about the future predictions (the far future--a billion or so years from now) for our sun, and they say that as the sun ages, it will begin to expand and get hotter, eventually making Earth too hot for life.

Is there a simple explanation for why the sun, after being stable for a few billion years, would start expanding and getting hotter?

 

[SteamKing]

I have read popular accounts about the future predictions (the far future--a billion or so years from now) for our sun, and they say that as the sun ages, it will begin to expand and get hotter, eventually making Earth too hot for life.

Is there a simple explanation for why the sun, after being stable for a few billion years, would start expanding and getting hotter?

When the sun gets sufficiently old, there will be no hydrogen left in the core to fuse to make energy to keep the sun from collapsing on itself. The core, where fusion normally takes place, will then contract sufficiently, raising the temperature and pressure for helium there to begin fusing. The temperature in the layers of hydrogen gas surrounding the core, where formerly no fusion took place, will rise sufficiently so that fusion of this material will begin, raising its temperature and pressure even further. As a result, the outer layers of the sun will expand, increasing the overall diameter of the sun. Eventually, this expansion proceeds until the sun becomes a red giant, after which fusion is no longer able to proceed, and the expanded outer layers of the sun are ejected into space, leaving a small white dwarf star.

https://www.e-education.psu.edu/astro801/content/l6_p2.html

 

[Alan McIntire]

I have read popular accounts about the future predictions (the far future--a billion or so years from now) for our sun, and they say that as the sun ages, it will begin to expand and get hotter, eventually making Earth too hot for life.

Is there a simple explanation for why the sun, after being stable for a few billion years, would start expanding and getting hotter?

The sun ISN"T exactly stable in luminosity.. It started out about 70% as luminous as it is now about 5 billion years ago, and has been wariming ever since
As the hydrogen in the core is converted to helium, the core of the sun gets hotter.

https://arxiv.org/pdf/1204.4449.pdf

See formula 1 for a pretty good approximation for the rate of warming of our sun

 

[D H]

When the sun gets sufficiently old, there will be no hydrogen left in the core to fuse to make energy to keep the sun from collapsing on itself. The core, where fusion normally takes place, will then contract sufficiently, raising the temperature and pressure for helium there to begin fusing.

While that is a commonly presented description of the transition of a one solar mass star from the main sequence to a red giant, that is not how it works. Read the link you yourself posted. There is no helium fusion during the red giant phase of a one solar mass star.

The innermost core of such a star is indeed helium, but it is inert (i.e., not fusing). The temperature at the core of a red giant is not yet high enough to trigger helium fusion. What happens instead is that hydrogen fusion occurs in the shell that surrounds that inert core. The fusion in the surrounding shell adds helium to the inert core. That inert core eventually does become hot enough for fusion to start, but that's only after spending about a billion years as a red giant. A one solar mass star then spends about 100 million years burning helium at the core. When that helium runs out, the star enters a second red giant phase, burning helium in a shell surrounding an inert carbon/oxygen core, which is in turn surrounded by a shell in which hydrogen fusion occurs. This final phase is very short lived, perhaps 10 million years or so.

Is there a simple explanation for why the sun, after being stable for a few billion years, would start expanding and getting hotter?

The Sun will become intolerably luminous long before it turns into a red giant. The cause is the ever growing amount of helium in the Sun's core. Stars on the main sequence are in a self-regulating balance between pressure, temperature, density, and fusion. At any point in a star, the weight of all the stuff above more or less equals the pressure at that point. At any point in a star, the temperature is the temperature at which heat flow from below and fusion at that point balances the heat flow going outward.

The increasing amount of helium in the core as a star gets older would act to decrease the fusion rate in the core if there wasn't some corresponding reaction. The reaction is that the star contracts as it ages, thereby increasing the temperature and density at the core. This keeps the innermost core stable, but at the cost of slowly increasing fusion throughout the core. The Sun gets brighter as it ages. Our Sun will have become intolerably hot long before it turns into a red giant about six or so billion years from now. Whether the Sun gobbles up the Earth as a red giant is a bit moot because the Earth will have already been dead for five billion years.

 

[Rubidium_71]

Wikipedia's entertaining Time Line Of The Far Future article states that Sol will become excessively luminous in just 600 million years from now.
https://en.wikipedia.org/wiki/Timeline_of_the_far_future

 

[Ken G]

The basic reason that the Sun gets a bit more luminous while fusing hydrogen in its core is that as helium builds up in the core, some of the free electrons get, in effect, gobbled up into neutrons. Reducing the free electrons allows light to escape more easily, raising the luminosity. But when hydrogen runs out in the core, fusion commences instead in a shell outside the core, and this fusion has a very different character because its temperature is not self-regulated, like core fusion, it is set by the escape speed at the edge of the inert helium core. That tends to be way higher than the self-regulated core fusion temperature, especially as helium piles up in the core, so the fusion rate in the shell just goes nuts. That's not a sustainable situation, so the only way to return the star to an equilibrium state is to drastically lower the pressure, and the amount of hydrogen, in the shell. That happens when the huge release of fusion energy causes the envelope of the star to expand, and an expanded envelope weighs much less, so the pressure in the shell ends up dropping dramatically and that gets the fusion rate under control. So you see, ironically, the luminosity of a red giant is so large because of the need to keep it from getting incredibly large.

 

[Chronos]

Convection patterns in stars change with age, so the core may become less active and promote fusion in outward layers..