What happens after a star dies?

In the image above, the name of the star is in the upper left corner of each image. Winds blowing outward from the star form structures such as disks, spirals and “roses,” consistent with the theory that a red giant star has a companion orbiting it. Red represents gas moving away from the observer and blue represents gas moving towards the observer. 1 AU is an astronomical unit, the distance from the Earth to the sun. By comparison, Neptune is 30 AU from the Sun. Companion stars may be closer than their host star, and they are not visible due to the glare of the host star.

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Alma has seen spirals or arcs around more than a dozen red giant stars, which almost certainly indicate material is being shed from the red giant and spiraling toward its companion star. The spirals matched computer simulations very well and couldn’t be explained by older stellar wind models. Deqin reported the initial findings in the journal Science in 2020 and expanded on them the following year in the Annual Review of Astronomy and Astrophysics.

In addition, Decin’s group may have discovered previously undetectable companion stars in the Alma image, two red giant stars, p1 Gruis and L2 Puppis. To make sure, she needs to monitor them over a period of time to see if the newly detected objects are moving around the host star. “If they move, I’m sure we have company,” Deqin said. Perhaps this discovery will win over the last skeptics.

Like crime scene investigators, astronomers now have “before” and “after” snapshots of planetary nebula formation. The one thing they were missing was CCTV footage of the event itself. Have astronomers ever hoped to catch a red giant star that’s turning into a planetary nebula?

So far, computer models have been the only way to “see” this centuries-long process unfolding from start to finish. They helped astronomers discover a dramatic scenario in which a companion star crashed into the host star after a long orbit and lost distance due to tidal forces. As it spirals toward the red giant’s core, it releases “a surprising amount of gravitational energy,” Frank said. Computer models suggest that this greatly accelerates the process by which the star releases its outer layers by as little as 1 to 10 years. If this is correct, and if astronomers know where to look, they can witness the death of a star and the birth of a planetary nebula in real time.

One candidate to watch is called V Hydrae. The very active red giant spews bullet-shaped plasmoids at its poles every 8.5 years, and it has also coughed up six large rings in its equatorial plane over the past 2,100 years. Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory who published the discovery of the ring in April, believes the red giant has not one but two companion stars. A nearby companion may have been skimming the red giant’s envelope and producing a plasma jet, while a distant companion in a dive-bombing orbit controlled the ring’s jet. If so, V Hydrae may be about to devour its closer companion.

Finally, what about our sun? The study of binary stars does not seem to be relevant to the fate of our star, since it is a single star. Deqin estimates that a star with a companion loses mass about 6 to 10 times faster than a star without a companion, because the companion is more efficient at breaking out of the red giant’s outer shell than the red giant’s own.

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 all sun-sized stars have some sort of companion star. According to Deqin, the companion star always affects the shape of the stellar wind, and if the companion star is close enough, it can significantly affect the mass loss rate. The Sun will likely eject its outer layers more slowly than those stars, and will remain in its red giant phase for several times as long.

But there’s still a lot unknown about the Sun’s final act. For example, even if Jupiter wasn’t a star, it might still be large enough to attract material from the sun and power the accretion disk. “I think we’re going to have a very small spiral created by Jupiter,” Deqin said. “Even in our simulations, you can see the effect it has on the solar wind.” If so, then our sun may also have a grand finale.

* This article appeared first In Knowable Magazine and republished under a Creative Commons license.

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