This Is What Will Happen To Our Sun After It Dies

An entire Universe of possibilities await stars like our own, even after they run out of fuel.

One of the most fundamental truths in the Universe is the transient nature of existence. Gravitational, electromagnetic, and nuclear forces constantly act upon matter, ensuring that everything we currently observe will eventually change. Even stars, the colossal entities that convert nuclear fuel in the cosmos, will ultimately exhaust their energy sources, including our Sun.

However, the demise of a star, known as stellar death—when it runs out of nuclear fuel—does not signify an absolute end. On the contrary, there are several fascinating developments in store for stars after their initial demise. While it is accurate that our Sun's energy source is limited, leading to a conventional stellar death, this phase is not the final chapter. This holds true for our Sun and other Sun-like stars. Here’s what lies ahead.

To be classified as a true star, a celestial body must be capable of fusing hydrogen into helium. When a gas cloud collapses to form a new star, it possesses significant gravitational potential energy in its diffuse state, which is converted into kinetic (thermal) energy during the collapse. This process heats the matter, and if it reaches sufficient temperature and density, nuclear fusion commences.

Through extensive studies of stellar formation, we have determined that a star must achieve an internal temperature of approximately 4 million K to initiate hydrogen fusion, requiring at least about 8% of our Sun’s mass—around 70 times the mass of Jupiter. This threshold is essential for a star to form.

When this mass and temperature threshold is surpassed, the star begins fusing hydrogen into helium, leading to one of three potential outcomes. These outcomes are determined solely by the star's mass, which influences the maximum temperature achievable in its core. Although all stars begin with hydrogen fusion, the subsequent fate is dependent on temperature. Specifically:

• If a star is of low mass, it will only fuse hydrogen into helium and will not reach the temperatures necessary for helium fusion into carbon. This outcome typifies M-class (red dwarf) stars, which are less than 40% of the Sun's mass and constitute the majority of stars in the Universe.
• If the star resembles our Sun, it will contract as the hydrogen in its core depletes, leading to helium fusion (into carbon) as the star expands into a red giant. It will end its life composed of carbon and oxygen, shedding lighter outer layers of hydrogen and helium. This process occurs for stars ranging from about 40% to 800% of the Sun's mass.
• For stars exceeding eight times the Sun's mass, they will not only fuse hydrogen into helium and helium into carbon but will also initiate carbon fusion, leading to oxygen and silicon fusion, culminating in a spectacular supernova.

These scenarios represent the most common fates for stars. Only a small fraction—about 0.1–0.2%—of all stars are massive enough to undergo a supernova, leaving behind neutron stars or black holes.

Stars with lower mass are the most prevalent in the Universe, accounting for roughly 75–80% of all stars, and they have the longest lifespans. Their lifetimes can range from approximately 150 billion to over 100 trillion years, with none having exhausted their fuel in our 13.8 billion-year-old Universe. Eventually, they will evolve into white dwarfs composed entirely of helium.

Sun-like stars, which make up about a quarter of all stars, will undergo an intriguing transformation when they exhaust the helium in their cores. They will evolve into a planetary nebula/white dwarf pair through a spectacular but gradual death process.

During the red giant phase, Mercury and Venus will likely be engulfed by the expanding Sun, while Earth's fate remains uncertain, influenced by various factors yet to be fully understood. The icy bodies beyond Neptune are expected to melt and evaporate, unlikely to survive the Sun's demise.

Once the outer layers of the Sun are expelled into the interstellar medium, only the remnants of charred planets orbiting the white dwarf will remain. The core, primarily composed of carbon and oxygen, will have approximately half the mass of the current Sun but will be about the size of Earth.

The white dwarf will remain hot for an extended period. Heat represents energy trapped within any object, which can only be lost through its surface. If we were to compress the energy of a star like our Sun into a smaller volume, it would heat up significantly. The red giant stars that give rise to white dwarfs are typically much cooler than the resulting dwarf itself. During contraction, temperatures can rise from as low as 3,000 K (for a red giant) to about 20,000 K (for a white dwarf). This temperature increase is due to adiabatic compression.

However, this white dwarf must eventually cool down, which will occur through its relatively small, Earth-sized surface. If we were to form a white dwarf today at 20,000 K, after 13.8 billion years, it would cool by only around 40 K, to 19,960 K.

Thus, we face a long wait to witness our Sun cool to the point of invisibility. Once the Sun exhausts its fuel, the Universe will provide ample time for this process. While galaxies in the Local Group will merge, and others will drift away due to dark energy, star formation will slow, with the lowest-mass red dwarfs depleting their fuel. Yet, the white dwarf will continue to cool.

Eventually, after a span of 100 trillion to 1 quadrillion years, the white dwarf that was once our Sun will fade from visibility and cool down to just a few degrees above absolute zero. This remnant will be known as a black dwarf, traversing through whatever remains of our galaxy alongside countless other stars and stellar remnants from our Local Group.

Yet, this is not the final chapter for our Sun. There are three potential outcomes that await it, contingent on various cosmic circumstances.

1.) Completely Unlucky: Approximately half of all stellar remnants in the galaxy originate from single star systems, similar to our Sun. Although multi-star systems are prevalent, with about 50% of all known stars existing in binary or trinary configurations, our Sun stands alone. This reality significantly diminishes the likelihood of merging with another star or being engulfed by one. If we don’t experience a merger, the remnants of our Solar System will likely be ejected from the galaxy after around 10¹? to 10¹? years.

2.) Lucky Enough to Revitalize: It may seem logical that once our Sun’s white dwarf cools, it will never shine again. However, there are numerous ways for it to gain a new lease on life and emit powerful radiation once more. This revival requires a new source of matter. If, in the distant future, our Sun:

• merges with a red dwarf or brown dwarf,
• accumulates hydrogen gas from a molecular cloud or a gaseous planet,
• or encounters another stellar remnant,

it can reignite nuclear fusion. The first scenario could result in millions of years of hydrogen burning; the second would lead to a nova, while the last could trigger a supernova explosion, obliterating both stellar remnants. If such an event occurs before ejection, it would be a spectacular cosmic display for remaining observers in our galaxy.

3.) Super Lucky, Devoured by a Black Hole: In the outer regions of our galaxy, approximately 25,000 light-years from the central supermassive black hole, only smaller stellar-mass black holes exist. These black holes present the smallest target area among massive objects in the Universe. Although extremely unlikely, these small black holes can occasionally interact with other matter. When they do, they can accelerate and funnel the matter into an accretion flow, consuming some and ejecting the rest. These active low-mass black holes are termed microquasars when they flare.

While it’s improbable for our Sun to meet such a fate, someone will eventually encounter this scenario, becoming food for a black hole in their final act.

In conclusion, every object in the Universe has a multitude of potential futures, and determining the fate of a specific object in the chaotic cosmos is a complex endeavor. However, by understanding the underlying physics and estimating probabilities and timescales for each type of object, we can predict likely outcomes.

For our Sun, we anticipate a transformation into a white dwarf in less than 10 billion years, followed by cooling to a black dwarf over the course of 10¹? to 10¹? years, and a possible ejection from the galaxy after 10¹? to 10¹? years. While this is the most probable trajectory, mergers, gas accumulation, collisions, or even consumption by a black hole are also possibilities. The future may remain unwritten, but the odds are in favor of a vibrant existence for trillions of years to come!

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.