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Proxima Centauri
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24-08-2016 6:35pm#1
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An M type star has quite a cool surface, under 3500 K, which is why they think this planet could be in the Goldilocks zone for liquid water even though it is close to its parent. The problem with M type dwarfs is that they produce a lot of X-ray flares that could zap any potential planetary life.Being so close to a red dwarf, does that mean it's very likely this planet has been scorched at some point in the past?
We see flares on our own star. They are magnetic phenomena. Charged material is funneled along magnetic field lines, which twist and stretch, then snap like an elastic band. In the process, the material is superheated and lofted into space along with bursts of X-rays. As a result, in our own sun the temperature increases from 5,700 K at the surface to two million degrees in the corona. This is counter-intuitive because we expect the temperature to be falling as we travel further from the Sun's surface. But the additional heating is provided by conversion of magnetic energy by a process called magnetic reconnection.
Where does the magnetic energy come from in the first place? It is generated by the rotation of the star itself, which consists of a highly charged plasma. All other things being equal, the magnetic field should be organised into regular field lines which emerge from the axial poles of the star (below left). But turbulent convection within the star's outer layers traps the field lines and winds them up, causing the magnetic field to distort and burst out through the surface (below right).
So then where does the convection come from? A star is heated by the fusion reactions going on in its core, themselves initiated by the temperatures and pressures caused by the ongoing gravitational collapse of the star. The more massive the star, the higher the internal temperature and pressure. The temperature determines the type of nuclear fusion reactions that occur. The temperature sensitivity of these reactions can determine the temperature gradient within the inner core. The rate of some reactions can be proportional to the seventeenth power of the temperature! This tends to make the reaction rate and the temperature drop off extremely rapidly with distance from the inner core.
In G type dwarf stars like our Sun, and even smaller K and M type dwarfs like Proxima Centauri, we only have to worry about the set of fusion reactions called the proton-proton chain which fuse hydrogen to helium. Core temperatures are on the order of fifteen million degrees. Within the core, energy is carried outward by radiation only. Photons from fusion reactions scatter off ions and slowly work their way outward through the bulk of the star. At higher distances from the core the density and viscosity of the plasma drop off because less of the star's weight is above you, and if there is a sufficient temperature gradient we get the onset of convection, like in a pot of boiling water or in this lava lamp:
Convective heat transport is chaotic, and produces charge separation between negative electrons and positively charged hydrogen ions. This convective zone is where the magnetic field gets distorted.
It follows immediately that if we have a core temperature of fifteen million degrees and a surface temperature of approximately zero (relatively speaking), that the highest temperature gradients will occur in the smallest stars. Thus, in our Sun the convective layer only extends 200,000 km deep from the surface, about 30% of its radius. But in an M type dwarf with its higher temperature gradient, the convective layer extends all the way down to the outer core. This traps the magnetic field much more effectively and results in higher magnetic energy released in flares at the surface, and a higher X-ray flux. That's why cooler stars don't necessarily have a more benign environment for planets in their Goldilocks zone.0 -
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Maximus Alexander is exactly right. A red giant like Betelgeuse is the same spectral type M as Proxima Centauri, but the two stars are on a completely different evolutionary track. The Hertzsprung Russell diagram is a piece of genius, but it originally just graphed spectral type (which is a good proxy for colour and temperature) against luminosity. This more modern one from Wikipedia fills in a whole lot of other detail inferred later from astrophysics:
All of the main sequence stars (the main diagonal on the diagram) are in the hydrogen burning phase. The only difference between them, by and large, is their initial mass which determines all their other properties. An individual star does not move (very much) on the main sequence during its lifetime. The red giants are out on their own, which begs the question why we don't see a continuous evolution off the main sequence into red giantdom on the HR diagram if main sequence stars turn into red giants. The short answer is that the evolutionary changes that occur when a star runs out of hydrogen fuel occur relatively rapidly so we don't often catch a star in the act of transition.
Nevertheless there is some structure to the red giant branch which you can see in another version of the HR diagram on that same Wikipedia page. This tells us about various transitions that occur in the later life of some stars. These changes are all mass dependent though, so the HR diagram should not be misconstrued as a picture of what happens to every star in its lifetime. And there is still the so-called "Hertzsprung gap" between the main sequence and the base of the red giant branch due to stars rapidly transitioning from the subgiant stage when they have consumed about 10% of their total hydrogen fuel. It is a surprising fact that a star that has burned hydrogen stably for many billions of years will undergo some later transitions in mere millennia. (Indeed, some of the more catastrophic changes to higher mass stars occur literally in seconds).
The red giant phase occurs when the star transitions from hydrogen to helium burning, and subsequent phases involve the fusion of successively heavier elements. Stars between about 0.5 and 8 solar masses will eventually end up with mainly carbon and oxygen in their cores and late in life will puff off most of their envelope (which will always have remained mostly consisting of hydrogen) leaving a so-called CO (Carbon-Oxygen) white dwarf. Stars below 0.5 solar mass such as Proxima Centauri never have sufficient mass to generate the temperatures to initiate helium burning and -- after trillions of years -- will eventually transition directly to He (Helium) white dwarfs.
It should be pointed out, of course, that a trillion years is orders of magnitude longer than the current age of the universe, so any He white dwarfs that exist must have gotten there by an unusual route. Funny things happen in binary star systems where one star may cannibalise another by gravitational attraction. A compact star such as a white dwarf may find itself next to an expanding red giant companion which overflows its Roche lobe -- meaning that material pours off the giant onto the white dwarf. This can cause nova explosions as material is transferred, or may even rejuvenate the white dwarf, returning it to the main sequence. Meanwhile a larger companion can be stripped of its hydrogen envelope leaving a prematurely decrepit He white dwarf.0 -
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