How a companion can change a star’s fate

When isolated stars form, their fates are pre-determined.

This region of space shows a portion of the plane of the Milky Way, with three extended star-forming regions all side-by-side next to one another. The Omega Nebula (left), the Eagle Nebula (center), and Sharpless 2-54 (right), compose just a small fraction of a vast complex of gas and dust found all through the galactic plane that continuously lead to the formation of newborn stars.

Stellar lifespans rely primarily on initial mass and heavy element content.

Supernovae types as a function of initial star mass and initial content of elements heavier than Helium (metallicity). Note that the first stars occupy the bottom row of the chart, being metal-free, and that the black areas correspond to direct collapse black holes. For modern stars, we are uncertain as to whether the supernovae that create neutron stars are fundamentally the same or different than the ones that create black holes, and whether there is a ‘mass gap’ present between them in nature. We must also consider that effects other than mass and metallicity (such as the presence of a companion) may indeed play major roles in determining the fate of massive stars, including in whether they can contribute to enriching the interstellar medium.

Below 7.5% of the Sun’s mass, you’re only a failed star: a brown dwarf.

The exoplanet Kepler-39b is one of the most massive ones known, at 18 times the mass of Jupiter, placing it right on the border between planet and brown dwarf. In terms of radius, however, it’s only 22% larger than Jupiter, as deuterium fusion doesn’t substantially change the self-compressed object’s size. Objects up to ~80 times the mass of Jupiter are still approximately the same size, with only higher-mass objects initiating nuclear fusion in their cores and becoming true stars.

Above that but below 0.4 solar masses, you’re a red dwarf.

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.

Fully convective, its ultimate fate is a helium white dwarf.

When lower-mass, Sun-like stars run out of fuel, they blow off their outer layers in a planetary nebula, but the center contracts down to form a white dwarf, which takes a very long time to fade to darkness. Lower-mass stars, unable to fuse helium after their core is out of hydrogen, will simply contract down to form a helium white dwarf, with no accompanying planetary nebula remnant at all.

Stars above that but below 8 solar masses are Sun-like.

Eventually, the evolution of the Sun will be the death of all life on Earth. Long before we reach the red giant stage, stellar evolution will cause the Sun’s luminosity to increase significantly enough to boil Earth’s oceans, which will surely eradicate humanity, if not all life on Earth. The exact rate of increase of the Sun’s size, as well as the details about its mass loss in stages, are still not perfectly known.

After helium fusion initiates, these red giants leave standard white dwarf remnants.

When the Sun has completely run out of its nuclear fuel, it will blow off its outer layers into a planetary nebula, while the center contracts into a hot, compact white dwarf star. Without a binary companion, however, the white dwarf will not produce any novae; singlet Sun-like stars don’t do that.

Higher-mass stars achieve carbon fusion and beyond, dying in core-collapse supernovae.

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.

Lower-mass supernovae leave neutron stars; others leave black hole remnants.

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.

But a companion can alter these fates.

Just as stars often exist in binary, trinary, and more populous multi-star systems, so too do brown dwarfs: failed stars. It’s possible that there are binary brown dwarf systems with sufficient separations to enable the inspiral and merger of these components a very long time from now, where they will ignite hydrogen fusion in the post-merger red dwarf that forms. If any orbiting worlds exist at the right distance around the newly formed red dwarf, life may eventually arise even quintillions of years into the future, or more.

Brown dwarfs can merge, creating a true star.

The globular cluster Messier 69 is highly unusual for being both incredibly old, with indications that it formed at just 5% the Universe’s present age (around 13 billion years ago), but also having a very high metal content, at 22% the metallicity of our Sun. The brighter stars are in the red giant phase, just now running out of their core fuel, while a few blue stars are the result of the mergers of initially lower-mass stars: blue stragglers.

Low-mass stars can merge, producing blue stragglers.

This view of part of the Milky Way showcases three zoom levels. At left, the individual star system known as Gaia DR3 4373465352415301632 is shown, which contains a binary companion of ~10 solar masses and an orbital period of 185.6 days (center). At right, an illustration of how the star might appear due to the lensing effect of the black hole is also shown. Many star systems are born in binaries, with the more massive, shorter-lived star becoming a stellar remnant (white dwarf, neutron star, black hole) while the original star still remains in its stellar phase.

The more massive companion evolves faster: creating a stellar corpse.

This illustration shows a neutron star with an accretion disk, siphoning mass off of a low-mass companion star. Many of these systems with neutron stars will have millisecond pulsars for their neutron stars, and the neutron star’s pulsing “jets” will strike, and slowly destroy, the companion star.

These remnants then siphon mass off of their remaining counterparts.

By siphoning mass off of a companion star, a stellar corpse like a white dwarf can eventually accrue enough material to exhibit a thermonuclear runaway event, resulting in a nova. Only if the white dwarf itself exceeds a critical mass threshold (the Chandrasekhar limit) or experiences a detonation event, will a type Ia supernova ensue. This classical picture was the dominant one of the 20th century for type Ia supernova, but here in the 21st century, that is changing.

Exotic, hydrogen-deficient stars can arise.

When a star destined for a supernova has a dense binary companion, that companion can steal enough mass to prevent that supernova from occurring. This mass siphoning by the denser star can lead to the eventual creation of white dwarfs dominated by heavier elements than the typical carbon-and-oxygen or oxygen-and-neon, such as magnesium-dominated white dwarfs.

Tidal disruption can destroy stars entirely.

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.

Binary companions may trigger direct-to-black-hole collapse.

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.

Swallowing neutron stars can create Thorne-Żytkow objects.

Many high-mass binary systems will produce a neutron star orbiting a giant or supergiant star, which will produce X-ray emissions initially. Over time, the neutron star could be absorbed into the supergiant star’s core, creating an exotic class of objects known as Thorne–Żytkow objects.

When stars expand, mass loss via siphoning becomes easier.

When two stellar objects orbit one another, it will be the denser object, even if it’s smaller and/or less massive, that has the capability of siphoning mass off of its companion. If the less dense star is very massive, it can actually lose enough mass to alter its fate and prevent a core-collapse supernova, leading to an unusual scenario where the star’s remnant is an exotic white dwarf.

Sufficiently stripped stars become exotic white dwarfs.

The more massive a white dwarf is, the smaller in radius it gets. The largest white dwarfs are the lowest in mass, and are only slightly smaller than a planet like Uranus or Neptune. The smallest white dwarf ever discovered, however, is comparable in size only to the Moon, very close to the maximum mass threshold for such an object. A white dwarf is typically composed of carbon-oxygen or oxygen-neon, but more exotic types can be made out of heavier elements if a dense companion strips away an evolving high-mass star’s outer layers.

Finally, white dwarf and/or neutron star mergers produce Universe-enriching cataclysms.

In the final moments of merging, two neutron stars don’t merely emit gravitational waves, but a catastrophic explosion that echoes across the electromagnetic spectrum. Whether it forms a stable neutron star or a black hole (like the 2019 merger), or a neutron star that then turns into a black hole (like the 2017 merger), will depend on factors like the total mass of the predecessor neutron stars and their combined spin.

Environment, not just initial mass, determines a star’s ultimate fate.

This simulation of a red supergiant’s surface, sped up to display an entire year of evolution in just a few seconds, shows how a “normal” red supergiant evolves during a relatively quiet period with no perceptible changes to its interior processes. At the centers of some red supergiants, neutron stars or white dwarfs may exist. These ‘stars-within-a-star’ get there via mergers, and can dramatically alter the fate of these red supergiants, preventing supernova explosions and ending their lives in under a million years.

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