Wonders of the Universe

THE LAST STARS

The nearest star to our solar system is Proxima Centauri. Although only a mere 4.2 light years away, Proxima Centauri is not visible to the naked eye from Earth and doesn’t even stand out against more distant stars in many of the photographs that have been taken of it. The reason for this is that Proxima Centauri is small, very small when compared to our sun – having just 12 per cent of the Sun’s mass – so to our eyes this star would appear to shine 18,000 times less brightly than our sun.

Proxima Centauri is a red dwarf star – the most common type of star in our universe. Red dwarfs are diminutive and cold, with surface temperatures in the region of 4,000K, but they do have one advantage over their more luminous and magnificent stellar brethren: because they’re so small, they burn their nuclear fuel extremely slowly, and consequently they have life spans of trillions of years. This means that stars like Proxima Centauri will be the last living stars in the Universe.

If we do in fact survive into the far future of the Universe, it is possible to imagine our distant descendants building their civilisations around red dwarfs in order to capture the energy of those last fading embers of stars. Just as our ancestors crowded around campfires for warmth on cold winter nights, so some time long in the future humans may take their warmth from a red dwarf as the last available energy in the Universe.

The rate of the fusion reactions in the cores of these red dwarfs that is needed to provide the thermal pressure to resist the inward pull of their weak gravity is very low, which enables them to live longer. Even so, these are still active stars, and their surfaces are whipped up into turmoil by the turbulent convective currents that constantly churn their interiors. Amongst all this activity, explosive solar flares occur almost continually, blasting bursts of light and X-rays out into space.

Ultimately, though, the frugality of these stars is no defence against the arrow of time. Four trillion years from now, at 300 times the current age of the Universe, Proxima Centauri’s fuel reserves will finally run out and the star will slowly collapse into a white dwarf. After trillions of years of stellar life and death, only white dwarfs and black holes will remain in the Universe, and then, in around 100 trillion years’ time, this age of the stars will draw to a close and the cosmos will enter its next phase: The Degenerate Era. And yet, even after 100 trillion years of light, the vast majority of the Universe’s history still lies ahead. Bleak, lifeless and desolate, our universe will go on, as it enters the dark

These computer-generated images reveal how Proxima Centauri will meet its end. Over the next four trillion years, this red dwarf will gradually collapse into a much dimmer white dwarf.

After trillions of years of stellar life and death, only white dwarfs and black holes will remain in the Universe, and then, in around 100 trillion years’ time, this age of the stars will draw to a close and the cosmos will enter its next phase: The Degenerate Era.

A white dwarf is visible amongst brighter, living stars in this enhanced image, taken by NASA’s Galaxy Evolution Explorer, of Z Camelopardalis, a binary star system.

NASA