okt 262013
 

October 26, 2013

The Fried Egg Nebula

IRAS 17163-3907

Image Credit: ESO/E. Lagadec

The Fried Egg Nebula is a nebula of 600 – 800 light-years across surrounding IRAS 17163-3907, which is probably a rare yellow hypergiant star, located some 13,000 light-years away from Earth in the southern constellation of Scorpius (the Scorpion).

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 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, before ending its life as a violent supernova explosion.

A yellow hypergiant is a massive star with an extended atmosphere, and an initial mass of 20 – 50 solar masses, but having lost as much as half that mass. They are amongst the most visually luminous stars, but also one of the rarest classes of stars in the Universe with just a handful known in our Milky Way galaxy. Yellow hypergiants are in an extremely active phase of their evolution, undergoing a series of explosive events. This image is the best ever taken of a star in this class.

IRAS 17163-3907 is around 1000 times the size of our Sun, has around twenty times its mass, and it shines some 500,000 times more brightly than the Sun. It is one of the brightest infrared sources in the sky, but due to intervening dust clouds its apparent visual magnitude is just 13.1. This central yellow hypergiant is surrounded by a huge dusty double shell. The star and its shells resemble an egg white around a yolky center, hence its nickname.

The inner shell is 4800 AUs in diameter (an Astronomical Unit is the mean distance between the Earth and the Sun: 149,597,870,700 meters) and the outer shell is 12,000 AU wide. The materials for the shells is ejected from the star during ejection events. The time period between expulsion of materials causing the first and the second outer shells is estimated at 435 years and the total mass in the ejecta is around four times the mass of the Sun, of which about 1% is silicate-rich dust.

At its most active, the star ejects materials equal to the mass of the Sun every few hundred years. The outer shell it’s estimated to contain 0.17 solar masses of dust and that it was produced in an outburst 17,000 years ago that expelled 7.1 solar masses of 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. Supernovae provide much-needed chemicals to the surrounding interstellar environment and the resulting shock waves can kick start the formation of new stars.

This image was taken in the mid infrared spectrum by the VISIR instrument in the Very Large Telescope in the Atacama Desert in northern Chile. The filters passed light of frequencies 12,810 nm (mapped to red in the image), 11,850 nm (mapped to green), and 8,590 nm (mapped to blue).

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