- Dana McKenzie
- (Dana Mackenzie)
Billions of years later, when the Sun approaches the end of its lifespan and the helium nuclei at its core begin to fuse, it will expand dramatically and become a so-called red giant. After swallowing Mercury, Venus, and Earth, it would become so large that it could no longer hold the outermost layers of gas and dust.
In its glorious finale, it will catapult these outer layers into space, creating a beautiful curtain of light that will glow like neon for thousands of years before disappearing.
The Milky Way is littered with thousands of these splendid remnants, known as planetary nebulae. This is the normal end phase for stars ranging from half the mass of the Sun to eight times the mass of the Sun. More massive stars have a much more violent ending, the kind of explosions known as supernovae. Planetary nebulae come in an amazing variety of shapes, such as the Southern Crab, Cat’s Eye and Butterfly. As beautiful as they are, they are also a mystery to astronomers. How did the Butterfly Nebula emerge from the seemingly unremarkable circular cocoon of a red giant star?
Both observations and computer models now offer an explanation that seemed odd 30 years ago: Most red giant stars have a much smaller companion star hidden in their gravitational pull. This second star is shaped into a planetary nebula by this process, much like a potter shapes a vessel on a turntable.
On the left of the image above, a near-infrared image shows the concentric shell of gas in the spectacular Southern Ring Nebula, a planetary-like nebula about 2,500 light-years away in the constellation Vela. nebula. These shells record the history of explosions from dying stars. On the right, a mid-infrared image easily distinguishes the dying star (red) at the center of the nebula from its companion (blue). All the gas and dust in the nebula was dissipated by the red star.
Previously the dominant theory of planetary nebula formation involved only one star—the red giant itself. With only a weak gravitational pull in the outer layers, it loses mass very rapidly at the end of its life, 1% per century. It also churns beneath the surface like a pot of boiling water, causing the outer layers to pulse in and out. Astronomers theorize that shock waves from these pulsations eject gas and dust into space, creating what is known as a stellar wind. However, it takes a lot of energy to completely expel this material without falling back into the star. It can’t be a mild wind, which needs the strength of a rocket blast to form a jet.
After the star’s outer layers escape, the much smaller inner layers collapse into a white dwarf. Hotter and brighter than the original red giant, the star illuminates and heats the escaping gas until the gas starts to glow on its own — and we see a planetary nebula. The whole process is very fast by astronomical standards, but slow by human standards, usually taking centuries to thousands of years.
Before the Hubble Space Telescope was used in 1990, “we were pretty sure we were on the right track,” said Bruce Balick, an astronomer at the University of Washington. . Then he and his colleague, Adam Frank of the University of Rochester in New York, saw the first Hubble image of a planetary nebula at a conference in Austria. “We went out for coffee, saw the photos, and we knew the research path had changed,” Barrick said.
Astronomers have assumed that red giants are spherically symmetric and that round stars should produce round planetary nebulae. But that’s not what Hubble saw — not even up close. “It’s clear that many planetary nebulae have bizarre axisymmetric structures,” said astronomer Joel Kastner of the Rochester Institute of Technology. Hubble revealed wondrous fronds, wings and other structures that are not circular but symmetrical around the nebula’s main axis, like turning on a potter’s turntable.
Instead of four legs, the Southern Crab Nebula has two bubbles in the above image from the Hubble Space Telescope. The center of the bubble shows two “knotted” gas jets that may glow when they encounter the gas between the stars. Located thousands of light-years from Earth in the constellation Centaurus, the southern crab appears to have experienced two gas-releasing events. An event about 5,500 years ago created the outer “hourglass,” and a similar event 2,300 years ago created a much smaller hourglass inside.
A 2002 article by Barrick and Frank in the Annual Review of Astronomy and Astrophysics addressed the debate over the origin of these structures at the time. Some scientists have proposed that axial symmetry stems from how red giant stars rotate or how their magnetic fields behave, but neither idea passes some basic tests. Both the rotation and the magnetic field should become weaker as the star gets larger, but the rate of mass loss in red giants accelerates towards the end of their lives.
Another way of saying it is that most planetary nebulae are not formed by a single star, but by a pair of stars – Orsola DeMarco, an astronomer at Macquarie University in Sydney Marco named it the “binary star hypothesis”. In this case, the second star is much smaller and thousands of times less luminous than the red giant, possibly as far away from the sun as Jupiter. So it destroys the red giant star far enough away without being engulfed. (Other possibilities exist, such as a subduction approach orbit in which a second star approaches the red giant every few hundred years, peeling off a shell from it.)
The binary star hypothesis nicely explains the first stage of the deformation of a dying star. When the companion star pulls dust and gas away from the host star, they are not immediately sucked into the companion star, but instead form a spinning disk of material, called an accretion disk, on the companion star’s orbital plane. This accretion disk is like a potter’s wheel. If the disk has a magnetic field, it will push any charged gas out of the plane of the disk and toward the axis of rotation. But even without a magnetic field, the material in the disk blocks the outward flow of the gas in the orbital plane, so the gas takes on a double-lobed structure and flows at a faster rate toward the poles. That’s exactly what Hubble sees in images of planetary nebulae. “Why go looking for a very complicated explanation when a companion star can explain the phenomenon so well,” DeMarco said.
In the upper left image, the Twin Jet Nebula in the constellation Ophiuchus, 2,400 light-years from Earth, shows an hourglass shape with two fast-moving jets of gas toward the poles. The gas may have been ejected from the central star 1,200 years ago. In the image to the right, the Cat’s Eye Nebula in Draco, 3,300 light-years from Earth, shows 11 concentric rings of dust that astronomers estimate is released every 1,500 years. The process by which the complex internal structures form is still anyone’s guess.
seeing is believing
However, some astronomers disagree that the companion star cannot be detected. As recently as 2020, a prominent astrophysicist told her, “You know, Leuven, it’s all about It’s amazing to see, the observations are so fascinating, and the current state-of-the-art models seem to be doing a good job of explaining the data, but ultimately, shouldn’t we just trust what we actually see?”
But over the past 10 to 15 years, that trend has steadily reversed. New telescopes have revealed that some red giants are surrounded by spiral structures and accretion disks before turning into planetary nebulae — as expected if a second star pulls material from the red giant. In some cases, astronomers may even have discovered the companion star itself.
DXN and her colleagues are particularly reliant on the Atacama Large Millimeter/submillimeter Array (Alma) in Chile, which came online in 2011. The Atacama consists of 66 radio telescopes that work together to produce images of astronomical objects. “If you want to understand dynamics and velocity, which gives us high spatial and spectral resolution, that’s important,” DXN said. For scientists to map stellar winds and accretion disks, velocity is a an important question.
Atacama found spirals, or arcs, around more than a dozen red giant stars, almost certainly a sign that material is being shed from the red giant and spiraling toward its companion star. These spirals are so close to computer simulations that they cannot be explained by older stellar wind models. DXN reported the initial findings in the journal Science in 2020, and further elaborated on them in the following year in the Annual Review of Astronomy and Astrophysics.
In addition, DXN’s team may have discovered in the Atacama images two previously undetectable companions of the red giant stars, namely p1 Gruis and L2 Puppis. To be sure, she needs to monitor them over time to see if the newly detected objects are orbiting the host star. “If they moved, I’m sure they had company,” says DXN. Perhaps this discovery will convince the last skeptics.
Like crime scene investigators, astronomers now have pre- and post-formation snapshots of planetary nebulae. The one thing they were missing was a video of the entire process. Do astronomers have any hope of capturing a red giant star that is turning into a planetary nebula?
So far, computer models have been the only way to “see” this centuries-long process from start to finish. They help astronomers pinpoint scenarios where the companion star, after orbiting the host star for a long time, loses distance due to tidal forces and falls into the host star. As it spiraled toward the red giant’s core, the companion star released “amazing gravitational energy,” Frank said. Computer models show that this greatly accelerates the process of the star breaking away from its outer layers, taking only 1 to 10 years. If this is correct, if astronomers know where to look, they can witness the death of stars and the birth of planetary nebulae in real time.
One candidate to watch is V Hydrae. The very active red giant spews bullet-shaped clumps of plasma at the poles every 8.5 years, and it has also coughed up six large rings on the equator for the past 2,100 years. Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory, who published the discovery in April, believes the red giant has two companion stars instead of one. A nearby companion may have brushed past the red giant’s outer shell and produced a plasma jet, while a distant companion controlled the ring’s jet in its dive-bombing orbit. If so, V Hydra may devour its closer companions.
Finally, what about our sun? The study of binary stars seems to have little to do with the fate of our star, since it is a single star. DXN estimates that a star with a companion loses mass 6 to 10 times faster than a star without, because a companion is much more efficient at peeling off a red giant’s shell than a red giant is at itself.
This means that data on stars with companion stars cannot reliably predict the fate of stars without companion stars, such as the Sun. About half of the sun-sized stars have some type of companion star. The companion star always affects the shape of the stellar wind, and if it’s close enough to the star, it can significantly affect the rate of mass loss, DXN said. The Sun will likely eject its outer layers more slowly than those stars, and remain in the red giant phase for several times longer.
But much remains unknown about the sun’s last activity. For example, although Jupiter is not a star, it is still large enough to attract material from the sun and power the accretion disk. “I think Jupiter will form a very small spiral. Even in our simulations, you can see its effect on the solar wind,” DXN said.
If so, then our sun may also have a gorgeous curtain call.