Red Supergiant Cauldrons let off Steam
These images show the water vapour clouds in shades of blue (S Persei) and red (VX Sagitarii), observed using MERLIN at 1.3 cm wavelength. The image of S Persei is about 370 AU across and the image of VX Sagitarii is about 680 AU across, where an AU is the distance between the Earth and the Sun. The Doppler shifts of the maser spectral lines mean that we can measure the speed and direction of the wind blown away from the star. The symbols show the positions of the hydroxyl masers observed using MERLIN and VLBI at 18.0 cm wavelength (the 'mainline' transition), colour coded according to velocity with respect to the star (blue is expanding towards us, red away from us etc.). The high resolution of Very Long Baseline Interferometry, using radio telescopes in Europe and the USA, confirmed that the hydroxyl masers are found in the water maser region and the appearance is not just a projection effect.