Blue Stragglers Found in Galaxy's Core.

[Suzy]

Hello everyone in the science forum,

I have some interesting reading for you'll.

http://www.nasa.gov/mission_pages/hu...straggler.html

NASA's Hubble Space Telescope has found a rare class of oddball stars called blue stragglers in the hub of our Milky Way, the first detected within our galaxy's bulge.

"Although the Milky Way bulge is by far the closest galaxy bulge, several key aspects of its formation and subsequent evolution remain poorly understood," Clarkson said. "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."

[mjc (Mark)]

"NASA's Hubble Space Telescope has found a rare class of oddball stars called blue stragglers in the hub of our Milky Way, the first detected within our galaxy's bulge.

The findings support the idea that the Milky Way's central bulge stopped making stars billions of years ago. It now is home to aging sun-like stars and cooler red dwarfs. Giant blue stars that once lived there have long since exploded as supernovae. "

What does that mean - in the bulge - away from the galactic centre?

I ask because apparently at least one young blue star has been detected before (Andrea Ghez 2003). One designated SO-2 has a tight orbit of around 130 - 1900 AU from the object at our galactic core (that's in the bulge - isn't it?) for which there is considerable evidence is where a black-hole resides.

See http://iopscience.iop.org/1538-4357/.../16990.web.pdf

Confused
Mark C.

[CraigS]

Hi Mark;

Looks like a definitional issue surrounding what defines 'Bulge'.

There's been a lot of work done on this and clearly, the drivers/dynamics of what goes on at the core, at ~130 AU from a SMBH, is different to the mechanisms further out in the 'bulgy bit'. (See below for links for further reading).

Also, its interesting that Clarkson states:

…. Such a bar-driven arrangement of the stars evolves over a timescale of several Gy, and can be transient with multiple generations of bar (e.g. Combes 2009). However, chemical evidence suggests the stars themselves mostly formed early and rapidly (e.g. Ballero et al. 2007; McWilliam et al. 2010). While the majority view now is that most stars in the Boxy/Peanut bulge of the Milky Way (which we call here “the bulge”) formed ∼10 Gy ago (e.g. Zoccali et al. 2003; Freeman 2008), the degree to which this star formation was extended over time, is presently only somewhat weakly constrained (Kuijken & Rich 2002; Zoccali et al. 2008). Thus, better estimates of the young stellar population of the bulge, and thus its evolution, are a natural dividend of constraints on the BSS {Blue Straggler Stars} fraction.

.. so anything they can deduce/discover would be better than almost nothing .. hence the 'excitement' they feel as a result of this work.

Here's a good paper, written by E. Athanassoula, which aims at classifying the structures of the 'bulge'. He wrote it "to avoid {the} considerable confusion" about the various interpretations of the term 'bulge', amongst his colleagues.

The same author, E. Athanassoula, also co-wrote a couple of other papers with M. Bureau in 2005 (mainly to do with Bar-Driven Evolution & 2D Spectroscopy and N-Body simulations of Disks, (if you're interested to do further reading).

Incidentally, all of the above references originally came from Clarkson's paper here, (the one this article is about), and is worth the read, as it puts the whole announcement into better context, (than, perhaps, the journo articles in the media).

Hope this helps.

Cheers & Rgds.

[mjc (Mark)]

Thank you Craig - The last reference has immediately caught my eye but will give them all an eye ball.

Yes it is probably a definitional problem.

Mark C.

[renormalised (Carl)]

No need to start trying to define what is and isn't a nuclear bulge here. You're only going to make matters worse for yourself because just about every paper written on the subject has a difference of opinion. If you follow what's the accepted definition of a nuclear bulge region of a galaxy, you'll be doing fine. If a new definition merges from the research then wait for it to emerge, don't try to second guess it when you don't have the background in the subject. You'll only confuse yourself.

Just for the record, you have galaxies with typical elliptical central concentrations of stars....classical bulges and others with a far more flattened spiral shaped structure in their centres...pseudobulges. Galaxies with pseudobulges have stars which orbit about the core much the same as the spiral arms of the galaxies in question.

SO-2 lies in the core of the galaxy, which by definition is at the centre of the bulge. However, not all core regions in galaxies are at the centres of the nuclear bulges...but that's another story.

Blue stragglers are formed from the collision and merging of two or more stars in tightly packed regions of stars....such as the environs of globular clusters and nuclear bulges of galaxies. Basically, two or so older stars come into gravitational contact with one another, spiral into or collide with one another and the resulting star which forms from this is both larger and hotter (hence bluer) than the previous two stars were. The new star which forms gets a new lease of life from the merger of its predecessors. The reason why it shows that the nuclear bulge of the galaxy is mostly populated by old stars is that this sort of collision normally occurs amongst the old Pop II stars of globular cluster and the nuclear bulge. These stars formed early and rather quickly (efficiently) in the formation of the galaxy. So, any of the larger stars have now gone and those that are left to merge are older stars...mostly late G and K and M class stars. The bulge is essentially the large elliptical mass of stars surrounding the central regions of the galaxy, excluding the central bar. Whilst the bulge stars have orbits about the centre that are similar to what occurs with stars in an elliptical galaxy (random, elliptical orbits), the stars, gas and dust in the central bar rotates as if it's a solid object. The bar does affect the orbits of those stars and gas/dust lying outside the bar to a certain extent.