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Rare star resembles fried egg
by Amanda Doyle
Posted: 28 September 2011

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A rare yellow hypergiant star surrounded by two dusty shells has been imaged by astronomers using ESO’s Very Large Telescope.

The picture was taken in the mid infrared by the VISIR instrument on the Very Large Telescope. It has been dubbed the Fried Egg nebula, due to the star and its surrounding shells resembling a yolk and egg white.

A rare yellow hypergiant star has been imaged with two surrounding dusty shells, composed of material ejected from the star. Image: ESO/E. Lagadec.

Although this star was discovered in 1976, it has only just been identified as a yellow hypergiant. It was originally thought to be a less massive star, until recently refined measurements of the distance put the star at a position four times further away than previously thought. The greater distance meant the star had to be very luminous and it was thus no ordinary star.

Hypergiants are among the most luminous class of stars and this yellow hypergiant, dubbed IRAS 17163-3907, is 500,000 times brighter than our Sun. The star is around 1000 times the size of the Sun; its exterior would fall just inside the orbit of Jupiter if it were to replace the Sun in our Solar System. The outer shell has a radius of 6,000 Astronomical Units, that is 6,000 times the distance between the Sun and the Earth, and would extend far into the outer Solar System.

Yellow hypergiants are an extremely rare class of star and only a handful have ever been identified. This is the clearest image ever taken of this type of star. Yellow hypergiants undergo frequent mass loss events, which is the source of the two dusty shells seen in this picture. The oxygen rich shells contain around four times the mass of the Sun and approximately 400 years passed between the expulsion of the first and second shell.

There is most likely a third unresolved shell closer to the star than the two shown in the picture. It is also possible that other shells exist more distant from the hypergiant, but the field of view of VISIR is not large enough to spot any. The material in these possible outer shells would not be very dense as it has had more time to disperse into space. The outer shells would also be cooler and thus emit light at longer wavelengths than VISIR is capable of detecting.

A star that begins its life with a mass between 8 and 40 times that as our Sun will end up like this yellow hypergiant. It will eventually become a Wolf-Rayet star, named after the discoverers of the first star of its type. Wolf-Rayet stars are very hot, rapidly rotating stars and IRAS 17163-3907 will end its life in a violent supernova explosion.