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‘Blue Stragglers’ in the Galactic Bulge

I’m fascinated by how much the exoplanet hunt is telling us about celestial objects other than planets. The other day we looked at some of the stellar spinoffs from the Kepler mission, including the unusual pulsations of the star HD 187091, now known to be not one star but two. But the examples run well beyond Kepler. Back in 2006, a survey called the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) used Hubble data to study 180,000 stars in the galaxy’s central bulge, the object being to find ‘hot Jupiters’ orbiting close to their stars.

But the seven-day survey also turned up 42 so-called ‘blue straggler’ stars in the galactic bulge, their brightness and temperature far more typical of stars younger than those around them. It’s generally accepted that star formation in the central bulge has all but stopped, the giant blue stars of the region having exploded into supernovae billions of years ago. Blue stragglers are unusual because they are more luminous and bluer that would be expected. They’ve been identified in star clusters but never before seen inside the core of the galaxy.

Image: Peering deep into the star-filled, ancient hub of our Milky Way (left), the Hubble Space Telescope has found a rare class of oddball stars called blue stragglers, the first time such objects have been detected within our galaxy’s bulge. Blue stragglers — so named because they seem to be lagging behind in their rate of aging compared with the population from which they formed — were first found inside ancient globular star clusters half a century ago. Credit: NASA, ESA, W. Clarkson (Indiana University and UCLA), and K. Sahu (STScI).

The galactic bulge is a tricky place to study because foreground stars in the disk compromise our view. But the SWEEPS data led to a re-examination of the target region, again with Hubble, two years after the original observations were made. The blue stragglers could clearly be identified as moving at the speed of the bulge stars rather than the foreground stars. Of the original 42 blue straggler candidates, anywhere from 18 to 37 are now thought to be genuine, the others being foreground objects or younger bulge stars that are not blue stragglers.

Allan Sandage discovered blue stragglers in 1953 while studying the globular cluster M3, leading scientists to ask why a star would appear so much younger than the stars around it. Stars in a cluster form at approximately the same time and should therefore show common characteristics determined by their age and initial mass. A Hertzsprung-Russell diagram of a cluster, for example, should show a readily defined curve on which the stars can be plotted.

Blue stragglers are the exception, giving the appearance of stars that have defied the aging process. One possibility is that they form in binaries, with the less massive of the two stars gathering in material from the larger companion, causing the accreting star to undergo fusion at a faster rate. More dramatic still would be the collision and merger of two stars — more likely in a region where stars are dense — which would cause the newly formed, more massive object to burn at a faster rate.

Scientists will use the blue straggler data to tune up their theories of star formation. Lead author Will Clarkson comments on the work, which will be published in the Astrophysical Journal:

“Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood. While the consensus is that the bulge largely stopped forming stars long ago, many details of its star-formation history remain controversial. The extent of the blue straggler population detected provides two new constraints for models of the star-formation history of the bulge.”

I’ll note in passing that Martin Beech (University of Regina) has suggested looking at blue stragglers in a SETI context, noting that some could be examples of astroengineering, the civilization in question using its technology to mix shell hydrogen into the inner stellar core to prolong its star’s lifetime on the main sequence. It’s an interesting suggestion though an unlikely one given that we can explain blue stragglers through conventional astrophysics. In fact, blue stragglers point to an important fact about the field some are calling ‘interstellar archaeology’ — gigantic astroengineering may be extremely difficult to tell apart from entirely natural phenomena, in which case Occam’s razor surely comes into play.

For the recent blue straggler discoveries, see Clarkson et al., “The First Detection of Blue Straggler Stars in the Milky Way Bulge,” in press at the Astrophysical Journal (preprint). On the possible application of blue stragglers to SETI, see Beech, “Blue Stragglers as Indicators of Extraterrestrial Civilizations?” Earth, Moon, and Planets 49 (1990), pp.177-186. And Greg Laughlin (UC-Santa Cruz) looks at blue stragglers as targets for photometric transit searches in this post on his systemic site.


Comments on this entry are closed.

  • Mike May 30, 2011, 13:53

    Blue stragglers in the Milky Way galactic bulge are another example of what I call “the more we look the more surprises we get”that is so characteristic of astronomy.
    Here’s a really wild idea, though already touched upon by better writers then me in science fiction. The article mentions Martin Beech’s idea of blue stragglers as astro-engineering by some pretty smart and capable ETs.
    Consider a type 4 civilization that is astro-engineering the entire universe for its purposes. With the evidence for its works being the puzzle of missing matter (dark matter) and dark energy.
    A little over the top I know but it’s fun to speculate. From a scientific approach it is a lousy idea because it basically amounts to using a “sky-hook” to cover our lack of knowledge.
    But on the other hand if there are very advanced ETs they may well be doing all kinds of huge engineering projects. As always, more data needed.

  • Adam May 30, 2011, 15:37

    All my fun suggestions already taken *sigh*

    Of course star-lifting may be too crude for would be stellar engineers, so perhaps we’re missing subtler possibilities. As Beech’s book on the issue discusses, mass-loss causes a star to get bluer, which is undesirable for one’s preferred spectral range perhaps. Mixing fresh material into the Core might prove easier.

  • Alex Ross May 30, 2011, 17:53

    Adam, what if the world on which they evolved orbited a bluish star? Changing the spectral type of the star via this astro-engineering process might actually be the case here. Of course, speaking totally hypothetically here. Given this is something to be looked at from an exobiological point of view.

  • David May 30, 2011, 19:54

    The main sequence lifetime, that is the total time a star steadily fuses hydrogen in its core,
    ranges from a few million to a few hundred million years for bluish stars. This span of time
    is unlikely to be sufficient for a technological civilization to arise. In fact it may be barely enough
    time for a rocky planet like our Earth to form.

  • Rob Henry May 30, 2011, 20:07

    There is some thing very very strange about the way blue stragglers are treated in astrophysics. It’s exactly as is everyone assumes that they must all be produced by stellar mergers, but no one can make these models fit the data. Given the amount of data we have on double stars and star density profiles, such a fit should be easy to achieve if that is the answer, yet every paper seems reluctant to compare their expected abundance due to merger with their actual occurrence. What am I missing, or would you have to kill me if you tell me?

  • Adam May 30, 2011, 22:09

    Hi Alex
    Good point. What actually happens, as I discovered after reading a bit more of Beech (GoogleBooks Previews, yay!) online, is that the star gets hotter as the metallicity of the core mix increases. Basic stellar physics, but you don’t normally hear about it because in regular stars the hot Core is surrounded by the cooler envelope that eventually balloons into a Red Giant. By star-lifting the Core is effectively exposed, though only a bit. Thus the blue-wards colour shift of the star is by a few thousand degrees, upwards of 10,000 K as the star depletes its hydrogen.

    Beech estimates a star’s lifetime can be extended 4-6 fold, while the star-lifted material can be used to kindle a few other low mass stars. Jupiter might serve as the first “hydrogen dump” and eventually become the Sun’s true binary companion. The Sun’s final mass depends on a number of factors, but one scenario had it reduced to ~0.3 solar masses – as the Sun shrank, the Earth’s orbit expanded and the Sun’s luminosity could rise to match. Thus L/d^2~constant, with a gradual blue-wards spectral shift.

    A natural fate for the gas giants, then, might be to become the cores of new stars, made out of star-lifted Sun-stuff. Just how that gets orbitally engineered is an exercise for the reader.

  • ljk May 31, 2011, 1:39

    Go to this article and scroll down the page about halfway to find the discussion and diagram on the possible artificial nature of blue stragglers:


    While I do not want us to declare every thing that seems odd in the Universe to be artificial, I also worry that too much conservatism will make us avoid something that was made by an intelligent mind. No doubt this has happened already.

  • Alex Ross May 31, 2011, 14:17


    Yes, I’m well aware of the age of stars by particular spectral group. But this is data in general. I think unless we study those stars up close and detailed, we can’t come to certainty. That being said, I think that its safe to assume that given the right conditions and set of circumstances, that a bluish star might not only potentially have an orbiting habitable planet, but an orbiting inhabited planet at that.

    But all these circumstances and factors to meet would probably be very, very rare (and rare could be an understatement here). But still, in astronomical terms, these kind of cases might occur more than a few times, even in our galaxy alone.

    What I’m trying to say is, maybe as life forms they developed complexity underground in a rogue planet, and got captured by the gravity of a young blue star. Life learns to adapt (we have evidence that this is the case, researching past life on Earth). By adaptation and accustoming to the new environment they would cope with the new-found conditions. Coping with the energy output of a blue star is quite a challenge. And we know how nature loves putting out challenges to life forms. A challenge is almost always necessary for a species to evolve. We also have some new models of evolution (i’ll try to find a link), referred to as ‘rapid evolution’, which if used in this case, it may actually solve the puzzle, if say, one day, we make contact with a civilization that claims it evolved under a blue sun.

    Also, the planet has to be a fine distance away from the star, not too small to loose its atmosphere in time, not too big to pose serious halt in the development processes, etc.

    Like I said, many factors, but it could happen in my honest opinion. It could be so rare that the chances of it happening are like winning a lottery (but there are people who do win it, despite the odds, right?).

    Oh, and, when I said bluish stars, I meant stars that are main sequence ranging from A9 to maybe A4 type. Some, according to calculations are expected to reach an age of nearly 3 billion years. I think its enough time for some intelligence to form. We had a lot of events on Earth that caused life to undergo ELEs, but the frequency of this might not be the case for other planets. Just my opinion.

    As so many things about the universe are strange and amaze me, I wouldn’t rule this off completely as a possibility.

    @ Alex

    Thank you.

    It does make sense to me. Only one thing is bugging me about that Jupiter-becoming-binary companion thing – I think it would need gas accumulation that would be equal to 19+ times the mass it has now. In order to ignite, and start fusion? Am just saying…

    Also, out of curiosity, does a symbiotic binary fit the proposed theory? Like, perhaps, having two A-class main sequence stars, one slightly larger than the other, orbiting very close to each other, and the larger of the two stealing material from the other, but on a slower rate as the size of the two stars is almost the same? It could potentially extend the life time of one of the stars, I think?

    @ ljk

    Thanks for the link. I find the things theorized there fascinating. Who would have thought that S type stars may not all be what it seems :).

    Also, the astro-engineering process is well explained. I firmly understand it now.

  • andy June 1, 2011, 15:25

    @Rob Henry: certain stages of binary star evolution are very tricky to model. Common-envelope stages are one example for which we cannot make good predictions of the outcome. For instance we can predict that the binary star Regulus A (the primary of which is essentially a blue straggler, despite not being a cluster member) will undergo a common-envelope stage when the primary evolves into its giant phase, but the outcome of this stage is uncertain. We do not know whether the result will be a rapidly-rotating merged star or a WD+sdB binary.