The ultimate fate of every star that’s ever lived

Given enough time, every star will eventually die.

Artist’s illustration (left) of the interior of a massive star in the final stages, pre-supernova, of silicon-burning. (Silicon-burning is where iron, nickel, and cobalt form in the core.) A Chandra image (right) of the Cassiopeia A supernova remnant today shows elements like iron (blue), sulfur (green), and magnesium (red). Ejected stellar material can glow due to heat in the infrared for tens of thousands of years, and the ejecta from supernovae can be asymmetric and can have segregated elements within it, as shown here. In the right environment, this asymmetric material can be unevenly incorporated into future generations of stars.

Stars are born whenever gaseous matter accumulates, fragments, and collapses.

Here, evaporating gaseous globules are seen at the edge of a star-forming region within the Orion Nebula, with newborn stars, Herbig-Haro objects, and many fainter sources of light, including protostars, brown dwarfs, and even planetary-mass objects found inside. As the gas continues to boil away, more and more of these lower-mass objects should be revealed.

Initiating hydrogen fusion in their cores officially triggers a star’s birth.

This ALMA observation of a high-mass protostar cluster, G351.77-0.54, has gotten down to ~120 AU spatial resolution, corresponding to 0.06 arc-seconds at the distance of these protostars. The gaseous material is fragmenting into at least four separate cores, a hint (now with further evidence) that core fragmentation, rather than anything having to do with a disk, is a major player in determining how many stars form in these high-mass star-forming regions. When nuclear fusion reactions initiate inside these protostar cores, they will officially become full-fledged stars.

The outward pressure from nuclear reactions holds the star up against gravitational collapse.

This cutaway showcases the various regions of the surface and interior of the Sun, including the core, which is the only location where nuclear fusion occurs. As time goes on and hydrogen is consumed, the helium-containing region in the core expands and the maximum temperature increases, causing the Sun’s energy output to increase. The balance between the inward-pulling gravity and the outward-pushing radiation pressure is what determines the size and stability of a star.

When insufficient pressure is created, the star directly collapses to a black hole.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. Direct collapse, while still under investigation, may be triggered by a stellar companion.

The most massive stars rapidly burn through their fuel, progressing onto fusing heavier elements.

The anatomy of a very massive star throughout its life, culminating in a Type II (core-collapse) Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive core-collapse supernovae typically result in the creation of black holes, while the less massive ones create only neutron stars.

Ultimately, they’ll go supernova, leaving a black hole or neutron star remnant.

In the inner regions of a star that undergoes a core-collapse supernova, a neutron star begins to form in the core, while the outer layers crash against it and undergo their own runaway fusion reactions. Neutrons, neutrinos, radiation, and extraordinary amounts of energy are produced, with neutrinos and antineutrinos carrying the majority of the core-collapse supernova’s energy away. Whether the remnant becomes a neutron star or black hole, ultimately, depends on how much mass remains in the core during this process.

Less massive stars, like the Sun, cannot fuse elements beyond helium.

As the Sun becomes a true red giant, expanding to over 100 times its current size as its interior contracts and heats up to fuse helium, the Earth itself may be swallowed or engulfed, but will definitely be roasted as never before. The Sun’s outer layers will swell, but the exact details of its evolution, and how those changes will affect the orbits of the planets, still have large uncertainties in them. Mercury and Venus will definitely be swallowed by the Sun, but Earth will be very close to the border of survival/engulfment.

They’re fated to die in a planetary nebula, leaving white dwarfs behind.

When our Sun runs out of fuel, it will become a red giant, followed by a planetary nebula with a white dwarf at the center. The Cat’s Eye Nebula is a visually spectacular example of this potential fate, with the intricate, layered, asymmetrical shape of this particular one suggesting a binary companion. At the center, a young white dwarf heats up as it contracts, reaching temperatures tens of thousands of Kelvin hotter than the surface of the red giant that spawned it. The outer shells of gas are mostly hydrogen, which gets returned to the interstellar medium at the end of a Sun-like star’s life.

The least massive stars, meanwhile, fuse only hydrogen in their cores.

The most straightforward and lowest-energy version of the proton-proton chain, which produces helium-4 from initial hydrogen fuel. Note that only the fusion of deuterium and a proton produces helium from hydrogen; all other reactions either produce hydrogen or make helium from other isotopes of helium. This reaction set occurs in the interiors of all young, hydrogen-rich stars, regardless of mass.

They live the longest, becoming pure helium white dwarfs: with no planetary nebula counterpart.

Energy produced in a star’s core must pass through large amounts of ionized material before reaching the photosphere, where it’s radiated away. Inside the Sun, there’s a large, non-convective radiative zone surrounding the core, but in lower-mass stars such as red dwarfs, the entire star can convect on timescales of tens or hundreds of billions of years (or longer), enabling red dwarf stars to fuse 100% of the hydrogen within them. Red dwarfs cannot fuse heavier elements than hydrogen, so when all their hydrogen has fused, they simply contract down to a helium white dwarf.

Stellar and brown dwarf mergers bump them up to greater masses, altering their fates.

When an orbiting body enters the photosphere of a massive star, the star will swell in size and brighten substantially, but will also cease spewing out dusty material; that was only a part of the pre-merger phase of the astronomical system in question. Stars often grow by mergers into more massive, shorter-lived stars.

Black hole encounters destroy stars through tidal disruption: gravitationally tearing them apart.

This illustration of a tidal disruption event shows the fate of a massive, large astronomical body that has the misfortune of coming too close to a black hole. It will get stretched and compressed in one dimension, shredding it, accelerating its matter, and alternately devouring and ejecting the debris that arises from it. Black holes with accretion disks are often highly asymmetrical in their properties, but far more luminous than inactive black holes that lack them.

Black holes ultimately decay into radiation via the Hawking process.

The event horizon of a black hole is a spherical or spheroidal region from which nothing, not even light, can escape. But outside the event horizon, the black hole is predicted to emit radiation. Hawking’s 1974 work was the first to demonstrate this, but that work has also led to a paradox that has not yet been resolved.

White dwarf mergers create Type Ia supernovae: annihilating them.

The two main ways to make a Type Ia supernova: the accretion scenario (left) and the merger scenario (right). Most white dwarfs that go supernova are significantly below the Chandrasekhar mass limit, strongly favoring the merger scenario for most Type Ia supernovae.

Meanwhile, lonesome white dwarfs and neutron stars simply fade to black: cold, non-luminous, but persisting eternally.

An accurate size/color comparison of a white dwarf (left), Earth reflecting our Sun’s light (middle), and a black dwarf (right). When white dwarfs finally radiate the last of their energy away, they will all eventually become black dwarfs. The degeneracy pressure between the electrons within the white/black dwarf, however, will always be great enough, so long as it doesn’t accrue too much mass, to prevent it from collapsing further. A similar process, albeit on longer timescales, should occur for neutron stars.

Only low-mass, isolated stellar corpses will persist eternally.

After the Sun dies, its remnant core will contract down to become a white dwarf. Over timescales of 100 trillion years, it will fade away, eventually becoming a black dwarf. Any surviving planets in orbit around it must survive gravitational encounters in order to remain, where gravitational radiation will eventually cause them to be devoured by the black dwarf. Black dwarfs should be the last surviving stellar remnants of all.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.