The Red Rectangle Nebula doesn't merely look modern-art cubist bizarre. Its very light is strange, its composition is food for philosophy, and it remains a hot venue of current research.
Located in the constellation Monoceros the Unicorn, its central 9th-magnitude binary star was discovered in 1915 by the famous double-star hunter Robert Grant Aitken. Things changed in 1973 when a rocket-borne infrared sky survey discovered the ruddy nebula surrounding the binary, soon labeled HD 44179 after the star’s catalog designation. It took years, and better instrumentation, including studies performed by the Hubble Space Telescope, to fully reveal how odd and intriguing is this dusty gas cloud 2,300 light-years away.
The first head-twister is immediately obvious. In a universe of vast swirling curves, where celestial straight lines are almost nonexistent and geometric shapes the stuff of dreams, we stare at a giant rectangle. Or perhaps it’s more like a squared-off bow tie or a woven “God’s eye.” Or maybe it’s a giant X with interconnected ladder-like steps. However you see it, the question remains: Why should gem-like symmetry materialize in an enormous gaseous nebula?
Then there’s its color. Spectrographs detect glowing hydrogen — the normal hum-drum white-bread emissions of nebulae everywhere. But here it’s diluted and dominated by a different shade of orange-red given off by glowing dust. Dust fluorescing? The dust in our homes thankfully doesn’t glow, and neither does the celestial variety. Physicists are only recently homing in on its composition, and it’s not what they expected.
Astronomers call this nebula’s alien illumination Extended Red Emission, or ERE. It was first discovered in 1975 right here in the Red Rectangle, and has since been observed elsewhere, even in other galaxies. It has been a mystery for more than 30 years. The odd glow is always associated with dust bathed in ultraviolet light, as well as with a kind of photoluminescence or afterglow. Researchers think complex carbon-rich molecules called polycyclic aromatic hydrocarbons (PAHs) are responsible for the ERE.
PAHs are always fascinating. You bite into them after you’ve overcooked your burger. You get a face full when you drive too closely behind a diesel truck. They are released during the burning of fossil fuel. And here they are in the Red Rectangle, including anthracene and pyrene, organic molecules vital for the formation of life. The big problem is that the same ultraviolet light that makes these PAHs glow should quickly destroy them. However, recent thinking says the thick dusty shielding surrounding the binary stars blocks the “hardest” ultraviolet (UV) emissions, allowing these complex organic hydrocarbons to remain intact, helped by newly discovered molecular forces that preserve them.