Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

A Tale of Two Populations - VLT FLAMES Finds Hints of Helium-Richest Stars Ever Seen

16.03.2005


On the basis of stellar spectra totalling more than 200 hours of effective exposure time with the 8.2-m VLT Kueyen telescope at Paranal (Chile), a team of astronomers [1] has made a surprising discovery about the stars in the giant southern globular cluster Omega Centauri.

It has been known for some time that, contrary to other clusters of this type, this stellar cluster harbours two different populations of stars that still burn hydrogen in their centres. One population, accounting for one quarter of its stars, is bluer than the other.

Using the FLAMES multi-object spectrograph that is particularly well suited to this kind of work, the astronomers found that the bluer stars contain more heavy elements than those of the redder population. This was exactly opposite to the expectation and they are led to the conclusion that the bluer stars have an overabundance of the light element helium of more than 50%. They are in fact the most helium rich stars ever found. But why is this so?



The team suggests that this puzzle may be explained in the following way. First, a great burst of star formation took place during which all the stars of the red population were produced. As other normal stars, these stars transformed their hydrogen into helium by nuclear burning. Some of them, with masses of 10-12 times the mass of the Sun, soon thereafter exploded as supernovae, thereby enriching the interstellar medium in the globular cluster with helium. Next, the blue population stars formed from this helium-rich medium.

This unexpected discovery provides important new insights into the way stars may form in larger stellar systems. The full text of this ESO Press Release 07/05, with all links and four photos, is available at http://www.eso.org/outreach/press-rel/pr-2005/pr-07-05.html

Two Populations

Globular clusters are large stellar clusters some of which contain hundreds of thousands of stars. It is generally believed that all stars belonging to the same globular cluster were born together, from the same interstellar cloud and at the same time. Strangely, however, this seems not to be the case for the large southern globular cluster Omega Centauri.

Omega Centauri is the galactic globular cluster with the most complex stellar population. Its large mass may represent an intermediate type of object, between globular clusters and larger stellar systems such as galaxies. In this sense, Omega Centauri is a very useful "laboratory" for better understanding the history of star formation.

However, it appears that the more information astronomers acquire about the stars in this cluster, the less they seem to understand the origin of these stars. But now, new intriguing results from the ESO Very Large Telescope (VLT) may show a possible way of resolving the present, apparently contradictory results.

Last year, an international team of astronomers [1], using data from the Hubble Space Telescope (HST), showed that Omega Centauri, unlike all other globular clusters, possesses two distinct populations of stars burning hydrogen in their centre. Even more puzzling was the discovery that the bluer population was more rare than the redder one: they accounted for only a quarter of the total number of stars still burning hydrogen in their central core. This is exactly the opposite of what the astronomers had expected, based on the observations of more evolved stars in this cluster.

Over Two Weeks of Total Exposure Time!

The same team of astronomers then went on to observe some of the stars from the two populations in this cluster by means of the FLAMES instrument on the Very Large Telescope at Paranal. They used the MEDUSA mode, allowing to obtain no less than 130 spectra simultaneously.

Twelve one-hour spectra were obtained for 17 stars of the blue population and the same number stars from the red one. These stars have magnitudes between 20 and 21, i.e., they are between 500,000 and 1 million times fainter than what can be seen with the unaided eye.

The individual spectra of stars from each population were then co-added. This produced a "mean" spectrum of a blue-population star and another of a red-population. Each of these spectra represents a total of no less than 204 hours of exposure time and accordingly provides information in unrivalled detail about these stars, especially in terms of their chemical composition.

The scientific outcome matches the technical achievement!

From a careful study of the combined spectra, the astronomers were able to establish that - contrary to all prior expectations - the bluer stars are more "metal-rich" (by a factor two) than the redder ones. "The latter were found to have an abundance of elements more massive than helium corresponding to about 1/40 the solar abundance [2] ", explains Raffaele Gratton of INAF-Osservatorio Astronomico di Padova in Italy. "This is indeed very puzzling as current models of stars predict that the more metal-rich a star is, the redder it ought to be".

Giampaolo Piotto (University of Padova, Italy), leader of the team, thinks that there is a solution to this celestial puzzle: "The only way we can explain this discrepancy is by assuming that the two populations of stars have a different abundance of helium. We find that while the red stars have a normal helium abundance, the bluer stars must be enriched in helium by more than 50% with respect to the other population!"

These stars are thus the most helium-rich stars ever found, and not by just a few percent! It took some 8 billion years for the Milky Way Galaxy to increase its helium abundance from the primordial 24% value (created by the Big Bang) to the present solar 28% value, and yet in a globular cluster that formed only 1 or 2 billion years after the Big Bang, stars were produced with 39% of helium!

Contamination from supernovae

The obvious question is now: "Where does all this helium come from?"

Luigi Bedin (ESO), another member of the team, suggests that the solution might be connected to supernovae: "The scenario we presently favour is one in which the high helium content originates from material ejected during the supernovae explosions of massive stars. It is possible that the total mass of Omega Centauri was just right to allow the material expelled by high-mass supernovae to escape, while the matter from explosions of stars with about 10-12 times the mass of the Sun was retained."

According to this scenario, Omega Centauri must therefore have seen two generations of stars. The first generation, with primordial helium abundance, produced the redder stars. A few tens of million years later, the most massive stars of this first generation exploded as supernovae. The helium-enriched matter that was expelled during the explosions of stars with 10-12 times the mass of the Sun "polluted" the globular cluster. Then a second population of stars, the bluer ones, formed from this helium-rich gas.

The scientists acknowledge that certain problems still remain and that the last word may not yet have been said about this unusual globular cluster. But the new results constitute an important step towards the solution of the biggest mystery of all: why is Omega Centauri the only one among the galactic globular cluster that was able to produce super helium-rich stars?

Notes

[1]: The team is composed of Giampaolo Piotto, Giovanni Carraro, Sandro Villanova, and Yazan Momany (University of Padova, Italy), Luigi R. Bedin (ESO, Garching), Raffaele Gratton and Sara Lucatello (INAF- Osservatorio Astronomico di Padova, Italy), Santi Cassisi (INAF- Osservatorio Astronomico di Teramo, Italy), Alejandra Recio-Blanco (Observatoire de Nice, France), Ivan R. King (University of Washington, USA), and Jay Anderson (Rice University, USA).

[2]: Helium is the second most abundant chemical element in the Universe, after hydrogen. The Sun contains about 70% hydrogen and 28% helium. The rest, about 2%, is made of all elements more heavier than helium. They are commonly referred to by astronomers as "metals".

Richard West | alfa
Further information:
http://www.eso.org/outreach/press-rel/pr-2005/pr-07-05.html

More articles from Physics and Astronomy:

nachricht NASA's James Webb Space Telescope completes final cryogenic testing
21.11.2017 | NASA/Goddard Space Flight Center

nachricht Previous evidence of water on mars now identified as grainflows
21.11.2017 | US Geological Survey

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Nanoparticles help with malaria diagnosis – new rapid test in development

The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.

Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

Previous evidence of water on mars now identified as grainflows

21.11.2017 | Physics and Astronomy

NASA's James Webb Space Telescope completes final cryogenic testing

21.11.2017 | Physics and Astronomy

New catalyst controls activation of a carbon-hydrogen bond

21.11.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>