Long term habitats

Red dwarfs… trillion year real estate

Red dwarf stars are the slow living members of the galactic community. Just why sheds light on some odd denizens of the galaxy. The maximum sustainable temperature in the core of a star is dependent on its mass, and the rate of fusion in that core is dependent on the temperature to about the 7th power – thus a star that’s half the Sun’s core temperature of 15.7 million K is burning protons some 2^7 (128) times slower in a lower mass core. Slowly burning its hydrogen a minimum mass red dwarf (0.08 solar masses) will last for about 13 trillion years before going out, almost literally as if switched off. Unlike heavier stars there’s no red giant stage, just a gradual brightening until its final decline a few billion years before the end of hydrogen fusing. Greg Laughlin, Fred Adams and Peter Bodenheimer modelled the long-term evolution of low mass Main Sequence stars… The End of the Main Sequence …and for stars in the 0.08 – 0.2 solar mass range the picture is much the same, trillions of years of slow brightening and an end that’s a bit brighter, then a ‘quick’ decline. In the 0.2 – 0.25 solar mass range the stars end more like red giants and less like helium-rich dwarf stars. Another feature of low mass stars is they convect almost all their material and thus end up fusing 98% of their hydrogen – unlike our Sun which ends up fusing only about 8% of its hydrogen on the Main Sequence. Why the convection trick, which seems so unfair? A cooler core and a denser star. Consider, a 0.1 solar mass star has a radius of 0.125 of the Sun, which means it’s 51 times denser than the Sun, with a gravity that’s 6.4 times higher too.

Below a few million degrees in the star’s core and the fusion reaction rate is ridiculously slow and essentially doesn’t happen, so the “star” is actually a brown dwarf, which only shines by virtue of gravitational collapse. The heavier the brown dwarf, the more it can collapse too, so lighter brown dwarfs run out of gravitational energy much quicker than heavier ones.

Brown dwarfs are, oddly enough, closely related to white dwarfs – star corpses – which also only shine via gravitational collapse, and then by the slow trickle of their massive internal heat capacity. White dwarf collapse is halted via electron repulsion – the so-called degeneracy pressure caused by electrons being crowded too much and gaining Heisenberg Uncertainty energy. Without that repulsive force the white dwarfs would collapse into black holes after radiating away a few trillion years worth of heat. According to work on Supersymmetry (SUSY) by L. Clavelli a white dwarf can ‘catalyse’ the transition from our broken SUSY world to a world of exact SUSY in which there’s no degeneracy pressure to stop the star from collapsing into a black hole. Such a transition is sudden and would produce a massive release of all its gravitational energy as gamma-rays – highly collimated gamma-rays just like a humungous laser, or gamma-ray burst.

But what if the energy release is slowed down? If the collapsing star radiated at the Sun’s current output – but a much higher temperature because it is so much smaller – then as its mass trickled away as radiation (4.5 million tons a second is a ‘trickle’ to a star) then it would last some 14 trillion years if it massed the same as the Sun. This is about 1,150 times the Sun’s expected life-time as a fusion-powered star, a vast improvement for intelligent life in our system. The question is: how do we control the rate of collapse so our Sun doesn’t become a massive laser? We only have a few billion years to find out.

Author: Adam

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