R Coronae Borealis is a star high in the northern sky. It is a variable star; its brightness changes. Over the course of a few weeks it can dim incredibly, then rise back in brightness over the ensuing months. Eventually lots of similar stars were found, and they are, as a set, called RCrBr stars (usually pronounced “R Core Bore”).
The leading theory is that RCrBr stars are old red giants, and they sometimes belch out vast clouds of matter off their surface. As these clouds expand, they cool. The material is rich in carbon (spectra of the clouds indicate this is true), so when the temperature drops, the carbon and other elements combine to form large, complex grains of dust. This dust is really good at absorbing visible light, so when the dust condenses out of the cloud, we see the starlight drop. Eventually, the cloud expands enough that it can’t adequately block the light, and the star appears to regain its strength.
This “dust puff” theory seems pretty good, but up until now it wasn’t directly observed – that is, the clouds themselves hadn’t been seen. But astronomers using the Very Large Telescope in Chile have some pretty good proof. RY Sagittarii (“Saj it airy eye”, because if you couldn’t pronounce R Core Bore, “Sagitarii” may sprain your tongue) is an RCrBr star, and the astronomers got these shots of it:
Strictly speaking these aren’t astronomical images; they are plots of brightness obtained using a technique called interferometry. It allows astronomers to see incredible details; data obtained this way can be far higher resolution than even Hubble images!
The image on the left shows the star RY Sag in May 2003. The star isn’t round, because we can see the dust clouds surrounding it. In September 2003, the image on the right was taken: things changed in just a few months! The clouds expanded, moved away from the star. As it happens, RY Sag underwent a dimming and brightening event around that time. These clouds seen here are part of that event (though too far off to the side to have done the actual deed). Mind you, these clouds are incredibly close to the star; they are roughly as far from the star as Pluto is from the Sun… but we’re viewing this scene from about 6000 light years away! That’s a long way off, and so the details in these images are pretty amazing. To give you an idea, about 1500 of these images could fit side-by-side across the face of the Moon as seen from Earth.
These data confirm the dusty puff model, but lots of question remain. How far from the star do the dust molecules form? One aspect of the puff theory says they form when the clouds are about twice the radius of the star away, some say 20 times. That’s a big difference, and involve different physics! Unfortunately, these observations can’t distinguish between the two… but they pave the way for future observations which can, so that’s cool.
The Galaxy is littered with dust; you can see it on a clear night, bifurcating the Milky Way with a stream of velvet. Where does it come from? We know RCrBr stars make dust, as do supernovae, and various other stars too. Eventually, we’ll have a better handle on the different aspects of this dark and smoky component of our Galaxy, and it’ll be due to observations like these.
Oh – if you ‘re interested, you can read the scientific paper about these results, too.
