After looking at 20 years' worth of fabulous pictures from the Hubble Space Telescope, you might get the feeling that each new release is a variation on a theme you've seen before: a supernova remnant, interacting galaxies, new stars cocooned in their placental nebulas, and so on.
But this is something different!
The dim spiral in this image is almost spooky in its perfect symmetry. The view was captured several years ago by HST's Advanced Camera for Surveys, but a few days ago it came to light (ouch! pun alert!) on a European website devoted to Hubble's discoveries.
"So what is it?" you ask.
First, I can tell you that it's real (not an artifact), it's about 3,000 light-years away in Pegasus, and it's not associated with the bright foreground star to its right.
Second, the feature itself is designated IRAS 23166+1655, which signifies that it was spotted by the Infrared Astronomical Satellite in 1983.
"So what is it?"
It's not a spiral galaxy but rather what astronomers call a pre-planetary nebula, created when an aging, swollen star starts to shed its outer layers into space — the beginning of its death throes. In this case the shedder is an extreme carbon star, one fortified with so much carbon that there's a sooty deposit in its photosphere thick enough to block the visible light trying escape from underneath. Astronomers only know a star is there because it's still hot and thus glowing brightly in the infrared.
"But why the spiral shape?"
This isn't just any old spiral; it's a perfect fit to an Archimedean spiral, something like a jet of water coming from a spinning lawn sprinkler.
The source of this pinwheel turns out to be part of a binary system, as revealed by near-infrared images taken with the Keck II telescope. As it spews matter into space, the dying star is also slowly twirling around an unseen companion.
Observers estimate that the ejected jet is moving outward at roughly 30,000 miles (50,000 km) per hour. Knowing that, and the angular spacing between successive rings (about 4½ arcseconds), observers conclude that they're spaced about 800 years apart. That is, if you took up a stationary position near the star, one of the outward-moving spiral's arms would sweep past you every 800 years.
As it turns out, the orbital period of the binary is also about 800 years. Consistent results!
Interestingly, the pinwheel is glowing faintly, but not from the stars hidden inside it. When Mark Morris (University of California, Los Angeles) and others took stock of this remarkable image in 2006, they concluded that the illumination source isn't the 12th-magnitude foreground star at the right — more likely we're seeing this ghostly pinwheel thanks to the combined glow of stars in the galactic plane.
If you want to explore the science behind this cosmic curiosity, here is the analysis by Morris's team, and here is a link to some work by Nicolas Mauron (CNRS, France) and Patrick Huggins (New York University).
Or you can skip the science and just be a little patient. "We have a much deeper HST observation with the new Wide Field Camera 3 now scheduled for this object in early October," Morris mentioned in an email, "so we're very excited about that."