Are the dancing stars too boring? Or maybe too fat...

Is the dance of the red supergiant a death struggle or a dazzling eruption?

Red supergiants "dance" because they contain too much gas, and the light they produce sways all over the place, making it difficult to invert them.

An artist's impression of an exoplanet orbiting a red supergiant star. The bright glow in the distance comes from a star of similar size.

(Photo source: Bixiu.com)

Scientists may finally be able to explain why some massive stars appear to dance across the sky even though they're not actually moving. According to a new study, these stars have unusual, bubble-like cores that make their surfaces wobble and change the amount of light they emit.

These dancing stars are called red supergiants, and they are huge objects that have expanded and cooled and are nearing the end of their lives. These stars have masses about eight times that of our sun and can be up to 700 times its diameter—a size equivalent to the sun's surface expanded beyond the orbit of Mars (in our sun's position, it would gradually engulf Mercury, Venus, Earth, and Mars). Yet even with their massive size, accurately determining the position of these slowly dying giants is a challenging task.

Typically, astronomers can determine a star's approximate location by identifying its optical center, the center of the light it emits—which usually matches up perfectly with its gravitational center, or barycenter. In most stars, the optical center has a fixed position. In red supergiants, however, these points wobble around in the star, constantly moving from side to side over time. The star's barycenter shows its exact location in the universe and doesn't wobble like the optical center does, but this movement of the optical center makes it difficult to pinpoint the gravitational center.

In the new study, researchers compared "dancing" red supergiants with smaller main sequence stars -- stars that are in a more stable phase of their lives. Using data from ESA's Gaia space observatory, the scientists looked at stars in the Perseus cluster -- a dense cluster of stars 7,500 light-years from our solar system.

"We found that the uncertainty in the position of red supergiants is much larger than for other stars," study co-author Rolf Kudriski, an astronomer at the University of Hawaii and director of the Institute for Astrophysical, Particle and Biophysics in Munich, Germany, said in a statement. To discover the underlying logic of these stars' wobble, the research team created maps of the surface intensities of red supergiants, measured and calculated the radiation they emit, and used a fluid dynamics simulator to represent the fluctuations in their three-dimensional shells.

The maps show that the surfaces of red supergiants are extremely dynamic, with many clumpy gas structures forming and disappearing over time, emitting more and denser energy than the rest of the surface. These short-lived but still powerful structures burn brighter than the rest of the star's surface, causing a shift in the star's optical center: if a bright structure flashes to the left of a red supergiant, its optical center will shift to the left.

One of the surface maps created during the study. The video shows how the surface of a red supergiant star changes over months and years. The light orange and yellow areas have greater intensity and produce more light than the lower-intensity red and black areas.

(Image source: A. Chiawasa et al. 2022)

The enormous size of red supergiants may explain why this happens. The outer shells of most stars are made up of thousands of adjacent convection cells - small, elongated regions of swirling gas (mostly hydrogen and helium) that transport hotter gas from the star's interior to the star's outer surface, where it cools and sinks back down, like bubbles in a lava lamp.

However, due to the large size of red supergiants, their surface gravity is much weaker than that of their cores. Therefore, their convective cells are much larger than those of other stars, accounting for about 20% to 30% of the radius of a red supergiant star itself, or 40% to 60% of its diameter. According to research, larger convective cells can transport more gas to the surface of the star, thus creating those strong and bright structures that are strong enough to change the optical center of the star.

The team's data showed that these surface structures vary in size, which determines how long they will stay nearby. "The largest structures typically last for months or even years, while smaller structures evolve within weeks," lead author Andrea Chiavassa, an astronomer at the Lagrange Laboratory in Rees and the Max Planck Institute for Astrophysics (MPIA) in Munich, said in a statement. "This suggests that the position of the optical center of these stars is constantly changing."

This video shows how the optical center of a red supergiant star moves over time, making it appear to wobble in space.

(Image source: A. Chiawasa et al. 2022)

Astronomers suspect that red supergiants play an important role in the evolution of galaxies; these massive stars eject vast quantities of gas and heavy elements, which are essential for the formation of new stars and exoplanets. Large, bright surface structures in supergiants likely contribute to the eruptions that release these important materials. Future studies of the "wobbles" of these stars may also help to reconstruct the exact processes that occur.

"The regularity of these red giant stars' dance across the sky can tell us more about their boiling outer crusts," study co-author Thelma de Minck, director of the MPIA, said in the statement. "We will be able to extract more crucial information about the dynamics of the stars and better understand the physical processes that cause these stars' intense convective motions."

BY: Harry Baker

FY: Nyarlathotep is outrageous

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