Making a 3D map of our galaxy would be easier if certain stars behaved long enough for us to calculate the distances to them. But red supergiants are the nimble kids on the block when it comes to determining their exact locations. This is because they look like they are dancing around, which makes it difficult to find their place in space. That wobbling is a trait, not a bug, in these massive ancient stars, and scientists want to understand why.
So, as with other challenging objects in the galaxy, astronomers have turned to computer models to find out why. In addition, they use the Gaia mission’s position measurements to get a grip on why red supergiants look like dancing.
Understand red supergiants
The population of red supergiants has several features in common. These are stars that are at least eight times the mass of the sun – they are huge. A typical one is at least 700 to 1,000 times the solar diameter. At 3500 K, they are much colder than our ~ 6000 K star, although it is difficult to measure these temperatures. They are super bright in infrared lightbut weaker Visible light than other stars. They also vary in their brightness, which (for some of them) may be related to the dancing movement. More on that in a moment.
If the sun were a red supergiant, the earth would not be there. This is because the star’s atmosphere would have reached Mars and starved our planet. The most famous examples of these stars are Betelgeuse and Antares. Red supergiants are found throughout the galaxy. There is a population of them that you can see at night in a nearby cluster called Chi Persei. It is part of the well-known double cluster.
The structure of red supergiants
So we have this population of stars that do not behave as expected and that are not suitable for simple measurements. Why is it like that? They have expanded so much that they end up with a very low surface gravity. Because of this, their convective cells (the structures that carry heat from the inside to the surface) become quite large. A cell covers as much as 20-30% of the star’s radius. It actually “interrupts” the star’s brightness.
The convection not only moves heat from the inside out, but also helps the star to shoot out material into nearby space. And, we are not talking small puffs of gas and plasma either. A red supergiant can send a billion times more mass into space than the sun does. All that action makes the star look frothy and as if its surface is boiling crazy. In essence, it makes the star’s position appear to be dancing in the sky.
Red supergiants on the whole
Red supergiant material becomes part of the chemical “inventory” of galaxies. The elements that these stars create continue to become new stars and worlds. So it helps to get a good understanding of how these stars lose their mass throughout life. Everything is part of the understanding star development in the Milky Way and its impact on the cosmic environment. This is why astronomers want to track the total mass that these aging stars blow out into space. They also measure the speed of the stellar wind and calculate the geometry of the cloud of “star objects” that enclose a red supergiant.
Now, what does this have to do with the dance act? Well, the boiling of the convection cells and the construction of a shell of material around the star contribute to its variation. That is, it affects its brightness over time.
One way astronomers use to determine the exact position of a star is to use its “photocentre”. It is the center of light of the star. If the star varies in brightness (for some reason), it shifts the photo center. It will not match the barycenter. (It is the common center of gravity between the star and the rest of its system. It is a component of distance measurements.) Essentially, the photo center varies as the star’s brightness changes. In combination with the action of the huge convection cells, the star appears to be dancing in space.
The dance changes the distance estimate
The red super giant’s “position problem” attracted Andrea Chiavassa (Laboratoire Lagrange, Excellence Cluster ORIGINS and Max Planck Institute for Astrophysics). She and astronomer Rolf Kudritzki (Munich University of Observatory and Institute of Hawai’i) and a research team created simulations of the boiling surfaces and the variation in the brightness of the red supergiant.
“The synthetic maps show extremely irregular surfaces, where the largest structures develop on time scales of months or even years, while smaller structures develop over several weeks,” says Chiavassa. “This means that the position of the star is expected to change as a function of time.”
In theirs Astronomy & Astrophysics the study, the team compared its model with stars in Chi Persei. That cluster was measured by the Gaia satellite, so the positions of most of its stars are very accurate. Well, all but the red supergiants. “We found that the positional uncertainty of red supergiants is much greater than that of other stars. This confirms that their surface structures change dramatically over time as predicted by our calculations,” Kudritzki explained.
This change in observable position provides a solution for understanding changing positions red supergiants. This in turn makes it difficult to measure the exact distance to many of these stars. The current model also provides clues to the development of these objects. But knowing what makes the stars dance offers a way to a solution when calculating their distance. Future models will help astronomers refine these distances and provide more insight into what happens to them stars as they age.
A. Chiavassa et al., Investigating red supergiant dynamics by photocenter displacements measured by Gaia, Astronomy & Astrophysics (2022). DOI: 10.1051 / 0004-6361 / 202243568
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