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Starts With A Bang

Astronomers Still Don’t Know What It Will Look Like When The Sun Dies

Planetary nebulae take a wide variety of shapes and orientations depending on the properties of the stellar system they arise from, and are responsible for many of the heavy elements in the Universe. Supergiant stars and giant stars entering the planetary nebula phase are both shown to build up many important elements of the periodic table via the s-process. (NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))

50% of stars are in Sun-like ‘singlet’ systems. The planetary nebulae we see just don’t line up.


Around 7 billion years from now, our Sun’s life will end.

As the Sun becomes a true red giant, the Earth itself may be swallowed or engulfed, but will definitely 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. (WIKIMEDIA COMMONS/FSGREGS)

Becoming a red giant, it eventually exhausts its core fuel.

In the early stages of a preplanetary nebula, hydrogen gas is expelled in a roughly spherical fashion, before transitioning to a bipolar shape. The spiral pattern is thought to emerge if the star ejecting the matter is part of a binary system, which is not uncommon. Approximately 50% of the stars in the Universe are parts of multi-star systems. (ESA/NASA & R. SAHAI)

Gravitation overcomes the decreased radiation, expelling the tenuous outer layers.

The Egg Nebula, as imaged here by Hubble, is a preplanetary nebula, as its outer layers have not yet been heated to sufficient temperatures by the central, contracting star. This will evolve into a planetary nebula when fusion in the outer layers of the central white dwarf ionizes and more fully illuminates the surrounding ejecta. (NASA)

The hot, contracting interior — forming a white dwarf — ionizes and illuminates the ejecta.

The rotten egg nebula, at lower right (and shown in detail in the inset box, as imaged by Hubble) is a preplanetary nebula that’s part of a larger star cluster that also contains a full-blown planetary nebula, at the upper left. Whereas planetary nebulae are emission nebula, preplanetary nebulae only reflect the light from their central star. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA (MAIN); ESA/HUBBLE & NASA ACKNOWLEDGEMENT: JUDY SCHMIDT (INSET))

These short-lived planetary nebulae shine for only tens of thousands of years before fading away.

The Dumbbell Nebula, as imaged here through an 8″ amateur telescope, was the first planetary nebula ever discovered: by Charles Messier in 1764. The shells of gas are slowly expanding and their definition remains constant over time, typical for a planetary nebula. (MIKE DURKIN; MADMIKED/FLICKR)

All stars born with 40%-to-800% the Sun’s mass experience similar fates.

The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. Our Sun is a G-class star, producing light with an effective temperature of around 5800 K and a brightness of 1 solar luminosity. Stars can be as low in mass as 8% the mass of our Sun, where they’ll burn with ~0.01% our Sun’s brightness and live for more than 1000 times as long, but they can also rise to hundreds of times our Sun’s mass, with millions of times our Sun’s luminosity and lifetimes of just a few million years. The most massive O-and-B-stars will go supernova, while the low-mass M-class stars will never fuse helium in their cores. All others, comprising about ~20% of all stars, will die in a planetary nebula/white dwarf combination, like our Sun. (WIKIMEDIA COMMONS USER LUCASVB, ADDITIONS BY E. SIEGEL)

Only ~3000 planetary nebulae exist among the Milky Way’s ~400 billion stars.

The famed planetary nebula NGC 7293, the Helix Nebula, and its central white dwarf, as imaged by Hubble. The central white dwarf is responsible for illuminating the outer layers, as the hot, ultraviolet radiation ionizes the outer material, which then emits light when the electrons fall back onto the ionized nuclei and cascade down the various energy levels. (NASA, ESA, AND C.R. O’DELL (VANDERBILT UNIVERSITY))

And yet, there’s a tremendous mystery surrounding them.

Nitrogen, hydrogen and oxygen are highlighted in the planetary nebula above, known as the Hourglass Nebula for its distinctive shape. The assigned colors distinctly show the locations of the various elements, which are segregated from one another. Complex molecules, including fullerenes containing 60 carbon atoms apiece, have been found in various planetary nebulae (and other locations) in space. (NASA/HST/WFPC2; R SAHAI AND J TRAUGER (JPL))

Somehow, 80% of them show evidence of directionality.

The Twin Jet nebula, shown here, is a stunning example of a bipolar nebula, which is thought to originate from either a rapidly rotating star, or a star that’s part of a binary system when it dies. We’re still working to understand exactly how our Sun will appear when it becomes a planetary nebula in the distant future. (ESA, HUBBLE & NASA, ACKNOWLEDGEMENT: JUDY SCHMIDT)

Many are bipolar nebulae, with two opposing lobes of ejecta.

This planetary nebula may be known as the ‘Butterfly Nebula’, but in reality it’s hot, ionized luminous gas blown off in the death throes of a dying star, and illuminated by the hot, white dwarf this dying star leaves behind. Our Sun is likely in for a similar fate at the end of its red giant, helium-burning phase. (STSCI / NASA, ESA, AND THE HUBBLE SM4 ERO TEAM)

Others display spiral structures within them.

The Red Rectangle Nebula, so called because of its red color and unique rectangular shape, is a preplanetary nebula in the Monoceros constellation. It is part of a binary star system, where one member is ejecting the hydrogen gas in the post-AGB phase. This system will someday evolve, but has not yet evolved, into a full fledged planetary nebula. (ESA/HUBBLE & NASA)

Still others are sculpted with odd, irregular shapes.

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. This is in contrast to the Stingray Nebula, which appears to be contracting. (NORDIC OPTICAL TELESCOPE AND ROMANO CORRADI / WIKIMEDIA COMMONS / CC BY-SA 3.0)

Merely 20% of planetary nebulae appear spherically symmetric: expected for singlet, Sun-like stars.

When seen from a certain orientation, this donut-shaped nebula, known as the Ring Nebula, provides a possible example of what our Sun might become approximately 7 billion years from now, when it dies in a planetary nebula. We expect spherical symmetry from singlet stars, but the number of spherical planetary nebulae is lower than we expect. (NASA, ESA, AND C. ROBERT O’DELL, VANDERBILT UNIVERSITY)

This is puzzling: 50% of all stars are singlets like our Sun.

This image from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star located about 3300 light-years away. The green color arises from emission line transitions in the ionized gas surrounding the dim, dying star. The spherical shape, however, is a relative rarity. (ESO / FORS INSTRUMENT)

Why, then, are only 20% of planetary nebulae spherically symmetric?

The red spiders nebula, shown here, has ripples and shock waves throughout its gas, due to the ultra-high temperature of its parent star: one of the hottest stars to form a planetary nebula in the known Universe. It is not yet known why this nebula has the morphology that it does, rather than being spherically symmetric. (ESA & GARRELT MELLEMA, LEIDEN UNIVERSITY, THE NETHERLANDS)

Perhaps large planets also carve irregular shapes.

This fiery swirl, known colloquially as the Eye of Sauron Nebula, is actually a planetary nebula known as ESO 456–67. The different gases and opacities translate into this stunning, multiwavelength view that looks right at you from across the galaxy. (ESA/HUBBLE AND NASA / ACKNOWLEDGMENT: JEAN-CHRISTOPHE LAMBRY)

Perhaps magnetic fields cause asymmetrical nebulae around singlet stars.

The planetary nebula shown here, NGC 2440, shows a large amount of ejected material blown off during the final stages of a dying red giant star’s life. The uncertainties in modeling the evolution of our Sun beyond the main sequence phase are too great to definitively draw conclusions about the survivability of planet Earth, or of the shape of our Sun’s eventual planetary nebula. (HUBBLE HERITAGE TEAM, ESA/NASA HUBBLE, AND HOWARD BOND (STSCI) AND ROBIN CIARDULLO (PENN STATE))

Or perhaps the more massive stars, shorter-lived and rapidly spinning, bias our statistics.

The Medusa Nebula, shown here, is faint, diffuse, and shows a complex structure indicative of its old age. Planetary Nebulae only persist for about 10,000 to 20,000 years, and this one is apparently nearing the end of its life. As the gas becomes neutral or too diffuse to shine and the central white dwarf cools, the nebula fades away entirely. (JSCHULMAN555 / WIKIMEDIA COMMONS / MT. LEMMON SKYCENTER)

Despite our knowledge, we still cannot predict the Sun’s eventual nebular structure.

The planetary nebula NGC 6369’s blue-green ring marks the location where energetic ultraviolet light has stripped electrons from oxygen atoms in the gas. Our Sun, being a single star that rotates on the slow end of stars, is very likely going to wind up looking akin to this nebula after perhaps another 7 billion years. (NASA AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))

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

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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