What Decides the Form of Planetary Nebulae? No matter’s Orbiting a Star When it Dies

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What Decides the Shape of Planetary Nebulae? Whatever's Orbiting a Star When it Dies

Planetary nebulae are some of the most beautiful objects in the galaxy, and span a variety of shapes and sizes. They are formed in the agony of stars like the sun, and new research is shedding light on how they get their distinctive and unique shapes. The answer: anything unlucky enough to orbit this dying star.

When stars like our sun near the end of their lives, they swell and turn red, becoming what astronomers call AGB stars, which stands for Asymptotic Giant Branch. The somewhat cryptic name comes from their position in the Hertzsprung-Russell diagram, which links the temperature and luminosity of stars in different phases of their life cycle.

When stars are in the AGB stage, they are incredibly unstable, especially when it comes to solar wind. The solar wind is a stream of gas that is constantly emitted by a star. However, during the AGB phase this increases and the star loses a decent part of its mass into interstellar space.

But for decades, astronomers had assumed that this stellar wind was spherical. After all, the stars themselves are spherical and send the solar wind out in roughly the same directions.

However, a number of new observations made with the Atacama Large Millimeter / Submillimeter Array (or ALMA) show a different picture: anything but a sphere.

The observations show that the winds that come from the stars follow complex patterns: sometimes a boring sphere, sometimes spirals, and sometimes even something that approaches the wings of a giant nebula butterfly. These observations show that previous assumptions about how much of this gas ends up in the interstellar mixture can vary by up to a factor of 10.

The observations also show why these fascinating shapes appear. Any orbiting companion, such as a smaller star or a massive planet, can stir and mix the passing winds, creating the shapes that eventually move further outwards. These star-blown winds eventually evolve into a planetary nebula that holds a memory of this original structure.

The observations can also paint a portrait of what is likely to happen to our own sun. In 5 billion years, our home star will become a AGB star and will sell winds and material at breakneck speed. But the interactions of Jupiter and Saturn will shape and shape these outflows into a weak but definite spiral.

It will be one last masterpiece that the sun created before the end of its life.

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