IRAS 17163−3907

Lore & Background

The star was discovered and catalogued as emission line star Hen 3–1379 in 1976, and classified as a proto-planetary nebula in 2003 after the 2.2 micron infrared sky survey. In 1989, it was analysed as a post-AGB star and assumed to have a luminosity of 9,600 L☉ and a distance around 1,000 pc. Although IRAS 17163-3907 is one of the brightest mid-IR sources in the sky, very few observations were made until 2011 when it was proposed to be at a much larger distance and hence to be a highly luminous object, probably a yellow hypergiant.

The picture of the star causing it to be dubbed the Fried Egg Nebula was taken in the mid infrared spectrum by the VISIR instrument on the Very Large Telescope in the Atacama Desert in northern Chile. The filters passed light of frequencies 12,810 nm (mapped to red), 11,850 nm (mapped to green), and 8,590 nm (mapped to blue). The star is about 500,000 times the luminosity of the Sun. The star is characterized by two visible dusty shells: the inner shell is 4,800 AU in diameter and the outer shell is 12,000 AU wide. The source of materials for the shells is material ejected from the star during ejection events common in hypergiants. 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 4 solar masses, of which about 1% is dust. At its most active the star ejects materials equal to the mass of the Sun every few hundred years. The shells are also discovered to be rich in silicates and hence oxygen. The star may contain more outer shells, but they were not detected by the VISIR instrument, as the field of view of the instrument is insufficient. Estimates of the outer shell are that it contains 0.17 solar masses of dust and was produced in an outburst 17,000 years ago that expelled 6-7 solar masses of gas.

Reader's Guide

The significance of IRAS 17163−3907 lies in its status as one of the brightest mid-infrared sources in the sky, yet it was scarcely observed until 2011 when its true distance and luminosity were recognized, reclassifying it as a yellow hypergiant. The photograph taken by the VISIR instrument on the Very Large Telescope is considered one of the best ever taken of a yellow hypergiant. The star's two visible dusty shells, with diameters of 4,800 AU and 12,000 AU, provide direct evidence of episodic mass ejection events common in hypergiants, with an estimated 435-year interval between the formation of the inner and outer shells. The properties of these shells are considered similar to those surrounding IRC+10420 (another evolved massive star suffering heavy mass loss) and, for the outer shell, to those around luminous blue variables (LBVs). This suggests both that IRAS 17163−3907 is in a pre-LBV stage and that the ring nebulae often surrounding LBVs may originate with mass loss during the red supergiant stage. The star is eventually expected to end its life in a supernova.

Did You Know?

• The star is nicknamed the 'Fried Egg Nebula' due to its appearance in mid-infrared images.
• The star ejects material equal to the mass of the Sun every few hundred years at its most active.