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Western Slope Skies: The Exotic Afterlife of Stars

The Helix Nebula, concealing a white dwarf
NASA/Spitzer/GALEX
The Helix Nebula, concealing a white dwarf

In a Western Slope Skies feature last fall, we discussed the cosmic origins of the chemical elements, including the process of stellar nucleosynthesis. From simple hydrogen and helium, stars construct more complex atoms through nuclear fusion, subsequently producing intense radiation pressure. In détente with a star’s compressive gravitation, this pressure shapes a star’s structure and size. As stars age, nucleosynthesis diminishes. Eventually gravity overcomes radiation pressure, precipitating stellar collapse and mass loss. A cinder-like stellar remnant emerges, composed of strange degenerate matter.

Roughly Sun-like stars collapse into an Earth-sized white dwarf. Highly compact, a teaspoon of white dwarf matter would weigh as much as an elephant. Gravity jams atoms together like marbles in a jar, producing electron degeneracy pressure. The resulting counterbalance sets a white dwarf’s maximum mass, the Chandrasekhar limit of about 1.4 solar masses. Frequently found within vivid planetary nebulae, white dwarfs will far outlast the Universe’s current age of 13.8 billion years, gradually freezing into a black dwarf.

Stars considerably larger than the Sun collapse into a city-sized neutron star. Far denser than a white dwarf, a teaspoon of neutron star matter would weigh 10 million tons. Gravity surpasses electron degeneracy pressure, merging atomic nuclei into neutrons. These exert their own degeneracy pressure, imposing a Tolman-Oppenheimer-Volkoff limit of about 2.9 solar masses. Among neutron star types are dervishing pulsars with sub-second rotational periods, manic magnetars with magnetic fields a quadrillion times greater than Earth’s, and vampiric members of x-ray binary systems, hungrily siphoning their companion star’s mass.

Theoretically, a highly massive stellar collapse could produce a quark star, formed from the gravitational shattering of protons and neutrons into constituent quarks. Also supported by degeneracy pressure, quark stars are expected to resemble unusually small, energetic neutron stars. A still greater collapse could hypothetically create an electroweak star, with a core temperature above a quadrillion Kelvin degrees. Internal electromagnetic and weak nuclear forces would merge under these conditions, as they were until briefly after the Big Bang. As yet, neither remnant has been confirmed through observation.

In the most extreme collapse scenario, a black hole of unfathomable gravitation results. Nothing can escape its interior, neither matter nor radiation. The heart of a black hole is the singularity, a region of infinitesimal size yet infinite density. Known physical laws break down here-- we are unsure of what truly happens. Yet black holes are real, having been observed in the hearts of galaxies and colliding with one another.

By some estimates, the Universe contains a trillion trillion stars. If so, the afterlife of stars must be a teeming, exotic realm.

You’ve been listening to Western Slopes Skies, produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and voiced by Michael T. Williams.