PDA

View Full Version : Discussion: Helium-Richest Stars Found



Fraser
2005-Mar-15, 05:30 PM
SUMMARY: European astronomers have found a group of stars in the Omega Centauri globular cluster which seem to be some of the most rich in helium ever seen. These are bluer stars, and astronomers would expect that they contained lighter elements, but the team found exactly the opposite - they have more heavy elements than red stars. One theory to explain this is that previous generations of stars exploded as supernovae, and seeded the region with helium and heavier elements. These blue stars then formed from this material.

View full article (http://www.universetoday.com/am/publish/helium_richest_stars.html)

What do you think about this story? Post your comments below.

scorpio711
2005-Mar-15, 05:56 PM
Wow !!! Very intriguing !!! First time a globular cluster shows (first) a population II stars (and second) with so much Helium !!!
Did this cluster have special characteristics (age, position, size ?) to have permitted such evolution ?
This story raises a lot of additional questions !!!!
Scorpio

antoniseb
2005-Mar-16, 07:13 AM
Originally posted by scorpio711@Mar 15 2005, 05:56 PM
Did this cluster have special characteristics (age, position, size ?) to have permitted such evolution ?
Omega Centauri is probably not a globular cluster, as far as it's origins are concerned. It is most likely the remaining core os a galaxy that the Milky Way has consumed. Thus, instead of having just one episode of star formation, as most globualrs had, this should have a normal galaxy's history, followed by a final period of furious star formation from when it hit the Milky Way.

astromark
2005-Mar-16, 08:00 AM
:) :rolleyes: :P :D :lol: I am not sorry for this, some body had to say it. . . .
We should go there it could be a laugh. .
Dispite the Helium the stars would still be hydrogen based a. . . or have I got that wrong too. :unsure: :blink: :rolleyes:

madaboutyou
2005-Mar-16, 01:34 PM
good for them

Greg
2005-Mar-16, 05:02 PM
I don't see why it is a surprise that a second generation of stars would have higher helium content than the first. What is interesting here is the inordinate degree to which helium is concentrated in the second generation. I would agree with the interpretation that the star cluster must have been just the right size to retain helium from the death throes of the large first generation stars while being too small to retain the heavier elements dispersed from their deaths

scorpio711
2005-Mar-16, 09:58 PM
Some interesting info on Omega Centauri here:
http://www.seds.org/messier/xtra/ngc/n5139.html
Omega Centauri is still named (same into the SIMBAD database) "globular cluster"... but extremely massive (5 million solar mass) and with a "multiple population".
Antoniseb, you are right :) (as usual ;) !), they mention the possibility that it's a remnant of a nucleus of a galaxy "eaten" by our MW !
So yes, I can imagine that a nucleus of a galaxy is very early generating numerous large stars thus resulting into high rate of supernovae... thus deeply enriching the interstellar medium in He (and other metals).
Would be interesting to have abundances of other metals !!! (Greg, I didn't see any mention that the heavier elements are not retained in the cluster?)
Scorpio

wstevenbrown
2005-Mar-17, 06:41 AM
Perhaps I am confused. If the cause of the He enrichment was supernovae, then the abundance of 'other metals' should be higher than 2%. Did they mean to say that the local ISM was enriched by pre-nova stellar winds, and the starburst was driven by the compression of SN blasts? How was the He enriched without an even greater enrichment of C, O, N, Si, Al, Mg, etc.?

A possible confound of the method: adding together all of the spectra of the 'blue' objects will completely swamp the contributions of the Blue Straggler component of the population, which ought to be considered separately. Admittedly, in most clusters and galactic nuclei, the Blue Stragglers are more common near the center (this sample seems to have been taken from the edge, for better image separation), but there are not many "globular clusters" with a mass of 5MSols.

I concur with Antoniseb-- this is the remnant nucleus of a captured galaxy. Possibly a large percentage of GC's are.

I don't think that stellar evolution models need to be scrapped. It is only that we need to know what peculiar collision/evolution timeline would produce this peculiar stellar population. Life's getting interesting. Best regards-- Steve :)

antoniseb
2005-Mar-17, 08:34 AM
Originally posted by wstevenbrown@Mar 17 2005, 06:41 AM
I concur with Antoniseb-- this is the remnant nucleus of a captured galaxy. Possibly a large percentage of GC's are.
Maybe. I tend to think that most globular clusters are artifacts of the formation of the central black hole in a galaxy. They mostly all fit into a certain size range that Omega Centauri is way outside of. They also all seem to have stars that formed at very close to exactly the same time (not counting blue stragglers).

An interesting question that I don't know the answer to which would support this idea would be whether there are any globular clusters orbiting the LMC and SMC. If there are none, or only a few captured strays, that would lend some credence to the idea that the LMC and SMC are remnants of a close encounter between M31 and the Milky Way a long time ago, and do not have central black holes of their own. It would also support (but far from prove) the idea that globulars are associated with the formation of the central black hole, as not all galaxies have them.

Guest
2005-Mar-17, 02:49 PM
An absence of globular clusters around the core of the SMC and LMC would be expected, but the reason could be either of the reasons iterated above or both. A lack of clusters would be expected if the primary process creating them is SMBH formation. If you believe in the accretion method of galaxy growth, then the lack of clusters could stem from the galaxy's small size from its formation leading to its inability to capture many objects or just bad luck in that it formed in isolation and remained small all of the time. More likely in my mind is that the SMBH formation generates the GCs and is a function of the initial size of the galaxy. THe increased size would make it more likely that the galaxy would capture other smaller galaxies, so the number of GCs cannot be easily used to differentiate which of the two methods (accretion or SMBH formation) is most likely generating the clusters. What would be most surprising is if the LMC or SMC have a proportional number of GCs compared to the milky way galaxy.

Planetwatcher
2005-Mar-19, 08:49 AM
Normally blue stars are hotter and larger then most other main sequence stars.
So would that be true of these as well?