In the picture above, the names of the stars are in the upper left corner of each picture. Winds blowing outward from the stars create a variety of structures such as discs, spirals, and “roses,” which is consistent with the theory that the red giant star has a companion in its orbit. Red indicates gases moving away from the observer and blue indicates gases moving towards the observer. 1 AU is one astronomical unit, or the distance from the Earth to the Sun. For comparison, Neptune is 30 AU from the Sun. Companion stars are probably closer than their primary and are not visible due to the primary star’s glow.
Alma saw spiral-shaped or arc-shaped structures around more than a dozen red giant stars; this is an almost sure sign that matter is pouring out of the red giant and spiraling towards its comrade. The spirals closely match computer simulations and are impossible to explain with the old star-wind model. Decin He reported the first findings in the journal Science in 2020. and expanded them the following year Annual Review of Astronomy and Astrophysics.
Also, Decin’s group may have seen previously undetected companions of two red giants, p1 Gruis and L2 Puppis, in Alma images. To be sure, newly detected objects need to track them over a period of time to see if they are moving around the primary star. “If they move, I’m sure we have comrades,” Decin says. Perhaps this discovery will win over the last skeptics.
Like crime scene investigators, astronomers now have “before” and “post” snapshots of the formation of a planetary nebula. The only thing missing is the equivalent of CCTV footage of the event itself. Is there any hope that astronomers can catch a red giant in the act of transforming into a planetary nebula?
Until now, computer models are the only way to “track” the centuries-long process from start to finish. They helped astronomers get home in a dramatic scenario where the companion star crashes into the primary star after orbiting for a long time and losing distance due to tidal forces. Frank says, as he spins toward the red giant’s core, the companion emits “an insane amount of gravitational energy.” Computer models show that this greatly speeds up the process computers go through. the star leaves its outer layers, that’s just one to 10 years. If this were true and astronomers knew where to look, they could witness the death of a star and the birth of a planetary nebula in real time.
One island to watch out for is called V Hydrae. This very active red giant star hurls clumps of lead-like plasma to its poles every 8.5 years and has also coughed up six large rings in the equatorial plane over the past 2100 years. Astronomer Raghvendra Sahai at NASA’s Jet Propulsion Laboratory, who published the discovery of the rings in April, said: the red giant has not one but two companion stars. A nearby comrade may already be scraping the red giant’s envelope and producing plasma ejections, while a distant comrade in dive bombing orbit may be controlling the rings’ ejection. If so, V Hydrae may be close to swallowing his best friend.
Finally, what about our Sun? Studies of binary stars may seem to have little to do with the fate of our star, because it is singular. Decin estimates that stars with companions lose mass about six to 10 times faster than those without them, because it is much more efficient for a companion star to pull on a red giant’s crust than the red giant pushes through its own crust.
This means that data on companion stars cannot reliably predict the fate of non-companion stars, such as the Sun. About half of sun-sized stars have some kind of companion. According to Decin, the companion will always affect the shape of the stellar wind, and if the companion is close enough, it will significantly affect the mass loss rate. The Sun will likely shed its outer layer more slowly than these stars and will remain in its red giant phase several times longer.
But not much is still known about the last act of the Sun. For example, while Jupiter is not a star, it may be heavy enough to pull material from the Sun and power an accretion disk. “I think we’re going to have a very small spiral created by Jupiter,” Decin says. “Even in our simulations, you can see the effect on the solar wind.” If so, then our Sun may well be lining up for a flashy grand finale.
* This article appeared at the beginning Featured in Knowable Magazine and republished under a Creative Commons license.
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