Digging into a Weird Stellar Spiral


Spirals are not a new shape to astronomers. We’ve all seen the gorgeous spiral galaxy galleries before. But they are still pretty cool when they show up in unexpected places, and a new radio telescope has found a weird spiral structure around a dying star.

Meet R Sculptoris. This red giant star has grabbed the headlines with this fascinating and gorgeous spiral structure. My favorite part? Yes, it’s a radio image. That means this material is glowing with radio light, which has far too long of a wavelength to be seen with our eyes, but was detected with an array of dishes in the Chilean desert — the Atacama Large Millimeter Array (ALMA).

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So how does a star produce radio waves? Typically, stars glow brightly in optical, infrared, or ultraviolet light because they are extremely hot. Objects that are thousands of degrees don’t typically give off much in the way of radio waves, at least not that are detectable at interstellar distances.

But this red giant is blowing off material as it is near the end of its lifetime. Some of that material is much cooler and can form molecules. These molecules, as it turns out, can glow brightly with radio spectral lines when they rotate. The specific fingerprint of each molecule is not unlike the spectral lines given off in optical light by a glowing neon tube, though the process of creating the light is a bit different.

With all these radio spectral lines out there, radio telescopes have been key to understanding the molecular history of our Galaxy and the Universe.

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In this star’s case, the astronomers picked up the spectral line of a rotating carbon monoxide (CO) molecule. CO is a fairly simple molecule so it’s quite common, but it is asymmetric, so it gives off energy as it spins. Since the gas is moving around the star, the spectral line gets redshifted or blueshifted based on how fast the material is moving. So, by breaking up the spectrum into channels, astronomers can determine the velocity structure of the material coming off the star. Each square in the image below shows material moving at a different speed.

Figure 1 from Maercker et al.

Okay, so what does this all mean physically about the star? It is undergoing a phase of “mass loss” meaning that material is sloughing off the star in its old age. This typically happens after the star’s core has run out of its main source of fuel for nuclear fusion, hydrogen.

But the strange spiral shape has been seen before in instances where there is a binary companion star. Using the data from ALMA for comparison, Shazrene Mohamed of the South African Astronomical Observatory made an astrophysical model of the system where R Sculptoris has a companion star that revolves around it once every 350 years. And it’s giving off a lot more material than previously thought, three times more. (I’ll direct you to the Bad Astronomer’s calculations to get a sense of just how much stuff that is.)

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The videos produced by the simulations show the 3-D structure of the shell of material around the star and the evolution of this material over a 2000-year time period. Note the “pulse” of material from the most recent outburst:

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The exquisite detail provided by the ALMA images made this type of analysis possible, and the telescope is only in “Cycle 0,” or early commissioning phases. Not even half of the full array was in place at the time of observations, and the array is scheduled to be completed next year with 66 antennas and a massive correlator, or back-end computer to bring all the dishes together to function as an interferometer.

As I lived in the same town as ALMA’s US headquarters for many years, we used to joke about how ALMA would “save the world” and “cure cancer” with the amount of anticipation that has surrounded the project long before a single antenna was built. But when I see images and science like this coming out of just the Early Science phase, I have to say that I’m impressed with the technology and hard work that has gone into its development, and I look forward to decades more of discovery with this telescope. 

Top Image credit: ALMA (ESO/NAOJ/NRAO)

Middle Image Credit: Maercker et al. 2012 in Nature, preprint at arXiv.org.

Video credit: Nature/M. Maercker et al./S. Mohamed/L. Calçada

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