Wednesday, September 28, 2011

Rare Yellow Hypergiant Star

This picture of the nebula around a rare yellow hypergiant star called IRAS 17163-3907 is the best ever taken of a star in this class and shows for the first time a huge dusty double shell surrounding the central hypergiant. The star and its shells resemble an egg white around a yolky center, leading astronomers to nickname the object the Fried Egg Nebula.
Credit: ESO/E. Lagadec

The monster star, known to astronomers as IRAS 17163-3907 [1], has a diameter about a thousand times bigger than our Sun. At a distance of about 13 000 light-years from Earth, it is the closest yellow hypergiant found to date and new observations show it shines some 500 000 times more brightly than the Sun [2].

"This object was known to glow brightly in the infrared but, surprisingly, nobody had identified it as a yellow hypergiant before," said Eric Lagadec (European Southern Observatory), who led the team that produced the new images.

The observations of the star and the discovery of its surrounding shells were made using the VISIR infrared camera on the VLT. The pictures are the first of this object to clearly show the material around it and reveal two almost perfectly spherical shells.

This chart shows the location of the rare yellow hypergiant star IRAS 17163-3907 within the constellation of Scorpius (The Scorpion). This map shows most of the stars visible to the unaided eye under good conditions and the location of the star itself is marked with a red circle. Although this object appears very bright in the infrared sky it is faint in visible light and hard to find in the very rich star-fields of the centre of the Milky Way.
The Fried Egg Nebula in the constellation of Scorpius
Credit: ESO, IAU and Sky & Telescope

If the Fried Egg Nebula were placed in the centre of the Solar System the Earth would lie deep within the star itself and the planet Jupiter would be orbiting just above its surface. The much larger surrounding nebula would engulf all the planets and dwarf planets and even some of the comets that orbit far beyond the orbit of Neptune. The outer shell has a radius of 10 000 times the distance from the Earth to the Sun.

This visible light wide-field image of the region around the yellow hypergiant star IRAS 17163-3907 was created from photographs taken through blue, red and infrared filters and forming part of the Digitized Sky Survey 2. The star appears close to the centre, indistinguishable from thousands of others. This very rich star field in the direction of the centre of the Milky Way also features the blue ring-shaped planetary nebula NGC 6337 to the upper left of centre, as well as several star clusters and some faint clouds of glowing gas. The field of view is approximately 2.9 degrees across.
Wide-field image of the sky around the Fried Egg nebula
Credit: ESO and Digitized Sky Survey 2

Yellow hypergiants are in an extremely active phase of their evolution, undergoing a series of explosive events — this star has ejected four times the mass of the Sun in just a few hundred years [3]. The material flung out during these bursts has formed the extensive double shell of the nebula, which is made of dust rich in silicates and mixed with gas.

This activity also shows that the star is likely to soon die an explosive death — it will be one of the next supernova explosions in our galaxy [4]. Supernovae provide much-needed chemicals to the surrounding interstellar environment and the resulting shock waves can kick start the formation of new stars.

The Very Large Telescope mid-IR instrument, VISIR, captured this delicious image of the Fried Egg Nebula through three mid-infrared filters that are here coloured blue, green and red [5].

Contacts and sources:
Notes

[1] The name indicates that the object was first spotted as an infrared source by the IRAS satellite in 1983 and the numbers show the star's place in the sky, in the heart of the Milky Way in the constellation of Scorpius (The Scorpion).

[2] IRAS 17163-3907 is one of the 30 brightest stars in the infrared sky, at the wavelength of 12 microns observed by IRAS, but it had been overlooked because it is quite faint in visible light.

[3] The total mass of this star is estimated to be roughly twenty times that of the Sun.

[4] After burning all their hydrogen all stars of ten solar masses or more become red supergiants. This phase ends when the star has finished burning all of its helium. Some of these high-mass stars then spend just a few million years in the post-red supergiant phase as yellow hypergiants, a relatively short time in the life of a star, before rapidly evolving into another unusual type of star called a luminous blue variable. These hot and brilliant stars are continuously varying in brightness and are losing matter due to the strong stellar winds they expel. But this is not the end of the star's evolutionary adventure, as it may next become a different kind of unstable star known as a Wolf-Rayet star (http://www.eso.org/public/images/wr124/), before ending its life as a violent supernova explosion.

[5] The three mid-infrared filters that were used passed light at wavelengths around 8590 nm (coloured blue), 11 850 nm (coloured green) and 12 810 nm (coloured red).

More information

This research is presented in a paper "A double detached shell around a post-Red Supergiant: IRAS 17163-3907, the Fried Egg nebula " by E. Lagadec et al., submitted to the journal Astronomy & Astrophysics.

The team is composed of E. Lagadec (ESO, Garching, Germany), A.A. Zijlstra (Jodrell Bank Center For Astrophysics, Manchester, UK), R.D. Oudmaijer (University of Leeds, UK), T. Verhoelst (Instituut voor Sterrenkunde, Leuven, Belgium), N.L.J. Cox (Instituut voor Sterrenkunde), R. Szczerba (N. Copernicus Astronomical Center, Torun, Poland), D. Mekarnia (Observatoire de la Cote d'Azur, Nice, France) and H. van Winckel (Instituut voor Sterrenkunde).

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world's most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 40-metre-class European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become "the world's biggest eye on the sky".

Links

- Research paper: A&A paper –
http://www.eso.org/public/archives/releases/sciencepapers/eso1136/eso1136.pdf

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