One might think that it would be hard to hide a star some 500,000 times more radiant than the sun, but distance and dust seem to have conspired to do just that.
A group of astronomers has identified such a star in the Triangulum Galaxy, a neighbor to our own Milky Way Galaxy about three million light-years away. The star is barely detectable in visible light, but it shines brightly in the mid-infrared bands accessible to NASA's Spitzer Space Telescope. In fact, it is the brightest mid-infrared object in the entire galaxy, a fact that seems to have escaped notice until researchers from Ohio State University and the National Observatory of Athens in Greece went digging through the Spitzer data. The team weighs in on what would cause a star to look so strange in a study in the May 1 issue of The Astrophysical Journal.
The star, to which they have attached the alluring name Object X, may be an aging, massive star that has shrouded itself in dust; a star at least 30 times as massive as the sun would fit the bill. The dust around it would absorb almost all of the optical light and reradiate it as longer-wavelength infrared light. Aging stars are known to create such cloaks by ejecting material into their surroundings, sometimes throwing off several suns' worth of matter at speeds in the hundreds of kilometers per second.
But the eruption is just a phase, and a brief one at that. The few known objects that astronomers have to compare to Object X—in particular, a hypergiant star in the Milky Way and one in Triangulum—have evolved on very short timescales, changing in brightness before astronomers' very eyes. Looking back at archived astronomical images, the researchers found that Object X has been obscured since at least 1949, and it could be due for an unveiling very soon—in a matter of decades. If that occurs, astronomers will get a rare glimpse into "a brief yet eventful evolutionary state," as the study's authors put it, and emerge with a better idea of how these massive stars age.
Astronomers of the much more distant future may get an even closer look in a few billion years, if humankind is still around to see it. In less than five billion years, as the sun is nearing the end of its life, our Milky Way Galaxy is expected to merge with the nearby Andromeda Galaxy, which may by that time have devoured Triangulum, including what remains of the mysterious Object X.
Telescopic photo of Triangulum: NOAO/AURA/NSF/T. A. Rector