This mosaic shows the Carina Nebula (left part of the image), home of the Eta Carinae star system. This part was observed with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory. The middle part shows the direct surrounding of the star system: the Homunculus Nebula, created by the ejected material from the Eta Carinae system. This image was taken with the NACO near-infrared adaptive optics instrument on ESO’s Very Large Telescope. The right image shows the innermost part of the system as seen with the Very Large Telescope Interferometer (VLTI). It is the highest resolution image of Eta Carinae ever.Credit: ESO/G. Weigelt
An international team of astronomers have used the Very Large Telescope Interferometer to image the Eta Carinae star system in the greatest detail ever achieved. They found new and unexpected structures within the binary system, including in the area between the two stars where extremely high velocity stellar winds are colliding. These new insights into this enigmatic star system could lead to a better understanding of the evolution of very massive stars.
Led by Gerd Weigelt from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, a team of astronomers have used the Very Large Telescope Interferometer (VLTI) at ESO’s Paranal Observatory to take a unique image of the Eta Carinae star system in the Carina Nebula.
This colossal binary system consists of two massive stars orbiting each other and is very active, producing stellar winds which travel at velocities of up to ten million kilometres per hour . The zone between the two stars where the winds from each collide is very turbulent, but until now it could not be studied.
The power of the Eta Carinae binary pair creates dramatic phenomena. A “Great Eruption” in the system was observed by astronomers in the 1830s. We now know that this was caused by the larger star of the pair expelling huge amounts of gas and dust in a short amount of time, which led to the distinctive lobes, known as the Homunculus Nebula, that we see in the system today. The combined effect of the two stellar winds as they smash into each other at extreme speeds is to create temperatures of millions of degrees and intense deluges of X-ray radiation.
The central area where the winds collide is so comparatively tiny — a thousand times smaller than the Homunculus Nebula — that telescopes in space and on the ground so far have not been able to image them in detail. The team has now utilised the powerful resolving ability of the VLTI instrument AMBER to peer into this violent realm for the first time. A clever combination — an interferometer — of three of the four Auxiliary Telescopes at the VLT lead to a tenfold increase in resolving power in comparison to a single VLT Unit Telescope. This delivered the sharpest ever image of the system and yielded unexpected results about its internal structures.
The new VLTI image clearly depict the structure which exists between the two Eta Carinae-stars. An unexpected fan-shaped structure was observed where the raging wind from the smaller, hotter star crashes into the denser wind from the larger of the pair.
“Our dreams came true, because we can now get extremely sharp images in the infrared. The VLTI provides us with a unique opportunity to improve our physical understanding of Eta Carinae and many other key objects,” says Gerd Weigelt.
In addition to the imaging, the spectral observations of the collision zone made it possible to measure the velocities of the intense stellar winds . Using these velocities, the team of astronomers were able to produce more accurate computer models of the internal structure of this fascinating stellar system, which will help increase our understanding of how these kind of extremely high mass stars lose mass as they evolve.
Team member Dieter Schertl (MPIfR) looks forward: “The new VLTI instruments GRAVITY and MATISSE will allow us to get interferometric images with even higher precision and over a wider wavelength range. This wide wavelength range is needed to derive the physical properties of many astronomical objects.”
 The two stars are so massive and bright that the radiation they produce rips off their surfaces and spews them into space. This expulsion of stellar material is referred to as stellar “wind,” and it can travel at millions of kilometres per hour.
 Measurements were done through the Doppler effect. Astronomers use the Doppler effect (or shifts) to calculate precisely how fast stars and other astronomical objects move toward or away from Earth. The movement of an object towards or away from us causes a slight shift in its spectral lines. The velocity of the motion can be calculated from this shift.
G. Weigelt, K.-H. Hofmann, D. Schertl, N. Clementel, M. F. Corcoran, A. Damineli, W.-J. de Wit, R. Grellmann, J. Groh, S. Guieu, T. Gull, M. Heininger, D. J. Hillier, C. A. Hummel, S. Kraus, T. Madura, A. Mehner, A. Mérand, F. Millour, A. F. J. Moffat, K. Ohnaka, F. Patru, R. G. Petrov, S. Rengaswamy, N. D. Richardson, T. Rivinius, M. Schöller, M. Teodoro, M. Wittkowski. VLTI-AMBER velocity-resolved aperture-synthesis imaging of η Carinae with a spectral resolution of 12 000. Astronomy & Astrophysics, 2016; 594: A106 DOI: 10.1051/0004-6361/201628832