The end of stellar evolution

With its current age of 14 billion years, the universe now lives in the midst of a Stelliferous Era, an epoch when stars are actively forming, living, and dying. Most of the energy generated in our universe today arises from nuclear fusion that takes place in the cores of ordinary stars. As the future unfolds, the most common stars in the universe - the low-mass stars known as red dwarfs - play an increasingly important role. Although red dwarf stars have less than half the mass of the Sun, they are so numerous that their combined mass easily dominates the stellar mass budget of the galaxy. These red dwarfs are parsimonious when it comes to fusing their hydrogen into helium. By hoarding their energy resources, they will still be shining trillions of years from now, long after their larger brethren have exhausted their fuel and evolved into white dwarfs or exploded as supernovae. It has been known for a long time that smaller stars live much longer than more massive ones owing to their much smaller luminosities. However, recent calculations show that red dwarfs live even longer than expected. In these small stars, convection currents cycle essentially all of the hydrogen fuel in the star through the stellar core, where it can be used as nuclear fuel. In contrast, our Sun has access to only about 10% of its hydrogen and will burn only 10% of its nuclear fuel while on the main sequence. A small star with 10% of the mass of the Sun thus has nearly the same fuel reserves and will shine for tens of trillions of years (Laughlin et al., 1997). Like all stars, red dwarfs get brighter as they age. Owing to their large population, the brightening of red dwarfs nearly compensates for the loss of larger stars, and the galaxy can maintain a nearly constant luminosity for approximately one trillion years (Adams et al., 2004).

Even small stars cannot live forever, and this bright stellar era comes to a close when the galaxies run out of hydrogen gas, star formation ceases, and the longest-lived red dwarfs slowly fade into oblivion. As mentioned earlier, the smallest stars will shine for trillions of years, so the era of stars would come to an end at a cosmic age of several trillion years if new stars were not being manufactured. In large spiral galaxies like the Milky Way, new stars are being made from hydrogen gas, which represents the basic raw material for the process. Galaxies will continue to make new stars as long as the gas supply holds out. If our Galaxy were to continue forming stars at its current rate, it would run out of gas in 'only' 10-20 billion years (Kennicutt et al., 1994), much shorter than the lifetime of the smallest stars. Through conservation practices -the star formation rate decreases as the gas supply grows smaller - galaxies can sustain normal star formation for almost the lifetime of the longest-lived stars (Adams and Laughlin, 1997; Kennicutt et al., 1994). Thus, both stellar volution and star formation will come to an end at approximately the same time in our cosmic future. The universe will be about 100 trillion (1014) years old when the stars finally stop shining. Although our Sun will have long since burned out, this time marks an important turning point for any surviving biospheres - the power available is markedly reduced after the stars turn off.

Continue reading here: The era of degenerate remnants

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