Will the Sun make a planetary nebula when it dies? | by Ethan Siegel | Starts With A Bang! | Apr, 2025

MarylandDigitalNews.comSCIENCE

posted on Apr. 14, 2025 at 2:33 pm

From their earliest beginnings to their final extent before fading away, Sun-like stars will grow from their present size to the size of a red giant (~the Earth’s orbit) to up to ~5 light-years in diameter, typically. The largest known planetary nebulae can reach approximately double that size, up to ~10 light-years across, but none of this necessarily means that the Sun is a typical, average star. (Credit: Ivan Bojičić, Quentin Parker, and David Frew, Laboratory for Space Research, HKU)

In around 7 billion years, we expect the Sun to run out of fuel, dying in a planetary nebula/white dwarf combination. Is that for certain?

Whenever a star is born, it expectantly follows a specific life cycle.

This Hubble Space Telescope image of open star cluster NGC 290, showcases a region where thousands of newborn stars were created 30–60 million years ago. They come in a wide variety of masses, where a combination of their initial mass and future interactions will determine their ultimate fates. (Credit: ESA and NASA; Acknowledgment: E. Olszewski (University of Arizona))

Stars are hot, dense balls of gas and plasma.

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 to “cross the main sequence” as its energy output increases. The balance between the inward-pulling gravity and the outward-pushing gas pressure, only slightly augmented by radiation pressure, determines the size and stability of a star, while the core’s temperature and element abundance determines the rate and species of fusion inside. (Credit: Wikimedia Commons/KelvinSong)

Inside their cores, nuclear fusion occurs: fusing light elements into heavier ones, liberating energy.

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. (Credit: Sarang/Wikimedia Commons)

Ultimately, all stars of all masses exhaust their core’s hydrogen: fusion’s primary fuel source.

After its formation some 4.6 billion years ago, the Sun has grown in radius by approximately 14%. It will continue to grow, doubling in size when it becomes a subgiant, but it will increase in size by more than ~100-fold when it becomes a true red giant in another ~7–8 billion years, total, all while growing in brightness by a factor of at least a few hundred. (Credit: ESO/M. Kornmesser)

For stars like the Sun (or more massive), the star evolves: swelling into a red giant.

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. (Credit: Fsgregs/Wikimedia Commons)

Meanwhile, hydrogen burning continues in a shell surrounding the core.

When a main sequence star, like the Sun, runs out of hydrogen in its core, its core becomes inert and the star expands into a subgiant, while hydrogen fusion continues in a shell surrounding the core. Eventually, the core contracts and heats up, where it can initiate helium fusion if the star’s core gets hot enough, which will only happen for sufficiently massive stars. (Credit: Thomas Kallinger/University of British Columbia/University of Vienna)

Eventually, the contracting core heats up sufficiently to fuse helium.

The prediction of the Hoyle State, an excited state of a carbon-12 nucleus, and the discovery of the triple-alpha process is perhaps the most stunningly successful use of anthropic reasoning in scientific history. This process is what explains the creation of the majority of carbon that’s found in our modern-day Universe: created in the hearts of evolved stars that fuse helium into carbon. The work of Hoyle, Fowler, and the Burbidges demonstrated that carbon was created via the process of stellar nucleosynthesis, rather than during the hot Big Bang. (Credit: E. Siegel/Beyond the Galaxy)

And then, critically, the giant star’s core runs out of helium fuel.

MarylandDigitalNews.comApril 14, 2025

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