ALMA Reveals the True Nature of Hubble’s Enigmatic Ghost Spiral

Appearing as a ghostly apparition in deep space, the LL Pegasi spiral nebula signals the death of a star — and the world’s most powerful radio observatory has delved into its deeper meaning.

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Left: HST image of LL Pegasi publicized in 2010. Credit: ESA/NASA & R. Sahai. Right: ALMA image of LL Pegasi. Credit: ALMA (ESO/NAOJ/NRAO) / Hyosun Kim et al.

When the Hubble Space Telescope revealed the stunning LL Pegasi spiral for the first time, its ghostly appearance captivated the world.

Known to be an ancient, massive star, LL Pegasi is dying and shedding huge quantities of gas and dust into space. But this is no ordinary dying star, this is a binary system that is going out in style.

The concentric rings in the star system’s nebula are spiraling outwards, like the streams of water being ejected from a lawn sprinkler’s head. On initial inspection of the Hubble observation, it was assumed that the spiral must be caused by the near-circular orbit of two stars, one of which is generating the flood of gas. Judging by the symmetry of the rings, this system must be pointing roughly face-on, from our perspective.

Though these assumptions generally hold true, new follow-up observations by the Atacama Large Millimeter/submillimeter Array (ALMA) on the 5,000 meter-high Chajnantor plateau in Chile has added extra depth to the initial Hubble observations. Astronomers have used the incredible power of ALMA to see a pattern in the rings, revealing the complex orbital dynamics at play deep in the center of the spiral.

“It is exciting to see such a beautiful spiral-shell pattern in the sky. Our observations have revealed the exquisitely ordered three-dimensional geometry of this spiral-shell pattern, and we have produced a very satisfying theory to account for its details,” said Hyosun Kim, of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan and lead researcher of this work.

Just as we read tree rings to understand the history of seasonal tree growth and climatic conditions, Kim’s team used the rings of LL Pegasi to learn about the nature of the binary star’s 800 year orbit. One of the key findings was the ALMA imaging of bifurcation in the rings; after comparing with theoretical models, they found that these features are an indicator that the central stars’ orbit is not circular — it’s in fact highly elliptical.

ALMA observation of the molecular gas around LL Pegasi. By comparing this gas distribution with theoretical simulations, the team concluded that the bifurcation of the spiral-shell pattern (indicated by a white box) is resulted from a highly elliptical binary system. Credit: ALMA (ESO/NAOJ/NRAO) / Hyosun Kim et al.

Probably most striking, however, was that Hubble was only able to image the 2D projection of what is in fact a 3D object, something that ALMA could investigate. By measuring the line-of-sight velocities of gas being ejected from the central star, ALMA was able to create a three-dimensional view of the nebula, with the help of numerical modeling. Watch the animation below:

“While the [Hubble Space Telescope] image shows us the beautiful spiral structure, it is a 2D projection of a 3D shape, which becomes fully revealed in the ALMA data,” added co-author Raghvendra Sahai, of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., in a statement.

This research is a showcase of the power of combining observations from different telescopes. Hubble was able to produce a dazzling (2D) picture of the side-on structure of LL Pegasi’s spirals, but ALMA’s precision measurements of gas outflow speed added (3D) depth, helping us “see” an otherwise hidden structure, while revealing the orbital dynamics of two distant stars.

A special thanks to Hyosun Kim for sending me the video of the LL Pegasi visualization!

On Mars, There’s No Asphalt

Curiosity's right-middle and rear wheels, bearing the scars of 488 sols of rough roving. Credit: NASA/JPL-Caltech
Curiosity’s right-middle and rear wheels, bearing the scars of 488 sols of rough roving. Credit: NASA/JPL-Caltech

If you’re like me, you hang off every news release and new photo from our tenacious Mars rover Curiosity. The awesome one-ton, six-wheeled robot is, after all, exploring a very alien landscape. But if there’s one thing I’ve learned from the mission, Mars is far from being a truly alien place. Sure, we can’t breath the thin frigid air, but we can certainly recognize similar geological processes that we have on Earth, and, most intriguingly, regions that would have once been habitable for life as we know it. This doesn’t mean there was life, just that once upon a time parts of Gale Crater would have been pretty cozy for terrestrial microbes. Personally, I find that notion fascinating.

But, way back in May, I noticed something awry with our beloved rover’s wheels. Curiosity’s beautiful aircraft-grade aluminum wheels were looking rather beaten up. Punctures had appeared. Fearing the worst I reached out to NASA to find out what was going on. After a friendly email exchange with lead rover driver Matt Heverly, I felt a lot more at ease: The damage was predicted; dings, scratches, even holes were expected to appear in the thinnest (0.75 mm thick) aluminum between the treads. On Mars, after all, there is no asphalt. Also, erosion is a slower-paced affair in the thin winds and dry environment — sharp, fractured rocks protrude, embedding themselves into the wheels at every slow turn.

Then, on Friday, in a news update on Curiosity’s progress, JPL scientists mentioned that they would be commanding the rover to drive over a comparatively smooth patch to evaluate the condition of the wheels as their condition is getting worse. But isn’t that to be expected? Apparently not to this degree. “Dents and holes were anticipated, but the amount of wear appears to have accelerated in the past month or so,” said Jim Erickson, project manager for the NASA Mars Science Laboratory at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

So what are we looking at here?

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All of the wheels are exhibiting wear and tear, but this particular ‘rip’ in aluminum is by far the most dramatic. But what does that mean for Curiosity? We’ll have to wait and see once JPL engineers have assessed their condition. Although this kind of damage has inevitably been worked into the the structural equations for the wheels’ load-bearing capabilities, whichever way you look at it, damage like this is not good — especially as Curiosity hasn’t even roved three miles yet.

But in the spirit of Mars exploration, Curiosity will soldier on regardless of how rough the red planet treats her.

Read more in my coverage on Discovery News, a location you’ll find me during most daylight (and many nighttime) hours: