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Out Of The Noise Of Parrots and the Milky Way

by WKT
January 15, 2002 ::

This 400 by 900 light-year mosaic of several Chandra images of the central region of our Milky Way galaxy reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. (Credit: NASA/UMass/D.Wang et al.)

What do parrots and the center of our galaxy have in common? The unexpected and admittedly somewhat tenuous connection can be found in two articles published the same week in two prestigious science journals.

The 4 January 2002 issue of Science magazine has an article on the "Fluorescent Signaling in Parrots" by Kathryn Arnold of the University of Glasgow and two colleagues, and the 10 January 2002 issue of the journal Nature reports on fluorescence observed in the Galactic Center in an article by astronomer Q. Daniel Wang of the University of Massachusetts, Amherst and two colleagues.

The Science article describes a study of the fluorescent plumage of parrots. Ultraviolet light is absorbed by the plumage on the crown and cheeks of budgerigar parrots and re-emitted as longer wavelength, yellow light. The experiments by Arnold et al. showed that both females and male budgerigars showed a significant sexual preference for fluorescent birds. In other words, when a parrot is out in the sunshine looking for a mate - and parrots are monogamous, so it is a serious search - she or he looks for one whose plumage fluoresces in the sun's ultraviolet light as an indicator of quality.

The astronomers' work on fluorescence may not be as sexy, but it is equally intriguing. Using images made by the Chandra X-ray Observatory, they produced a stunning, high-energy panorama of the central regions of our Milky Way galaxy.

Figure 1. Budgerigar's head (A) under white light and (B) under UV illumination to induce yellow fluorescence. (C) Crown irradiated with UV light only (dashed line), resulting in human visible fluorescent emission (solid line). (D) Normalized visual difference between the emission spectrum of plumage, measured as radiant emission from feathers (solid line) and the spectral sensitivities of the four single cones classes of the budgerigar's retina (dashed lines) (4).
(Credit: K.Arnold et al., Science, 295, 92)

Among other things, they found fluorescence, not from the plumage of parrots, but from iron atoms in a vast cloud of gas that pervades the central region of our Galaxy. The fluorescence occurs when high energy particles or X-rays knock electrons out of the inner parts of the iron atoms and excite the atom to a higher energy level. The atoms almost immediately return to their lower energy state with the emission of a longer wavelength, fluorescent X-ray. This process is similar to the one that causes parrot plumage to fluoresce.

The origin of this Milky Way fluorescent signaling is a mystery. It is well known that the disks of gas swirling toward neutron stars and black holes produce iron fluorescence, but there are not enough bright neutron star and black hole sources in the Galactic Center region to explain the observations. Nor is gas falling into the supermassive black hole at the center of our Galaxy producing enough X-rays.

One possible explanation is that the center of the Galaxy was wracked by an unusually large outburst from a supernova or the supermassive black hole a few hundred years ago. If so, fluorescent iron atoms could be sending a signal that something really big happened there in the past. Meanwhile, astronomers will continue to explore this and other evolutionary pathways that could have led to the Milky Way's fluorescence.

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