Hubble imaging of the preplanetary nebula AFGL 4104 and a yellow hypergiant observed with AMBER/VLTI

Image credit: Image: NASA, ESA, and R. Sahai (Jet Propulsion Laboratory)

This cosmic butterfly is a nebula known as AFGL 4104, or Roberts 22. Caused by a star that is nearing the end of its life and has shrugged off its outer layers, the nebula emerges as a cosmic chrysalis to produce this striking sight. Studies of the lobes of Roberts 22 have shown an amazingly complex structure, with countless intersecting loops and filaments. A butterfly’s life span is counted in weeks; although on a much longer timescale, this stage of life for Roberts 22 is also transient. It is currently a preplanetary nebula, a short-lived phase that begins once a dying star has pushed much of the material in its outer layers into space, and ends once this stellar remnant becomes hot enough to ionise the surrounding gas clouds and make them glow. About 400 years ago, the star at the center of Roberts 22 shed its outer shells, which raced outwards to form this butterfly. The central star will soon be hot enough to ionize the surrounding gas, and it will evolve into a fully fledged planetary nebula.

In astrophysical terms, the timescale for this stage of stellar evolution is incredibly short. The imaging is from as far back as 1999, using WFPC2. In more recent development, the yellow hypergiant HR 5171A has been observed interferometrically and revealed to be the primary of a massive interacting contact binary system containing a much lower mass companion embedded within the dense wind of the primary. The total system mass is around 40 solar masses, with the primary containing ~ 90% of the total mass. The radius of this stellar component is around 6 astronomical units or 1300 solar radii, making HR 5171A one of the largest yellow stars known.

Above: the field around HR 5171, courtesy ESO/Digitized Sky Survey 2. Such rare stars are also crossing the Hertzsprung-Russell diagram on short (104 to 105 yr) evolutionary timescales, as a consequence of extreme mass loss. Coincidentally, similar timescales as short as 104 yr apply to the formation of massive stars, which are still deeply embedded in dark molecular clouds even when already on the main sequence.

Update: Kevin L. Luhman of Pennsylvania State University and discoverer last year of a binary brown dwarf at only 2 parsecs distant, has performed an astrometric search for very nearby companions to the Sun and has ruled out “Planet X”. The paper is “A Search for a Distant Companion to the Sun with the Wide-Field Infrared Survey Explorer”, in the Astrophysical Journal vol. 781, (2014) doi:10.1088/0004-637X/781/1/4.


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