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Starts With A Bang

How big will the Sun become when it dies?

Our Sun will continue to grow, becoming a red giant and then a planetary nebula. Here’s how large it will get.
The Helix Nebula, shown here, offers a potential preview of the planetary nebula/white dwarf combo that our Sun will someday become approximately 8 billion years from now. This nebula, itself, is presently between 3-and-4 light-years in diameter, and our Sun may yet eventually reach an even larger size. (Credit: NASA, ESA, C.R. O'Dell (Vanderbilt University), and M. Meixner, P. McCullough)
Key Takeaways
  • When our Sun runs out of hydrogen fuel in its core, it will expand, engulfing Mercury, Venus, and maybe even the Earth.
  • When it runs out of helium in its core, however, it will create a planetary nebula many light-years across.
  • The Sun’s matter will extend over ~5 light-years before it’s finished dying: far larger than previously known.

Although it shines almost perfectly constantly, the Sun imperceptibly changes over time.

solar flare
A solar flare from our Sun, which ejects matter out away from our parent star and into the Solar System, can trigger events like coronal mass ejections. Although the particles typically take ~3 days to arrive, the most energetic events can reach Earth in under 24 hours, and can cause the most damage to our electronics and electrical infrastructure. (Credit: NASA/Solar Dynamics Observatory/GSFC)

Each second, its core converts over 4 million tons of mass into energy.

This cutaway showcases the various regions of the surface and interior of the Sun, including the core, which is where nuclear fusion occurs. As time goes on, the region of the core where nuclear fusion takes place expands, causing the Sun’s energy output to increase. (Credit: Wikimedia Commons/KelvinSong)

Over time, the core grows, driving increases in energy output, luminosity, and — very slowly — size as well.

The changes in a one solar-mass star’s luminosity, radius, and temperature over its lifetime, from the start of nuclear fusion in its core 4.56 billion years ago until its transition into a full-fledged red giant, which is the beginning of the end for Sun-like stars. (Credit: RJHall/Wikimedia Commons)

Today, the still-growing Sun is about 14% bigger than at birth.

The present sizes of the planets, today, remain unchanged compared to their sizes 4.5 billion years ago, in the early stages of the Solar System. The Sun, however, has grown by a significant margin over that time. In the earliest stages of our Solar System, you could line up only 96 Earth’s across the diameter of the Sun. Today, you can fit 109 Earths there instead: an increase of ~14%. (Credit: NASA/Lunar and Planetary Institute)

After another ~5 billion years, it becomes a subgiant, expanding to double its current size.

When stars fuse hydrogen to helium in their core, they live along the main sequence: the snaky line that runs from lower-right to upper-left. As their cores run out of hydrogen, they become subgiants: hotter, more luminous, cooler, and larger. Procyon, the 8th brightest star in the night sky, is a subgiant star. (Credit: Richard Powell)

About 2.5 billion years later, it swells into a red giant, fusing helium internally.

red giant
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. (Credit: ESO/M. Kornmesser)

It will reach ~300 million km in diameter, engulfing Mercury, Venus, and possibly Earth, too.

red giant
As the Sun becomes a true red giant, the Earth itself may be swallowed or engulfed (Mercury and Venus definitely will), but will certainly be roasted as never before. The Sun’s outer layers will swell to more than 100 times their present diameter, but the exact details of its evolution, and how those changes will affect the orbits of the planets, still have large uncertainties in them. (Credit: Fsgregs/Wikimedia Commons)

But the Sun achieves true enormity upon completing its red giant phase.

red giant
The dying red giant star, R Sculptoris, exhibits a very unusual set of ejecta when viewed in millimeter and submillimeter wavelengths: revealing a spiral structure. This is thought to be due to the presence of a binary companion: something our own Sun lacks but that approximately half of the stars in the Universe possess. (Credit: ALMA (ESO/NAOJ/NRAO)/M. Maercker et al.)

After reaching the asymptotic giant branch, winds expel nearly all the remaining hydrogen.

planetary nebula
This compact, symmetric, bipolar nebula with X-shaped spikes is known to have a binary system at its core, and is at the end of its asymptotic giant branch phase of life. It has begun to form a preplanetary nebula, and its unusual shape is caused by a combination of winds, outflows, ejecta, and the central binary at its core. (Credit: H. Van Winckel (KU Leuven), M. Cohen (UC Berkeley), H. Bond (STScI), T. Gull (GSFC), ESA, NASA)

Outflows, companions, and winds shape, shock, and collimate this stellar ejecta.

planetary nebula
Near the end of a Sun-like star’s life, it begins to blow off its outer layers into the depths of space, forming a protoplanetary nebula like the Egg Nebula, seen here. Its outer layers have not yet been heated to sufficient temperatures by the central, contracting star to create a true planetary nebula just yet. (Credit: NASA and the Hubble Heritage Team (STScI/AURA), Hubble Space Telescope/ACS)

The matter reaches into the Oort cloud, illuminated as a preplanetary nebula.

planetary nebula
When the central star heats up to about temperatures of ~30,000 K, it becomes hot enough to ionize the previously ejected material from a dying star, creating a true planetary nebulae. Here, NGC 7027 has just recently crossed that threshold, and is still rapidly expanding. At just ~0.1-to-0.2 light-years across, it is one of the smallest and youngest planetary nebulae known. (Credit: NASA, ESA, and J. Kastner (RIT))

The core contracts and heats up further, eventually ionizing the expelled material.

planetary nebula
Normally, a planetary nebula will appear similar to the Cat’s Eye Nebula, shown here. A central core of expanding gas is lit up brightly by the central white dwarf, while the diffuse outer regions continue to expand, illuminated far more faintly. The extended halo of matter beyond the typical planetary nebula was formed over ~100,000 years, due to previously ejected material. The entire nebula spans ~4 light-years. (Credit: Nordic Optical Telescope and Romano Corradi (Isaac Newton Group of Telescopes, Spain))

This shining planetary nebula phase lasts approximately 10,000 to 20,000 years.

planetary nebulae
From their earliest beginnings to their final extent before fading away, stars will grow from the size of the Sun 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. (Credit: Ivan Bojičić, Quentin Parker, and David Frew, Laboratory for Space Research, HKU)

Planetary nebulae grow over time, typically reaching ~5 light-years across.

planetary nebula
One of the largest planetary nebulae known at nearly 10 light-years in diameter, Sharpless 2-188 is still expanding, but isn’t as asymmetric as it appears. Its fast velocity relative to the interstellar medium, which is also full of gas, gives the asymmetric appearance, but the nebula itself is nearly spherical in shape. (Credit: T.A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOIRLab/NSF/AURA)

Finally, the material cools, becoming neutral, invisible, and fading away.

planetary nebula
This animation shows how significant the fading of the Stingray Nebula has been since 1996. Note the background star, just to the upper left of the central, fading white dwarf, which remains constant over time, which confirms that the nebula itself is dimming significantly. (Credit: NASA, ESA, B. Balick (University of Washington), M. Guerrero (Instituto de Astrofísica de Andalucía), and G. Ramos-Larios (Universidad de Guadalajara))

Rejoining the interstellar medium, that expelled material contributes to future stellar and planetary generations.

The interstellar medium, normally invisible except for the light it absorbs, can become illuminated by either reflecting starlight or being excited and emitting its own light. Here, the previously-enriched interstellar medium is revealed by the hot, new stars in a central young star cluster. (Credit: Gemini Observatory/AURA; Travis Rector/University of Alaska-Anchorage)

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


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