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    What happens when a star dies?

    In the picture above, the names of the stars are in the upper left-hand corner of each image. Winds blowing outward from the stars create a variety of structures, such as disks, spirals and “roses”, that are consistent with the theory that the red giant star has a companion orbiting it. Red indicates gases that are moving away from the observer, and blue indicates gases moving toward the observer. 1 AU is one astronomical unit, or the distance from Earth to the Sun. For comparison, Neptune is 30 AU from the Sun. The companion stars are likely closer than that to their primary stars, and they are not visible due to the glare of the primary.

    Final act

    Alma has seen spiral-shaped or arc-shaped structures around more than a dozen red giant stars, almost certainly a sign that matter is being shed from the red giant and spiralling toward its companion. The spirals closely match computer simulations and are impossible to explain with the old stellar-wind model. Decin reported the initial findings in 2020 in Science and expanded on them the following year in Annual Review of Astronomy and Astrophysics

    In addition, Decin’s group may have spotted the previously undetectable companions of two red giants, p1 Gruis and L2 Puppis, in Alma images. To make sure, she needs to monitor them over a period of time to see if the newly detected objects are moving around the primary star. “If they move, I’m sure that we have companions,” says Decin. Perhaps this discovery will win over the last sceptics. 

    Like crime scene investigators, astronomers now have “before” and “after” snapshots of the creation of a planetary nebula. The one thing they lack 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 turning into a planetary nebula? 

    So far, computer models are the only way to “watch” the centuries-long process unfold from beginning to end. They have helped astronomers home in on one dramatic scenario, in which the companion star plunges into the primary after a prolonged period of orbiting it and losing distance due to tidal forces. As it spirals toward the red giant’s core, the companion sheds “an insane amount of gravitational energy”, says Frank. The computer models show that this hugely accelerates the process through which the star lets go of its outer layers, to just one to 10 years. If this is correct, and if astronomers knew where to look, they could witness the death of a star and birth of a planetary nebula in real time.

    One candidate to keep an eye on is called V Hydrae. This very active red giant star ejects bullet-like clumps of plasma toward its poles every 8.5 years, and it has also coughed out six large rings in its equatorial plane over the last 2,100 years. Raghvendra Sahai, an astronomer at Nasa’s Jet Propulsion Laboratory who published the discovery of the rings in April, believes that the red giant has not one but two companion stars. A nearby companion may already be grazing the red giant’s envelope and producing the plasma ejections, while a distant companion in a dive-bombing orbit controls the ejection of the rings. If so, V Hydrae may be close to swallowing up its closer companion. 

    Finally, what about our Sun? Studies of binary stars might seem to have little relevance for our star’s fate, because it is a singleton. Stars with companions lose mass about six to 10 times faster than those without, Decin estimates, because it’s much more efficient for a companion star to pull off a red giant’s shell than for the red giant to push off its own shell.

    This means that data on stars with companions cannot reliably predict the fate of stars without companions, like the Sun. Roughly half of the stars that are the Sun’s size have companions of some sort. According to Decin, the companion will always affect the shape of the stellar wind, and it will significantly affect the mass-loss rate if the companion is close enough. The Sun will most likely eject its outer layer more slowly than those stars and will stay in its red-giant phase several times longer.

    But a great deal is still unknown about the Sun’s last act. For example, even though Jupiter is not a star, it could still be hefty enough to attract material from the Sun and power up an accretion disc. “I think we’ll have a very small spiral created by Jupiter,” Decin says. “Even in our simulations, you can see its impact on the solar wind.” If so, then our Sun too might be in line for a showy grand finale.

    * This article originally appeared in Knowable Magazine, and is republished under a Creative Commons licence.

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