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The Darkening of Centauri A

Take a look at the image of Alpha Centauri in the Centauri Dreams logo. It’s the bright object at far left, not the single star that it appears but a triple system whose glare masks its two major components. Centauri A is a G2 star much like our Sun, while Centauri B is a K1. The two are separated by an average of 23 AU, with an orbital period of some eighty years. Indiscernible in the image is Proxima Centauri, an M-class dwarf which is actually the closest star to Earth.

Bright, nearby and highly studied, the Centauri stars would seemingly be well characterized. But new results from the European Space Agency’s XMM-Newton x-ray satellite show anomalies. Unlike optical wavelengths, where the larger Centauri A dominates, Centauri B is the brighter object in x-ray emissions. What’s odd is that repeated monitoring of the two by XMM-Newton shows that Centauri A faded by no less than an order of magnitude in x-rays during the two-year observing range, a behavior out of keeping with all prior observations of the star at these wavelengths.

Below are side by side views of Centauri A and B in x-ray wavelengths. Note in the image on the left that Centauri A shows up just above and left of the brighter Centauri B in these wavelengths. The later image, on the right, shows a dramatic drop in x-ray brightness.

X-ray views of Alpha Centauri

Image (click to enlarge): Two EPIC MOS images of Alpha Centauri A+B, taken in March 2003 (left) and Feb. 2005 (right). The separation of Alpha Centauri A+B is in March 2003 around 12″, in Feb. 2005 somewhat above 10″. Alpha Cen B is the X-ray brighter object down right and exhibits a comparable X-ray luminosity in both exposures. In contrast Alpha Cen A, a star very similar to our Sun, is only visible in the left image. It has fainted in X-rays by at least an order of magnitude, a behaviour never observed before despite several observations of the Alpha Centauri system over the last 25 years. Image and caption credit: Robrade, Jan and ESA.

From a recent paper on this work:

…a strong decrease of the total emission measure is necessary to explain our findings. While smaller differences in the long term evolution of X-ray luminosity may be explained by the use of the various instruments, the decline seen over the XMM-Newton campaign can only be explained by a X-ray activity cycle or an irregular event. While no definite statement can be made about an irregular event, the scenario of an activity cycle would require, that all previous X-ray measurements were made when α Cen A was near the ’high state’ of its cycle. Putting all the observation dates together, this would require a cycle with a duration of ~ 3.4 years from maximum to maximum.

A definitive statement on the cause of this x-ray ‘fainting’ is impossible without further data. We do know that our own Sun has a coronal activity cycle with a period of some eleven years. Centauri A’s similarity to the Sun makes further study of Solar x-ray emissions a possible key to the mystery, but it seems remarkable that such a cycle would have gone unnoticed until now. It will be interesting to see what this ongoing observation program comes up with next. Also interesting: the XMM-Newton work has observed a flare on Centauri B, confirming its nature as a flare star.

The paper is Robrade et al., “X-rays from Alpha Centauri – The darkening of the solar twin,” accepted by Astronomy & Astrophysics, with preprint available online. Thanks to Ian Jordan (Space Telescope Science Institute) for the heads-up on this work.

Comments on this entry are closed.

  • Phil December 27, 2006, 18:54

    Fascinating. Has this sort of work been done on other nearby solar analogues?


  • Marc December 27, 2006, 20:35

    That’s interesting. Perhaps it’s a planet emitting x-rays? For example, the planet is going through a revolution and, at this moment, it is behind the star now. Just a guess.

  • Administrator December 27, 2006, 20:51

    Phil, the study of x-ray emissions and coronal cycles in general on stars other than the Sun seems to be in its infancy. So we have a lot to learn about how Centauri A works and much to investigate about other nearby stellar systems.

    As for Marc’s question, it’s pretty clear that what these astronomers are looking at is a cycle that occurs on the star itself. The real problem comes in trying to figure out just what the cycle is, and more particularly, why it’s been concealed up to now, given the amount of study that has gone into these stars.

  • Adam December 28, 2006, 5:45

    Hi Paul

    Stellar engineering?


  • Administrator December 28, 2006, 8:46

    Implying a Type II civilization around these stars. For that matter, both Centauri A and B are older than the Sun, about which more later today. Great fodder for an SF tale, don’t you think?

  • andy December 28, 2006, 8:48

    You never know, Alpha Centauri A might have just gone into a Maunder minimum phase. When we do eventually get there we’ll find any Earthlike planet has just gone snowball and wiped out all the interesting creatures…

  • Aristotle Pagaltzis December 29, 2006, 0:47

    Proxima Centauri is only the second closest star to Earth, actually. :-)

  • sar April 17, 2007, 9:03

    *wow i have learned so much about alpha centauri a..it is amazing.. im doing a research project on it…any advice for me? or some helpful facts?

  • MyNameisToby April 17, 2007, 10:00

    If you have learned so much, what else do you need to know? This isn’t a place to have someone else do your homework for you.

  • Administrator April 17, 2007, 12:42

    Re learning more about Alpha Centauri, do use the ‘search’ function here. I recommend especially looking into the recent work of Laughlin and Wertheimer as well as Quintana and Lissauer. These two teams have investigated the Centauri stars intensively and we’re learning more about them all the time. Of particular note is the question of whether habitable planets might exist around any of the three stars, and the answer there is still unknown, but the situation around Centauri B especially seems more promising than previously thought. You’ll find more about this in the archives, and good luck with your project!

  • ljk May 18, 2007, 9:35

    The Milky Way Near the Southern Cross

    Credit & Copyright: Yuri Beletsky

    Explanation: The glow of the southern Milky Way and the well-known Southern Cross are featured in this colorful skyscape recorded in April over La Frontera, Chile. The Southern Cross (Crux) itself is at the right of the 20 degree wide field of view, topped by bright, yellowish star Gamma Crucis. A line from Gamma Crucis through the blue star at the bottom of the cross, Alpha Crucis, points toward the south celestial pole.

    Against faint Milky Way starlight, the dark expanse of the Coal Sack Nebula lies just left of the cross, while farther left along the Milky Way are the bright stars Hadar and Rigil Kentaurus, also known as Beta and Alpha Centauri.

    Blazing in the lower left, Alpha Cen is the closest star to the Sun, a mere 4.3 light-years distant. In fact, yellowish Alpha Cen is actually a triple star system that includes a sun-like star. Seen from Alpha Cen, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia.