Surprising spiral pattern hints at ‘elliptical’ binary
New discoveries

Surprising spiral pattern hints at ‘elliptical’ binary

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The stunning spiral pattern in this ALMA image is produced by a binary star – two stars orbiting each other. The size and shape of the spiral provides astronomers with information about the binary’s orbit. Apparently, this orbit is not circular, but elliptical – very elongated.

The spiral pattern was already observed by the Hubble Space Telescope in 2006. Now, Taiwanese and American astronomers have also studied the beautiful structure with ALMA. The spiral consists of glowing gas, which also includes various molecules. ALMA was able to observe the millimeter radiation emitted by those molecules.

ALMA could also measure the velocity of the gas (towards us or away from us). Thus, it was possible to learn about the three-dimensional shape and motion of the spiral pattern – something that Hubble couldn’t do.

So how does the spiral form? Embedded in the center is an old star. It has swollen to a size 200 times as large as our own Sun. As a result, the star’s outer layers are blown into space, at velocities of some 50.000 kilometers per hour.

But apparently, the old star (known as LL Pegasi) has a companion – it is a binary star. The orbital motion of this lightweight binary is producing the beautiful spiral pattern in the gas outflow.

If you look closely at the photo, you can see that the spiral splits in two every now and then. This can only be explained if the binary star is in an elongated orbit, completing one revolution every 800 years.

A few billion years from now, our Sun will also become a bloated giant star, shedding its outer layers into space. But the Sun has no binary companion, so it will not produce a spiral pattern.


The spiral pattern is officially known as IRAS 23166+1655. It consists of gas and dust blown into space by a red giant star. The star is knowns as LL Pegasi; it is located in the constellation Pegasus, the Winged Horse. It can’t be seen with an optical telescope: its light is absorbed by the dust. The spiral pattern was first observed by the Hubble Space Telescope in 2006.


The ALMA observations of LL Pegasi and its spiral outflow were carried out by a team of Taiwanese and American astronomers. The team was led by Hyosun Kim of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taipei, the capital of Taiwan. Hyosun worked together with colleagues at the ASIAA institute, and with Raghvendra Sahai of the Jet Propulsion Laboratory in Pasadena (USA) and Mark Morris of the University of California at Los Angeles. They have published their results in the journal Nature Astronomy.