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What makes the luminous star known as Object X look so dim?

The views expressed are those of the author and are not necessarily those of Scientific American.


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Triangulum Galaxy M33One 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





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  1. 1. jtdwyer 4:45 pm 04/23/2011

    This crew of astronomers really missed out! Think of all the publicity they could have gotten if they’d only called it ‘Dark Star’!

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  2. 2. Moodie-1 6:30 pm 04/23/2011

    @John Matson

    "A group of astronomers has identified such a star in the Triangulum Galaxy…", "…it is the brightest mid-infrared object in the entire galaxy…". A bit ambiguous, I’d say. The brightest mid-infrared object in which galaxy, ours or the Triangulum galaxy? Please specify.

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  3. 3. jtdwyer 12:19 am 04/24/2011

    I agree it’s a bit murky, but it is in the Triangulum galaxy.

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  4. 4. TMEubanks 7:22 pm 04/25/2011

    Could this be a Dyson Sphere ? This has almost exactly the signature expected from a Dyson Sphere so, while unlikely, I think that the possibility should be considered.

    In a Dyson sphere (or swarm) a civilization surrounds an entire star to capture most or all of its luminosity; severely cutting down on its optical luminosity but accentuating the IR luminosity, which is just what is seen here.

    It is true that this hypergiant star would seem like an unusual candidate for a Dyson envelopment (as it will have a short lifetime), but any civilization that could do this would be by definition a Kardashev Type II (with K = 2.6 from this star alone, it might well be a Type III), and thus would be considerably beyond our technical abilities. I would hesitate to speculate on how such a civilization would choose stars to envelope. As Object X would produce as much energy as a Dyson envelopment of 500,000 Suns, it might, for example, be more efficient, if you needed such vast amounts of energy, to get it from one star rather than many.

    So, I would urge that an attempt be made to rule this out as a large example of astroengineering, both by monitoring its behavior with time, and by looking for artificial features in its IR spectrum.

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  5. 5. MikePeel 7:11 pm 04/26/2011

    I suspect that it could be due to the environment of the star, rather than dust thrown out from the star. If there is a large dust cloud along the line of sight between us and the star, which is far enough from the star to not have been disrupted by the star’s formation process, and yet close enough to the star that the dust has been heated up by the star’s emission – then that could explain an excess of infrared emission with a lack of optical emission.

    That’s assuming that it is a star that is generating this emission, though, rather than e.g. an AGN generating synchrotron emission, or a particularly luminous HII region / star formation region (but the latter would probably be variable over longer time periods than 10 days).

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  6. 6. Wayne Williamson 6:55 pm 04/29/2011

    Remember the movie with that title;-)

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  7. 7. Quinn the Eskimo 9:39 pm 05/1/2011

    Installed CFL’s to save electricity. Go Green!

    Even in the galaxy their dim watted waste.

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