~10¹⁴

Last of the Stars

Stellar evolution can be thought of as a giant cosmic recycling program. Clouds of gas and dust condense and contract through gravitational forces, eventually growing into spherical masses—stars—with deep internal pressures and temperatures high enough to initiate nuclear fusion. After their hydrogen or helium or other nuclear fuel is consumed, stars die in gentle to spectacular ways depending on their mass, expelling much of that mass out into deep space. Those stellar remains—clouds of gas and dust—can then condense and contract through gravitational forces to form new stars. It’s a beautiful cycle of stellar life.

Each time a star dies, however, some significant fraction of its mass does not get recycled back into space, but remains behind as a slowly cooling white dwarf star (in the case of low-mass stars) or another stellar remnant, such as a neutron star or a black hole (in the case of higher-mass stars). Thus, over time, all the material in the universe involved in star formation is eventually trapped in these final, nonrecycled stellar remnants. With the vast majority of stars living their lives as medium- to low-mass main sequence stars, the vast majority of the final trapped observable mass of the universe could end up in the form of white dwarf stars.

A typical main sequence star’s lifetime is about 10 billion, or 10¹⁰ years, although lower-mass stars, like those near the theoretical lower limit of nuclear fusion (around 8 percent of the mass of the Sun), can have lifetimes as long as 10 trillion (10¹³) years. Astronomers estimate (with considerable uncertainty) that by the time the universe is about 100 trillion (10¹⁴) years old, or about 10,000 times as old as it is now, almost all the observable mass of the cosmos will have been trapped in white dwarfs, with a small amount trapped in red dwarfs and other remnants, such as neutron stars and black holes. Star formation will thus come to an end, and the universe will enter a very different, final era of development.

The cosmos will slowly begin to fade to black. After they cool, white dwarfs should theoretically become black dwarfs—stellar remnants with temperatures eventually approaching absolute zero. But no one really knows how long it will take the universe’s last stars to shut off; in some theories, dark matter or weakly interacting nuclear forces may help keep these last embers of the once-glorious stars dimly shining for 10¹⁵ to 10²⁵ years or more.

SEE ALSO White Dwarfs (1862), Main Sequence (1910), Neutron Stars (1933), Nuclear Fusion (1939), Black Holes (1965), End of the Sun (~5–7 Billion), How Will the Universe End? (The End of Time).

Hubble Space Telescope photograph of ancient (12–13 billion years old) white dwarf stars in the Milky Way galaxy. The 2002 photo is from part of the globular cluster M4 in Scorpius.